GB2049325A - Tuning a trap circuit - Google Patents

Abstract

A network for attenuating the adjacent channel sound carrier signal in a television receiver includes first (60) and second (70) trap circuits intermediate the R.F. mixer (30) and the first I.F. amplifier (100). The trap circuits are tuned to frequencies in the vicinity of and above and below respectively the nominal frequency of the adjacent channel sound carrier and are mutually coupled to produce a composite response curve exhibiting maximum attenuation in the vicinity of the nominal frequency location of the adjacent channel sound carrier. The trap circuits provide attenuation in excess of that required to eliminate adjacent channel sound carrier interference over a bandwidth sufficient to encompass expected carrier deviation due to frequency modulation, carrier mislocation, and trap mistuning. A method of tuning at least one of the trap circuits to the nominal frequency is provided by comparison in a process controller 20 which adjusts inductors of the network until the null frequency of the trap circuit is aligned with the nominal frequency so as to provide the required network characteristic. <IMAGE>

Description

SPECIFICATION Tuning a trap circuit This invention relates to tuning a trap circuit, e.g. in television intermediate frequency (I.F.) selectivity networks, and more particularly, to a network and method of tuning same, which attenuates the sound carrier signal of the channel adjacent to the desired television channel.

In a television receiver, signals at radio frequencies (R.F.) are received, amplified, and converted to intermediate frequencies by heterodyning with the output of an oscillator in a mixer. The frequency of the oscillator signal is controlled by the television channel selector so that the mixer will convert the R.F. signals of a selected television channel to specific l.F. frequencies. In the typical NTSC system, the sound carrier of the selected channel is converted to 41.25 MHz, the color carrier is converted to 42.17 MHz and the picture carrier is converted to 45.75 MHz. However, the mixer will indiscriminately convert all of the received R.F. signals to differing intermediate frequencies, including those of channels above and below the selected channel.The picture carrier of the upper adjacent channel is converted to 39.75 MHz, and the sound carrier of the lower adjacent channel is converted to 47.25 MHz. If the upper and lower adjacent channel signals have an appreciable amplitude with respect to the signals of the selected channel, (e.g., within 30 db), they can interact with the signals of the selected channel to produce distortions within the band of frequencies of the selected channel signals. For instance, the 47.25MHz adjacent channel sound carrier is only 1.5 MHz away in frequency from the 45.75 MHz picture carrier of the selected channel. The 47.25 MHz sound carrier can intermodulate with the 45.75 MHz picture carrier as a result of nonlinear operation ofthe l.F.

amplifier, to produce an undesired signal 1.5 MHz lower than the picture carrier, at 44.25 MHz. This undesired signal would be detected as a video information signal by the video detector in the television receiver, thereby creating an interfering pattern in the television image on the kinescope.

Even if the l.F. amplifier is operating linearly so as to prevent intermodulation distortion, the presence of the adjacent channel sound carrier can still cause problems in the video detector. If the adjacent channel sound barrier has an appreciable amplitude after l.F. amplification, it can be detected as a video information signal by the video detector and will appear at 1.5 MHz in the detected baseband signal.

The adjacent channel sound carrier will thus appear as visible interference in the television picture. This interference will be visible even when the sound carrier is as much as 30 db lower in amplitude than the selected channel picture carrier.

To prevent the above-described distortion and interference, television receivers customarily employ circuits which remove, or trap out, the lower adjacent channel sound carrier, as well as the upper adjacent channel picture carrier, prior to l.F. signal processing. The above problems, which involve the adjacent channel sound carrier, are prevented by maintaining at least a 40 to 45 db differential between the adjacent channel sound and selected channel picture carriers in the television receiver.

Although the sound carrier is normally broadcast at an amplitude which is 3 to 6 db lower than that of the picture carrier of the same channel, the adjacent channel sound carrier can have a larger amplitude than that of the selected channel picture carrier when the received selected channel signal is weaker (i.e., from a more distant station) than that of the lower adjacent channel. It is therefore often necessary to attenuate the adjacent channel sound carrier by more than 40-45 db in order to ensure the required amplitude differential between these two carriers.

Trap circuits for attenuating the adjacent channel sound and picture carriers are normally coupled between the mixer stage and the first l.F. amplifier of the television receiver. A typical arrangement is illustrated in RCA Television Service Data File 1978, C-2, for the CTC-87 Series Chassis. The l.F. signals from the tuner and mixer are first applied to a bridged-Ttrap, comprising the parallel combination of a capacitor and an inductor. An intermediate tap on the inductor is coupled to an l.F. signal grounding point by a resistor. The trap is adjusted by moving two cores in the inductor, one of which adjusts the inductance of the trap, and a second which adjusts the Q and hence the bandwidth and depth of attenuation of the trap. Signals at the lower adjacent channel sound frequency are sharply attenuated by this bridged-T trap.The l.F. signals are then applied to a parallel L-C trap circuit, including an adjustable inductor having a single core for tuning the circuit to attenuate the upper adjacent channel picture carrier at 40 MHz. The l.F. signals are then applied to the first l.F. amplifier.

A bridged-T trap such as that used in the CTC-87 chassis is capable of attenuating signals by as much as 70 db at its center tuned frequency. However, the response characteristic of the trap has sharp "skirts" which define the bandwidth of the trap at varying levels of attenuation. At a 45 db level of attenuation, for instance, the bandwidth of the trap is approximately32 KHz, which means that signals at frequencies within 16 KHz of the center tuned frequency will be attenuated by 45 db or more; frequencies further away from the center frequency will be attenuated by less than 45 db.

The 32 KHz bandwidth can, under certain conditions, result in insufficient trapping of the adjacent channel sound carrier due to frequency shifts of the carrier. Frequency modulation of the sound carrier causes frequency deviations of the carrier over a 50 KHz range centered around its nominal center frequency. The frequency locations of the l.F. carriers may be in error by as much as 50 KHz, even with television receivers which employ automatic fine tuning circuits. And the center tuned frequency of the adjacent channel sound trap is generally adjusted to a tolerance of 20 KHz of its desired center frequency. When these tolerances combine, it is seen that the adjacent channel sound carrier can be anywhere within 95 KHz of the frequency location of the adjacent channel sound carrier trap.When the carrier frequency differs from the center tuned frequency of the CTC-87 trap by 70 KHz, it will be only 29 db lower than the level of the selected channel picture carrier, thereby causing video detection of the adjacent channel sound carrier and possible intermodulation distortion in the television receiver. The 29 db difference will be even less if the adjacent channel sound carrier is received from a stronger broadcast signal than that of the selected channel.

