GB1591797A

GB1591797A - Tuning circuit arrangement

Info

Publication number: GB1591797A
Application number: GB4331676A
Authority: GB
Original assignee: Texas Instruments Ltd
Current assignee: Texas Instruments Ltd
Priority date: 1976-10-19
Filing date: 1976-10-19
Publication date: 1981-06-24
Also published as: JPS5391501A

Description

(54) TUNING CIRCUIT ARRANGEMENT (71) We, TEXAS INSTRUMENTS LIMITED, a British Company, of Mantdn Lane, Bedford; do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a tuning circuit arrangement and is particularly, but not exclusively, suited to application in television receivers in which there are a large number of signal channels of the same bandwidth and spaced apart by the same frequency difference.

Most television receivers now produced are tuned by the use of variable capacitance (varactor) diodes to which an adjustable d.c. voltage is applied to vary the capacitance and so select the various channels required. The variable d.c.

voltage could, for example, be produced by means of a potentiometer connected across an accurately stabilised d.c. voltage source of about 30 volts, and may include a mechanical or electronic switch. It has also been proposed to employ a more electronic approach in which the voltage to be applied to the diode is represented by a digital word of 12 to 14 bits which is applied to a digital to analogue converter which uses the stabilised d.c. voltage as a reference. In such arrangements each television channel required must be represented by a potentiometer adjusted either mechanically or by the selection of the appropriate digital word in a memory. Such systems have the disadvantage that a large number of potentiometers either in the form of mechanical adjustments or adjustable words must be available to enable all available channels to be selected.Although only a few channels are tunable at any one location over the whole country, there are used some 40 or so channels in bands 4 and 5 alone. This means that a complex mechanical system must be provided to enable the channels to be selected or a large programmable memory. must be provided to provide the digital words. This difficulty has been solved by making available to the user only a selected few of the channels which must be set up by the user to the channels available to him.

Typically, television receivers provide between 8 and 20 channels which can be tuned.

A programmable read only memory could be used to store the digital words representing the selected channels, but this would be relatively expensive and would mean that the receiver could only be used in a particular locality once the memory had been programmed.

A lower cost alternative would be to use a read only memory matched to the TV tuner so that appropriate voltage requirements for each channel are built into the memory. Unfortunately, the characteristics of the tuners vary sufficiently so that it would not be possible to utilise a read only memory in this way with any degree of success.

It is an object of the present invention to provide an arrangement suitable for tuning a television receiver in which some at least of the above difficulties are alleviated.

According to the present invention there is provided a tuning circuit arrangement having a resonant circuit including a varactor diode the capacitance of which is variable under the influence of an applied d.c. voltage to vary the resonant frequency of the circuit, means for generating the applied voltage in response to pulse signals, input means entering an input value, and microprocessing means responsive to the input value for generating the pulse signals, wherein the microprocessing means includes storage means for storing a plurality of digital signals corresponding respectively to a plurality of predetermined input values and a processing unit for interpolating among or extrapolating beyond the stored digital signals in response to the input value from the input means to produce the pulse signals corresponding to the input value from the input means.The processing unit may be programmed to utilise linear interpolation and extrapolation or another standard mathematical curve fitting technique to effect the interpolation and extrapolation.

The invention is especially but not exclusively suited to the function of tuning a UHF (bands 4 and 5) television receiver for which there are 48 channels for television signals, each occupying a bandwidth of 8 MHz, so that the resonant circuit needs to be tunable from 470 MHz to 854 MHz. The channels are numbered from 21 to 68 and consequently the-input means may take the form of a keyboard by means of which the two digits of the channel numbers may be entered.

Alternatively, the keyboard may have keys for the different programmes available, e.g. BBC 1, BBC 2, ITA, with connections adjustable by the dealer, for example, to produce the digits of the channel numbers. The input means or the microprocessing means may include a converter or a separate converter may be provided for convering the programme name or number into the corresponding channel number or frequency. The pulse signals should be such as to establish the voltages to be applied to the varactor diode with such accuracy that the required channel is tuned sufficiently closely as to be within the pulling-in range of an automatic frequency control circuit when all of the circuit tolerances have been taken into account.For example, the pulse signals may take the form of a parallel digital signal having 12 to 14 bits, so that the microprocessing means should use 16bit words or be arranged to use double length working with 8-bit words.

Alternatively, the pulse signals may take the form of a pulse train which when smoothed by a low pass filter produces the voltage for the varactor diode without the need for a digital to analogue converter. As particularly high speeds of processing are not required any of several commercially available microprocessors would be suitable; the specific embodiment described uses a Texas Instruments TMS 9940.

The arrangement may also include a digital display for showing the number of the channel selected, and may, for example, use conventional seven segment digital display elements employing light-emitting diode techniques.

Since the tuning operation of a television/receiver would occupy only small part of the time which the receiver is switched on, the microprocessing means would have spare processing capacity which would be available for use for other functions in the receiver.

In order that the invention may be fully understood and readily carried into effect it will now be described with reference to the accompanying drawings, of which: Figure 1 is a block diagram showing one example of the present invention, and Figure 2 shows an example of the graph relating applied voltage to the frequency of a tuner.

In Figure 1, a keyboard 1 which may, for example, be of the type employed in a pocket calculator, has its column conductors connected through four input connections 2 in parallel to microprocessor 3. The microprocessor 3 has four scanning outputs 4 which are connected to the row conductors of the keyboard 1.

Two of the scanning outputs of the microprocessor 3 are also applied via drivers 5 and 6 to drive the anode electrodes of respective seven segment display digits 7 and 8. The cathode electrodes of the display digits 7 and 8 are driven via respective drivers 9 by seven of the eight output connections 10 of the microprocessor 3. The connections 10 also provide address data (eight bits in parallel) for a programmable read-only memory 11, the output data from which is applied in four bit parallel form to the input connections 2 of the microprocessor 3. The PROM 11 may be a Texas Instruments type SN 74S 287. A "chip select" signal is applied to the PROM II via a conductor 12 from the microprocessor 3 so that the PROM 11 does not produce spurious outputs in response to data on the connections 10 intended for the display digits 7 and 8. The microprocessor 3 used in this embodiment is a Texas Instruments TMS 9940 and has altogether thirty-two general purpose input/output connections of which only twenty-two are used, the actual allocation of the connections to the inputs and outputs depending entirely on the software. Three connections 13, 14 and 15 are connected to a tuner 16 to effect band selection; and a group of 17 of ten connections are spare. The tuner 16 includes switches responsive to the band selection signals to switch into the resonant circuits the appropriate inductors and capacitors.

The Texas Instruments TMS 9940 includes a counter separate from the main data processing facility, and this is used, as explained below, to produce a train of width modulated pulses on an output connection 18, which when smoothed by a low pass filter 19, results in a steady voltage on a conductor 20 for causing one or more varactor diodes in the tuner 16 to tune resonant circuits to required frequencies. A crystal 23 is provided to keep the clock oscillator frequency at the required value.

Another output connection 21 of the microprocessor 3 is used to convey a signal for disabling an automatic frequency control (A.F.C.) signal to a switch 22 in the input for the A.F.C. signal of the tuner 16. The A.F.C. signal would typically be obtained from the received signal by means of a frequency demodulator in the receiver and is arranged to effect adjustment of the frequency to which at least one resonant circuit (which may determine the frequency of the local oscillator) is tuned