This problem becomes worse when the television receiver is receiving signals from a CATV system.

First, the sound and picture carriers are generally transmitted at equal amplitudes by the CATV broadcast equipment, which eliminates the usual 3-6db signal differential which is characteristic of freely radiated broadcast signals. Second, the CATV broadcast equipment locates all of the carriers in the frequency spectrum of the CATV system. It is possible that the CATV equipment may not always maintain the nominal 1.5MHz spacing between channels. Thus it is to be expected that the adjacent channel sound carrier may not be located at 47.25 MHz in the television receiver, but may deviate above and below this frequency depending on the performance of the particular CATV system.

Finally, automated tuning and alignment equipment is increasingly used in the manufacture and assembly of television receivers. Such automated equipment is capable of tuning trap inductors to their desired values, such as the single coil inductor in the 40 MHz trap in the CTC-87 chassis. However, the inductor of the bridged-T trap in the CTC-87 chassis employs two coaxially aligned cores for adjusting the inductance and the Q of the inductor.

This inductor is adjusted by first adjusting the inductance core, then adjusting the 0 core, then readjusting the inductance core to account for variations caused by the Q adjustment. Such a procedure is undesirable in an automated tuning and alignment system due to its complexity. It is such more desirable to use single core inductors in an automated tuning and alignment system, in which only a single adjustment is required for each inductor.

A trap or filter circuit may be characterized mathematically by a transfer function containing poles and zeroes. These poles and zeroes are related to maximum points (i.e., minimum attenuation) and minimum points (i.e., maximum attenuation) in the characteristic response curve of the circuit. When an inductor in a filter circuit is adjusted, the frequency locations of the poles and zeroes of the circuit are effectively adjusted to develop the desired response curve for the circuit.

When several filter circuits are coupled together, they may not exhibit a response which is the product of their individual responses, but can interact with each other to develop a response which is characteristic of the unique combination. Thus, tuning one filter circuit in the combination may not merely alter one specific part of the response characteristic, but can affect the frequency locations of numerous poles and zeroes and consequently the overall shape of the response characteristic. Although such a filter combination is susceptible to rigorous mathematical analysis of the filter transfer functions to predict the results of such adjustment, this analysis is often complex and time-consuming. It is usually easier to apply a test signal to a filter combination while the filters are being tuned.The results of the tuning on the test signal are monitored, until a point is reached atwhich the combination exhibits the desired characteristic response.

This trial and error method of filter circuit tuning may be advantageously employed in the assembly and adjustment of a television receiver. When automated by the use of computer-controlled tuning mechanisms and test signals, complex filter circuits can be quickly and easily adjusted to exhibit a desired response. A complex filter circuit which is adjusted in this manner is the intermediate frequency passband shaping network to be described in greater detail below.

In accordance with the principles of the present invention, a network is provided for attenuating the adjacent channel sound carrier signal in a television receiver and a method -is provided for tuning a trap circuit in the network.

In accordance with the principles of the present invention, a method of tuning a trap circuit to a desired frequency is provided, comprising the steps of: (a) applying a constant amplitude signal, of successively different frequencies in a progression of frequency increments, over a given frequency range including the desired frequency, to the trap circuit, while (b) detecting the amplitude of the signal as modified by the trap circuit at each of the different frequencies, and (c) sequentially storing the detected amplitudes;; (d) comparing successive ones of the stored amplitudes until an amplitude difference is found which is greater than a predetermined minimum difference and is of a polarity indicative of an increase in amplitude with the progression, and is a successor of an amplitude difference which is greaterthan the predetermined minimum difference and is of a polarity indicative of a decrease in amplitude with the progression; and (e) adjusting the trap circuit in accordance with the frequency difference between the frequency of inflection indicated by the first named amplitude difference and the desired frequency.

Steps (a) through (d), hereinafter referred to as the slope-search method, will result in the development of the response curve of the filter circuit, despite the presence of noise and nonlinearities in the automated alignment system.

In accordance with a further aspect of the present invention, a method of tuning two cascaded trap circuits to locate their characteristic null frequencies at first and second given frequencies above and below (or below and above) a desired centerfrequency is provided, comprising the steps of: (a) tuning the two trap circuits until their null frequencies are located at approximately the desired center frequency; (b) adjusting one of the trap circuits so as to locate its null frequency at a third frequency above (or below) the first given frequency; (c) adjusting the other of the trap circuits so as to locate its null frequency substantially at the second given frequency; and (d) adjusting the trap circuit specified in step (b) so as to locate its null frequency substantially at the first given frequency.

In a preferred embodiment of the network, first and second trap circuits are serially coupled intermediate the R.F. mixer and the first l.F. amplifier. One of the trap circuits is tuned to a frequency in the vicinity of and above the nominal frequency of the adjacent channel sound carrier, and the other is tuned to a frequency in the vicinity of and below that of the adjacent channel sound carrier. The two trap circuits are mutually coupled to a degree necessary to produce a composite response curve exhibiting a substantially constant level of maximum attenuation in the vicinity of the nominal frequency location of the adjacent channel sound carrier.The trap circuits provide attenuation in excess of that required to eliminate adjacent channel sound carrier interference over a bandwidth sufficient to encompass expected carrier deviation due to frequency modulation, carrier mislocation, and trap mistuning. In the illustrative embodiment of the present invention, the trap circuits comprise bridged-Ttrap circuits, which utilize only single core inductors, thereby simplifying the automated tuning and alignment ofthe television receiver.

In the drawings: Figure 1 illustrates, partially in block diagram form and partially in schematic diagram form, a low impedance network constructed in accordance with the principles of the present invention; Figure 2 illustrates, partially in block diagram form and partially in schematic diagram form, a high impedance network constructed in accordance with the principles of the present invention; Figure 3 illustrates response curves of a single adjacent channel sound trap and a double adjacent channel sound trap; Figure 4 illustrates a typical response curve for the networks of Figures 1 and 2; and Figure 5 illustrates a typical response curve for an overcoupled pair of adjacent channel sound traps.

Figure 6 illustrates, partially in block diagram form and partially in schematic diagram form, apparatus arranged to perform the method of the present invention; and Figures 7through 16 illustrate waveforms explaining the operation of the apparatus of Figure 1 in accordance with the tuning method ofthe present invention.

Referring to Figure 1, a network constructed in accordance with the principles of the present invention is shown as a part of the input circuitry of a television receiver. R.F. television signals are received by an antenna 10 and coupled to an R.F.

amplifier 20. The amplifier signals are then applied to an oscillator and mixer 30, where the signals are converted to l.F. frequencies. In the typical NTSC system and in this example, the selected television channel is converted to frequencies centered around 44 MHz. The upper adjacent channel picture carrier will then be located at approximately 39.75 MHz, and the lower adjacent channel sound carrier will be located at approximately 47.25 MHz.

The oscillator and mixer 30 is coupled to an input attenuator 40. The input attenuator 40 isolates the oscillator and mixer 30 from subsequent reactive circuit elements, and provides a proper terminating impedance for the oscillator and mixer 30. Typically, the l.F. signals are coupled to the attenuator 40 by a 50 ohm coaxial cable, which must be terminated by a 50 ohm load to prevent signal reflections. The input attenuator also performs an impedance transformation to match the coaxial cable to the input impedance of the next stage of reactive elements. In the present example, the input attenuator 40 provides the necessary impedance transformation to match a 50 ohm cable from the oscillator and mixer to a selectivity network having an impedance of approximately 40 ohms.The input attenuator 40 consists of a shunt resistor 41, the parallel combination of a capacitor 45 and the serial combination of a capacitor 42 and a resistor 43, and a second shunt resistor 44.

The input attenuator 40 is coupled to a first selectivity network 50, consisting of a capacitor 56 shunted to ground, and the serial combination of an adjustable inductor 52 and a capacitor 54. The first selectivity network 50 is coupled by two adjacent channel sound traps 60 and 70 to a second selectivity network 90. The second selectivity network 90 consists of the series connection of a capacitor 92 and an adjustable inductor 92, and the series combination of a resistor 96 and a capacitor 98 shunted to ground. The two selectivity networks cooperate to shape the l.F. passband of the selected television channel. The picture and color carriers are normally located on the upper and lower slopes of the passband response curve and are attenuated by 3 db relative to the center band gain. The sound carrier is usually located 20 db down on the lower slope of the response curve.Adjustable inductors 52 and 94 are adjusted to shape the passband.

The second selectivity network 90 is coupled to a first l.F. amplifer 100. In addition to band shaping, the second selectivity network 90 also provides an impedance transformation of the l.F. signals from a low impedance to a high impedance which better matches the high input impedance of the first l.F.

amplifier. The I.F. signals are amplified by the first l.F. amplifier 100 and then applied to a second l.F.

amplifier (not shown) for further amplification and signal processing.

Coupled between the first and second selectivity networks 50 and 90 are first and second adjacent channel sound traps 60 and 70. These two traps are each arranged in a bridged-T configuration. The first adjacent channel sound trap 60 is comprised of a resistor 62 coupled in parallel with two serially connected capacitors 64 and 66. An adjustable inductor 68 is coupled from the junction of the two capacitors 64 and 66 to ground. The second adjacent channel sound trap 70 is similarly arranged and includes the parallel combination of a resistor 72 and serially connected capacitors 74 and 76. An inductor 78 is coupled from the junction of the capacitors 74 and 76 to ground.

An upper adjacent channel picture trap 80 is coupled to ground from the junction of the second adjacent channel sound trap 70 and the second selectivity network 90. The adjacent channel picture trap 80 includes the series combination of a capacitor 82 and an inductor 84 coupled to ground. The trap 80 is a high 0 trap which is tuned to approximately 40 MHi Frequencies in the vicinity of this frequency, including the upper adjacent channel picture carrier and many of its sidebands, are severely attenuated by this trap.

The network of Figure 1 is a low impedance filter network, with the traps having an impedance of approximately 10 to 15 ohms. A high impedance equivalent of the network of Figure 1 is illustrated in Figure 2, comprising a network having an impedance in the range of 200 to 700 ohms, depending upon component values. The performance ofthe two networks is substantially the same, but the low impedance circuit is advantageous in that it uses no tapped inductors and has capacity values with high Q.

Referring to Figure 2, the I.F. signals produced by the oscillator and mixer 30 are applied to a first selectivity network 150 by an inductor 140. The first selectivity network is comprised of the parallel combination of an adjustale inductor 152 and a capacitor 154 coupled between the I.F. signal path and ground. As in Figure 1, the first selectivity network 150 cooperates with a second selectivity network 190 to provide shaping of the I.F. passband.

The second selectivity network 190 is comprised of the parallel combination of an adjustable inductor 192 and two serially coupled capacitors 194 and 196.

The second selectivity network is coupled between the I.F. signal path and ground. l.F. signals are taken from the junction of the capacitors 194 and 196 and applied to the first l.F. amplifier 100.

The first selectivity network 150 is coupled to the second by way of two adjacent channel sound carrier traps 160 and 170 and an adjacent channel picture carrier trap 180. The first adjacent channel sound trap 160 consists of the parallel combination of a capacitor 162 and an adjustable inductor 164 coupled in series with the I.F. signal path. A resistor 166 is coupled from a tap on the inductor 164 to ground. The second adjacent channel sound trap 170 is similarly comprised of the parallel combination of a capacitor 172 and an adjustable inductor 174 disposed in series with the I.F. signal path, and a resistor 176 coupled from a tap of the inductor 174 to ground.The adjacent channel picture trap 180 is coupled in series between the second adjacent channel sound trap 170 and the second selectivity networK 190, and consists of the parallel combination of a capacitor 182 and an adjustable inductor 184. The adjacent channel picture trap 180 is a high Qtrap,tuned to approximately 40 MHz.

For reasons explained previously, it is desirable for the adjacent channel sound trap to attenuate the adjacent channel sound carrier by at least 40 to 45 dub over a 190 kc bandwidth centered on the nominal 47.25 MHz frequency location of the sound carrier.

Waveform 200 in Figure 3 illustrates a typical response curve for a single bridged-T sound trap, such as that used in the RCA CTC-87 chassis. While the trap has a depth of approximately -70 db at center frequency, it is seen to have a 190 kc bandwidth at an attenuation level of only db.

Thus, intermodulation distortion and sound carrier detection interference can occur in this system when the adjacent channel sound carrier varies over the expected range of 190 kc around the nominal adjacent channel sound carrier frequency location.

Ideally, it would be desirable to attenuate the adjacent channel sound carrier through the use of two consecutively coupled traps, each tuned to a centerfrequency of 47.25MHz. Such traps would produce the response curve 300 shown in Figure 3.

Response curve 300 is seen to have a maximum depth of approximately -90 db at center frequency, and has a 190 kc bandwidth at the -43 db level. This arrangement would adequately prevent the intermodulation distortion and sound carrier detection interference referred to previously.

However, in order to produce the response illustrated by waveform 300, it is necessary that both traps be independently tuned to the center frequency of 47.25 MHz, with no mutual coupling between the two traps. If construction of the trap circuits results in a small degree of mutual coupling between the traps, they will no longer exhibit the response shown by waveform 300, but will begin to exhibit characteristics of a double-tuned circuit.

Depending upon the degree of mutual coupling, the attenuation at center frequency will decrease considerably, thereby producing a double-humped response curve with frequencies of maximum attenuation above and below the center frequency. The mutual coupling can also cause the center frequency ofthe response curve to shiftto a lower frequency, thereby displacing the entire response curve to a lower frequency.

The two traps are desirably located between the oscillator and mixer circuit and the first l.F. amplifier, to attenuate the adjacent channel sound carrier prior to the I.F. amplifiers, where intermodulation distortion can occur. The two traps must thus be in close physical proximity to each other. This will cause some degree af mutual inductive coupling, either due to the close physical proximity of the inductor coils, or through the ground plane connecting the two traps. While it may be possible to construct a specific circuit with virtually no mutual coupling between the two traps, such a possibility must be discounted when the trap circuits are manufactured in quantity, such as occurs in the mass production of a television receiver. It is therefore necessary to allow for variations in mutual inductive coupling in - the design of a network which includes two adjacent channel sound traps.

In accordance with the principles of the present invention, and as noted above, the two adjacent channel sound traps of Figures 1 and 2 are not tuned to the nominal 47.25 MHz frequency of the adjacent channel sound carrier, but are tuned to frequencies above and below this frequency, respectively.

Advantage is taken of the small amount of mutual inductive coupling between the two traps to produce a characteristic response curve with a substantially flat bottom, centered at the nominal frequency location of the adjacent channel sound carrier. Such a response curve is illustrated as waveform 400 in Figure 4. The response curve 400 has a 190 kc bandwidth at an attenuation level of approximately -43 db, which is sufficient to prevent intermodulation distortion and sound carrier detection interference in the following I.F. amplifying stages and video detector.

The double traps of Figures 1 and 2 are easily aligned during adjustment of the television receiver to achieve the characteristic response shown in Figure 4. Initially, the two traps are randomly tuned in the range of 40 to 50 MHz. At first, both traps are tuned to 47.25 MHz. One trap is then tuned to a significantly higher frequency above 47.29 MHz, and the other is precisely tuned to 47.21 MHz. At this time, the response curve of the two traps will not have a flat bottom, but will exhibit lesser attenuation at frequencies intermediate the two trap settings, thereby presenting a double-humped appearance.

The higher tuned trap is then slowly returned toward its final nominal setting of 47.29 MHz. As this tuning proceeds, the double-humped response is gradually changed to that of the flat-bottomed response curve 400. Finally, when the flat-bottomed response is attained as the higher frequency trap reaches the vicinity of 47.29 MHz, the tuning process is terminated. This entire process is more fully explained below.

The Q's of the traps of the present invention are determined primarily by the component values of the traps, and will vary slightly from one trap to the next depending upon component tolerances. These Q variations do not affect the performance of the circuit, since the traps nominally have a combined attenuation depth in excess of -60 db. Although Q variations may decrease this depth slightly, such changes will not cause the trap depth to rise to the critical level of -45 db when the two traps are tuned in the vicinity of 47.25 MHz. By contrast, tolerance variations in the CTC-87 trap circuit can cause 0 changes which can decrease the trap depth to -35 db. These 0 changes must therefore be compensated by carefully adjusting the 0 adjustment core of the trap inductor.

Unlike the two-core inductor of the CTC-87 trap, the bridged-Ttraps of the present invention each require only a single core adjustment. Adjustment of the cores changes the tuning frequencies of the traps without any significant effect on their Q's. The two traps may be adjusted simultaneously, which facilitates automated adjustment of a television receiver using the traps.

Care must be taken during the initial layout and construction of the double-trap network of the present invention to ensure that the final assembly does not result in overcoupling of the two traps.

Overcoupling will cause the two traps to exhibit the characteristic response shown by waveform 500 in Figure 5. Waveform 500 is similar to the response of a typical double-tuned circuit as compared to that of waveform 400. More significantly, waveform 500 has a lesser attenuation level at the nominal frequency of the adjacent channel sound carrier than at frequencies above and below that frequency. However, the overcoupling problem is readily averted if careful attention is given to considerations such as inductor shielding and ground connections of the two traps.

Referring to Figure 6, a system is shown which automatically tunes five circuits. The filter circuits in this example comprise the intermediate frequency passband shaping network of a television receiver as described above.

The selectivity networks 50 and 90 and the trap circuits 60, 70, and 80 may be tuned to specific frequencies by adjusting the position of cores located in the inductors of the circuits. Each core has a threaded, cylindrical shape and may be screwed up and down through the field in the center of an inductor coil winding, thereby varying the inductance of the inductor. In the trap circuits 60,70, and 80, moving the core changes the frequency at which the respective trap provides maximum signal attenuation, the null frequency.

Test signals are applied to the filter network of Figure 1 by a connection from programmable frequency generator 24 to the input attenuator 40. The frequency generator 24 is controlled to apply constant amplitude test signals in sequential incremental frequency steps over a range of frequencies which includes the tuning frequencies of the filter traps. It is assumed in the following example that the filter traps 60 and 70 will be initially tuned to frequencies in the range of 40 to 50 MHz, which is the frequency generator range in this example.

A detector 26 is coupled to an output of the first I.F.

amplifier 100 to detect the amplitudes of the test signals as modified by the filter network. The values ofthe detected amplitudes are converted from analog form to digital form by an analog-to-digital converter 28. This digital data is then supplied to an input of a process controller 20.

A process controller 20, which may be a general purpose digital computer, controls the tuning process by stepping the frequency generator 24 through its incremental frequency steps. The controller sends a digital signal to a digital-to-analog converter 22, which converts the signal to an analog control voltage and applies it to the frequency generator. In addition, the process controller calculates the adjustments required to tune the filter circuits to the desired frequencies. After making these calculations, the process controller 20 sends control signals to a translator and stepping motor unit 30 by way of control lins 32,34, and 36. The translator receives the control signals and activates one of the stepping motors in the unit. The motors adjust the threaded cores of inductors 52, 68,78,84, and 94 as indicated by broken lines 240-248. The cores are screwed up and down through the fields in the centers of the inductor coils to adjust the frequencies of the filter circuits. The control signal on line 32 selects one of the inductors and the control signals on lines 34 and 36 determine whether the cores are to be screwed clockwise (down) or counterclockwise (up). If desired, directional control lines for each stepping motor may be provided from the process controller, enabling simultaneous adjustment of the inductors.

The apparatus of Figure 6 may be implemented to tune filter trap circuits 60 and 70 in the following manner. It is assumed, for purposes of this example, that the two traps are to be tuned to approximately 47.25 MHz and are to be critically coupled so as to exhibit a broad bandwidth, flat-bottomed response curve illustratively shown in Figure 15. Prior to tuning, the inductor cores are located so that approximately half of the core body is inserted into the upper portion of the coil winding, to a tolerance expressed as a given number of coil turns. When the cores are so located, the inductors are tuned to approximately the center of their adjustment ranges, and each trap circuit will exhibit a response curve with a null point of maximum attenuation located between 40 and 50 MHz in the frequency spectrum.

The two traps will exhibit a composite response 150 shown in Figure 7. The two null points IMN and IMNH, are seen to be located above and below 47.25 MHz in this example, but it is understood that the two null points may be located anywhere in the 40 to 50 MHz range.

The first step in the tuning process is to sweep the filter traps with a constant amplitude signal from the frequency generator, 24 over the 40 to 50 MHz range.

The frequency generator will perform this function under the control ofthe process controller 20, in incremental frequency steps, which in this example are 100 KHz. The filter traps will produce output signals of varying amplitudes over the range of sweep frequencies, which are detected by the detector 26, converted to digital data by AID converter 28, and stored in a sequential array by the process controller. The amplitude data, which is correlated to the frequency steps, is representative of the response curve of the two filter traps.

The process controller next analyzes the data to determine the null points of the two traps. If the filter circuit included only a single filter trap, it is possible that only one null point would be present, which could be easily found by selecting the minimum value of the amplitude data. This cannot be done, however, when two null points are present, as is the case of response curve 150. The two null points, IMNL and IMNH, can have different amplitudes due to various factors. For instance, the Q's of the trap circuits may differ at their initial frequency settings.

The null points are at signal levels which approach the noise level of the system, and may be 70 db lower in amplitude than the test signal. The null points will not be at a fixed frequency location, but will vary slightly in frequency and amplitude with system noise. Also, first I.F. amplifier 100, the detector 26 and the AID converter 28 may exhibit nonlinear responses and quantizing errors. Furthermore, the trap which produces IMNL may be adjacent in frequency to a pole of another filter circuit, such as the second selectivity network 90, which shapes the upper edge of the I.F. passband in the television receiver, in the vicinity of the picture carrier (45.75 MHz). In this case, IM N, would be expected to be at a higher amplitude than IMNH.A minimum point search would consequently locate IMNH immediately, but the next minimum point could be point 152; and the third minimum point could be point 154, both of which are lower in amplitude than IM NL. Therefore, a more sophisticated method must be used to positively identify IMNand IMNH.

In accordance with the principles of the present invention, a slope search technique is provided for locating the null point of one or more trap circuits.

Referring to Figure 8, the amplitude data is partially shown as a series of points V1 through VN.

The process controller operates on these data points by first subtracting V1 from V2, the result of which is representative of the slope of the response over the range of the first frequency step. The result is first compared to a given tolerance value, which in this example is 0.1. If the slope is less than this tolerance, the result is automatically assumed to be noiseinfluenced and is ignored. If the slope is greater than the tolerance, the result is presumed to be valid, and the sign of the result is examined to determine whether the slope is positive or negative. In this example of Figure 8, V2 - V1 yields a positive slope, which is recorded by the processor.

The data points are then incremented to determine the slope of V3 - V2. The foregoing process is repeated forthis calculation, and, if the result is valid, the slope is compared with the previous valid slope calculation. If, as in this result, the slope remains the same (i.e., positive), the data points are again incremented and the foregoing process is continued.

It may be seen from Figure 8 that the process will soon be incremented to data points V4 and V5, which will yield a negative slope result, as shown by slope line 160. The processor will note this slope change and record point V4 as the peak of the response curve, IMXL. Succeeding slope calculations should therefore yield negative results until the null point is passed. Taking this fact into account, the succeeding calculations of V7 - V6, V8 - V7, etcetera, are reversed and calculated as V6 - V7, V7 - V8, etcetera, so that the process controller can continue to look for a positiveto-negative sign change of the slope calculation.

The process continues, until a slope change is noted with the calculation V12 - V13, as indicated by slope line 170. Point V12 is then recorded by the process controller as IMNL, the null point of a trap. At this point in the process, the points may again be reversed in the calculation, and the process controller can continue the process to find the next response peak and the second null point. However, in the interests of efficiency, the process controller in the present example halts the slope search at this point, which is termed the low slope search. The process controller then begins a high slope search.

This search follows the same procedure as the low slope search, but begins in a descending sequence from the last points recorded, at 50 MHz. Referring to Figure 7, it may be seen that the high slope search will first find the high frequency peak point IMXH, and then the high frequency null point IMNH. The low and high slope searches have been found to be efficient in practice, because the initial data points analyzed are frequently on the negative slopes which descend into the null points. Thus, IMXL and IMXH are often not found since they often do not exist within the 40 to 50 MHz search range. The calculation reversals at these points are therefore omitted. More significantly, IMNL and IMNH have been found to be frequently located in the vicinity of 40 MHz and 50 MHz, respectively.The high-low slope search technique will therefore find the null points without the necessity of analyzing the large number of data points located between IMNL and IMNH.

The effect of the slope tolerance comparison is illustrated by Figure 9, which shows the peak of a waveform 180 containing a significant amount of noise. Without the tolerance comparison, slope calculations would result in V2 being recorded as a peak point, V3 a null point, and V4 a second peak point. in this example however, only the determination that V4 iS a peak point is valid; the other two findings are actually noise-induced results. But when the slopes are compared to the tolerance, it can be seen that the calculation V3 - V2 iS less than the tolerance and would be ignored as invalid.The next valid comparison would be V4 - Vg, which has the same slope sign as V2 -V. A slope change would not occur until the calculation of V8 - V4 or, if this value does not exceed the tolerance, the calculation of V6 V5.

At the end of the high and low slope searches, the process controller will have determined the frequency locations ofthe two null points IMNL and IMNH of Figure 2. The process controller will then proceed to tune both trap circuits 60 and 70 to 47.25 MHz. Since the controller knows the existing frequency locations of the null points, it can calculate how much to move the null points to center them at 47.25 MHz, but it does not know at this point which inductor corresponds with which null point. The controller will nevertheless assume that inductor 68 corresponds to point IM NL and inductor 78 corresponds to point IMNH, and will proceed to adjust the two inductors accordingly.

The inductors are adjusted by the translator and stepping motor in increments of two hundred steps per core revolution. At this point, each core step is assumed to result in a change of 2.75 KHz in the frequency of the null point of the trap.

As previously mentioned, the null points can initially be located anywhere in the frequency spectrum of 40 to 50 MHz. Five of these possibilities are illustrated in Figure 10. Both null points can be located below 47.25 MHz (waveform 102), or both can be located above 47.25 MHz (waveform 104).

The null points can be located on either side of 47.25 MHz (waveform 106), or one can be located at 47.25 MHz with the other point being either above (waveform 108) or below (waveform 110)47.25 MHz. In addition, the lower frequency null point of each possibility can correspond to filter trap 60 and the higher frequency null point to filter trap 70, or the two can be reversed. This ambiguity as to which null point relates to which filter trap means that the five waveforms of Figure 10 have five complementary waveforms. And, of course, it is possible for both null points to be initially located at 47.25 MHz. Thus, there are eleven possible combinations of null point locations which must be resolved successfully by the automated tuning system.

By proceeding in accordance with the following procedure, the automated tuning system of the present invention will tune the two filter traps to 47.25 M Hz without irretrievably tuning one of the filter traps out ofthe 40 to 50 MHz range. If this were to happen, the null point of the mistuned trap would be beyond the range over which the frequency generator provides null point location information to the system, and that trap could conceivably become lost to the tuning system.

For purposes of the present example, the procedure of the present invention will be explained in conjunction with the tuning of the initial waveform 102 shown in Figure 10, although it is understood that this procedure can be followed with any of the initial conditions of Figure 10. After performing a slope search, the process controller will calculate the number of core steps required to tune each of the two traps to 47.25 MHz. In this example, the process controller assumes that one core step results in a change of 2.75KHz in the location of the null point of a trap. If the number of core steps required exceeds fifty, the core is turned only fifty steps. If the required number of core steps is less than fifty, the core is turned only three-quarters of the required number of steps to prevent the possibility of overshooting 47.25 MHz.If the target frequency is overshot, the two null points will be reversed with respect to their respective filter traps in the subsequently produced response curve if the null points are closing on the target frequency from opposite directions. This would cause the system to mistune the traps away from the target frequency during the next adjustment.

After moving the two traps through one or more fifty step increments, the null points will begin to approach 47.25 MHz, as shown by waveform 120 of Figure 11. During this time, the system assumes that the lower frequency null point corresponds to filter trap 60, and the upper one to filter trap 70. This assumption may not be correct; and in this example, it is incorrect. It will be seen that the process controller will eventually discover this mistake and will correct it.

When the two null points are located as shown in waveform 120, the system will make a slope search and calculate that NP60 of trap 60 must be tuned X steps and NP70 of trap 70 must be tuned Y steps to reach the target frequency. The process controller will command the translator and stepping motors to tune the traps accordingly. However, since the assumed correspondence of the inductors to the null points was incorrect, NP60 will actually by moved Y steps, and NP70 will be moved X steps. This will result in the generation of the response curve of waveform 122 after the next slope search. But since the null points NP60 and NP70 are located closer to 47.25 MHz than they were in waveform 120, the system does not recognize its mistaken assumption as yet. The process controller now calculates that NP60 of trap 60 must be tuned X' steps, and NP70 of trap 70 must be tuned Y' steps, and that the cores of the two traps must be adjusted in opposite directions (i.e., clockwise and counterclockwise). The translator and stepping motors are again activated, and since the assumed trap and null point correspondence was wrong, NP60 and NP70 will move away from 47.25 MHz by Y' and X', respectively. After the next slope search, the system will discover that the two null points have diverged, as shown in waveform 124, and will reverse its assumption. Since the process controller now knows that NP70 is the lower frequency null point and NP60 the higher of the two, the system can now proceed to converge the two traps on 47.25MHz, as shown in waveform 126.

The two traps are tuned until the two null points NP60 and NP70 are both located within a certain frequency range W (e.g., 180 KHz), centered on 47.25 MHz, as shown in Figure 12. If the actual core step sizes are greater than the assumed value of 2.75KHz, further tuning at this point would only cause the two null points to overshoot the target frequency during subsequent tuning steps. The system would then mistakenly tune the two traps in the wrong direction, an error which would again have to be discovered and corrected. To prevent this osci 11 atory tu ning, the process is halted when the two null points are located within the range W.

In the tuning example of Figure 11, the mistakenly assumed trap and null point correspondence was corrected until relatively late in the turning process, after the assumed NP70 passed above the center frequency of 47.25 MHz. However, it is possible that the mistaken assumption will be discovered earlier, and may even be corrected after the first tuning adjustment. For instance, if the two traps are initially tuned as illustrated by waveform 106 of Figure 10, the two traps will diverge from 47.25 MHz after the first tuning adjustment if the assumed correspondence is wrong. The error would be discovered and corrected at this time. Furthermore, the individual null points can cross over the center frequency several times before tuning is completed, which may create the erroneous correspondence condition several times during the process.However, the method of the present invention will discover all of these errors, correct them, and eventually tune the two traps to the desired center frequency.

The automated tuning system now proceeds to move the two null points to precise frequency locations in order to develop the desired flatbottomed response curve. The system will no longer mistakenly tune the wrong trap, since they are both tuned to approximately 47.25 MHz. The precision of the system is increased by making minimum point searches, rather than slope change searches. The minimum point search technique will precisely locats the null points of the traps, since the results of the search include the effects of stray reactances on the circuits, which cannot be readily calculated during tuning. The resolution of the system is also increased by incrementing the frequency generator in smaller frequency steps during the search.

The first step of this final tuning process is to tune trap 70 up in frequency by one hundred and ten steps. The resultant response curve 130 is as shown in Figure 13. Next, trap 60 is turned down in frequency until NP60 is located at 47.21 MHz. During this tuning step, the frequency generator sweeps only a 400 KHz range, centered on 47.21 MHz, in steps of 5 KHz. This tuning continues until the minimum amplitude point is located at approximately 47.21 MHz, producing response curve 140 of Figure 14. NP70 is now moved to its final frequency location. Trap 70 is first tuned down in frequency by ten core steps. This ten step rotation is necessary in order to take the backlash out of the inductor, since the direction of core rotation is being reversed from the previous one hundred and ten step adjustment upward.A minimum search is now made to determine the precise frequency location of NP70. The core of trap 70 is then turned forty steps down in frequency and the frequency location of NP70 is again determined by a minimum search. The process controller next calculates the frequency change Ad of NP70 over this forty step change. The process controller will now calculate exactly how many steps are required to tune the null point of trap 70 to its final frequency location of 47.29 MHz. At this point, the minimum point search technique cannot be used, because the final response curve will have numerous noise-affected minimum points between 47.21 and 47.29 MHz and the resultant data would not determine the precise location of NP70.The process controller will thus command the translator and stepping motors to turn the core of trap 70 the number of steps calculated to locate NP70 MHz, thereby producing the response curve 200 of Figure 15.

In accordance with a further aspect of the present invention, the aforedescribed method is modified to tune a single trap circuit, such as the 40 MHz adjacent channel picture carrier trap 80 of Figure 1. A typical waveform 300 containing the null point NP80 of trap 80 is shown in Figure 16. NP80 can initially be located anywhere within the frequency range of 35 to 44 MHz, which is the frequency range swept by the frequency generator 24. As in the case of the double traps, a minimum search cannot be used to locate NP80, since the search results may show a minimum amplitude point at the lower end ofthe frequency range, as indicated by point 302. The search results may also show a second minimum point 306 located at a frequency higher than that of NP80.This minimum point 306 is caused by the adjacent location 304 of a pole of the first selectivity network 50, which may not as yet have been properly tuned. NP80 is located by a low slope search, starting at 35 MHz, which prevents the discovery of minimum point 306 before NP80 is found. The low slope search is conducted in frequency steps of .25 MHz and, as before, amplitude differences are compared to a minimum tolerance value and invalid difference values are disregarded.

The low slope search is halted when a negative-topositive slope transition occurs, in the vicinity of NP80. Once NP80 has been found, the process controller determines the adjustment necessary to locate the null point at the desired frequency of 40 MHz. The core of inductor 84 is rotated in the proper direction to tune the trap, limited as before to a maximum of 50 steps. During subsequent slope searches and adjustments, NP80 will begin to close in on 40 MHz. This process of slope searching and adjusting is halted when NP80 is located within a first frequency range extending from 30.5 to 40.5 MHz.

Once NP60 is located in the first frequency range, this range is swept by the frequency generator 24 and sampled in increments of 25 KHz to find the minimum amplitude point in the range. This minimum search is followed by a tuning adjustment, and the process is repeated until the minimum amplitude point is located in a second frequency range extending from 39.9 to 40.1 MHz. When this occurs, the adjacent channel picture trap is satisfactorily tuned, and the system can proceed to align the selectivity networks 50 and 90.

Claims (17)

1. In a television receiver including means for converting received radio frequency signals to intermediate frequency signals, the output of said converting means including a signal component of a selected television channel at a first intermediate frequency, and a signal component of an adjacent television channel at a second intermediate frequency subject to appearance within a given range of frequencies centered about a nominal frequency location; intermediate frequency signal processing apparatus comprising: means for amplifying said intermediate frequency signals; and a network coupled intermediate said converting means and said amplifying means, including in cascade: a first trap circuit tuned to a third frequency which is within said given range of frequencies and above said nominal frequency location; and a second trap circuit tuned to a fourth frequency which is within said given range of frequencies and below said nominal frequency location.

2. The apparatus of Claim 1 wherein said signal component at said first intermediate frequency is a picture carrier and said signal component at said second intermediate frequency is a sound carrier of an adjacent television channel.

3. The apparatus of Claim 1 or 2 wherein mutual coupling between said first and second trap circuits is provided to a degree establishing an overall response characteristic for said network which provides substantially the same attenuation for signals in the band of frequencies lying between said third and fourth frequencies.

4. The apparatus of Claim 3 wherein the level of attenuation provided in said band of frequencies lying between said third and fourth frequencies is of such magnitude that the response of said network over said range of frequencies is sufficiently low relative to the response of said network at said first frequency as to substantially preclude development in said amplifying means of inter-modulation products of an interfering level from said adjacent channel signal component.

5. The apparatus of any preceding claim, wherein said first and second trap circuits each include an adjustable inductor having a single tuning core for adjusting the inductance of said inductor.

6. The apparatus of Claim 2 wherein said fist trap circuit comprises: a first resistor serially coupled in said signal path; first and second serially coupled capacitors coupled in parallel with said first resistor; and a first adjustable inductor, having a core for varying the inductance of said inductor, and coupled between the junction of said first and second capacitors and a point of reference potential; and said second trap circuit comprises: a second resistor, serially coupled to said first resistor in said signal path; third and fourth serially coupled capacitors coupled in parallel with said second resistor; and a second adjustable inductor, having a core for varying the inductance of said inductor, coupled between the junction of said third and fourth capacitors and a point of reference potential.

7, The apparatus of Claim 2 wherein said first trap circuit comprises: a first capacitor serially coupled in said signal path; a first adjustable inductor, having an intermediate tap and a core for varying the inductance of said inductor, and coupled in parallel with said first capacitor; and a first resistor, coupled between said intermediate tap of said first adjustable inductor and a point of reference potential; and said second trap circuit comprises: a second capacitor serially coupled to said first capacitor in said signal path; a second adjustable inductor, having an intermediate tap and a core for varying the inductance of said inductor, and coupled in parallel with said second capacitor; and a second resistor, coupled between said intermediate tap of said second adjustable inductor and a point of reference potential.

8. Apparatus of Claim 2 further including a first selectivity network coupled to an output of said converting means; a second selectivity network coupled to an input of said means for amplifying said intermediate frequency signals; and a third trap circuit tuned to the nominal frequency of said adjacent channel picture carrier; said first trap circuit being coupled to said first selectivity network, said second trap circuit being coupled to said first trap circuit; and said third trap circuit being coupled between said second trap circuit and said second selectivity network, wherein said first and second selectivity networks are tuned to shape the passband of said selected television channel.

9. A method of tuning a trap circuit to locate the frequency at which the trap provides maximum attenuation at a desired frequency comprising the steps of: (a) applying a constant amplitude signal, of successively different frequencies in a progression of frequency increments, over a given frequency range including said desired frequency, to said trap circuit, while (b) detecting the amplitude of said signal as modified by said trap circit at each of said different frequencies, and (c) sequentially storing said detected amplitudes;; (d) comparing successive ones of said stored amplitudes until an amplitude difference is found which is greater than a predetermined minimum difference and is of a polarity indicative of an increase in amplitude with said progression, and is a successor of an amplitude difference which is greaterthan said predetermined minimum difference and is of a polarity indicative of a decrease in amplitude with said progression; and (e) adjusting said trap circuit in accordance with the frequency difference between the frequency of inflection indicated by said first named amplitude difference and said desired frequency.

10. A method of tuning two cascaded trap circuits to locate their characteristic null frequencies at first and second given frequencies above and below a desired center frequency comprising the steps of: (a) tuning said two trap circuits until their null frequencies are located at approximately said desired center frequency; (b) adjusting one of said trap circuits so as to locate its null frequency at a third frequency above said first given frequency; (c) adjusting the other of said trap circuits so as to locate its null frequency substantially at said second given frequency; and (d) adjusting said one trap circuit so as to locate its null frequency substantially at said first given frequency.

11. The method of Claim 10, wherein step (a) comprises the steps of: (a1) applying a constant amplitude signal, of successively different frequencies in a progression of frequency increments, over a given range of frequencies including said desired center frequency, to said trap circuits, while (a1) detecting the amplitude of said signal, as modified by said trap circuits, at each of said different frequencies, and (a3) sequentially storing said detected amplitudes; (a4) comparing said stored amplitudes to determine the null frequencies of said trap circuits; and (a5) adjusting said trap circuits in accordance with the frequency differences between said null frequencies and said desired center frequency so as to locate said null frequencies at approximately said desired center frequency.

12. The method of Claim 10, wherein step (c) comprises the steps of: (c1) applying a constant amplitude signal, of successively different frequencies in a progression of frequency increments, over a given range of frequencies which includes said desired center frequency and said second given frequency, while (c2) detecting the amplitude of said signal as modified by said trap circuits, at each of said different frequencies; and (c3) sequentially storing said detected amplitudes; (c4) adjusting the other of said trap circuits in accordance with the difference between said second given frequency and the frequency at which said detected amplitudes exhibits a minimum value.

13. The method of Claim 10 or 12,wherein step (d) comprises the steps of: (d1) adjusting said one trap circuit to a fourth frequency which is lowerthan said third frequency and higher than said first given frequency; (d2) adjusting said one trap circuit to a fifth frequency which is lower than said fourth frequency and higher than said first given frequency; (d3) adjusting said one trap circuit so as to locate its null frequency substantially at said first given frequency in accordance with the difference between said fifth frequency and said first given frequency, on the basis of the adjustment of said one trap circuit from said fourth to said fifth frequency.

14. The method of Claim 11, wherein step (a4) comprises the steps of: (a4-1) comparing successive ones of said detected amplitudes, beginning with the first stored amplitude, until an amplitude difference is found which is greater than a predetermined minimum difference and is of a polarity indicative of an increase in amplitude with said progression, and is a successor of an amplitude difference which is greaterthan said minimum difference and is of a polarity indicative of a decrease in amplitude with said progression; and (a4-2) comparing successive ones of said detected amplitudes in reverse of the order of step (a4- 1), beginning with the last stored amplitude, until an amplitude difference is found which is greater than a predetermined minimum difference and is of a polarity indicative of an increase in amplitude with said reverse order succession, and is a successor of an amplitude difference which is greater than said minimum difference and is of a polarity indicative of a decrease in amplitude with said reverse order succession; and wherein step (a5) comprises adjusting said two trap circuits in accordance with the corresponding frequency difference between the frequency of inflection indicated by said first named amplitude difference of step (a4-1) and said desired center frequency, and the frequency difference between the frequency of inflection indicated by said first named amplitude difference of step (a4-2) and said desired center frequency, respectively.

15. The method of Claim 11, wherein step (a5) comprises the steps of: (a5-1) adusting one of said trap circuits in repetitive incremental frequency steps in accordance with the frequency difference between the current value of one of said null frequencies and said desired center frequency, while adjusting the other of said trap circuits in repetitive incremental frequency steps in accordance with the frequency difference between the current value of the other of said null frequencies and said desired center frequency, while determining updated null frequencies after each of said incremental steps, until said null frequencies are located within a second given frequency range including said desired center frequency or until one of said frequency differences increases after an incremental adjustment, as compared with the pre-, vious respective frequency difference, and, in the instance of said increase; (a5-2) adjusting said other of said trap circuits in repetitive incremental frequency steps in accordance with the frequency difference between the current value of said one of said null frequencies and said desired center frequency, while adjusting said one of said trap circuits in repetitive incremental frequency steps in accordance with the frequency difference between the current value of said other of said null frequencies and said desired center frequency, while determining updated null frequencies after each of said incremental steps, until said null frequencies are located within said second given frequency range, or until one of said frequency differences increases after an incremental adjustment, as compared with the previous respective frequency difference, and, in the instance of said latter increase, repeating step (a5-1).

16. The method of Claim 14, wherein step (a5) comprises adjusting each of the respective trap circuits by: (1) a first predetermined frequency increment if the corresponding frequency difference is greater than said first predetermined frequency increment, and (2) less than the corresponding frequency difference if the frequency difference is equal to or less than said first predetermined frequency increment and greater than a second predetermined frequency increment which is smaller than said first predetermined frequency increment.

17. Trap tuning method or apparatus, substantially as hereinbefore described with reference to Figure 1,2or6.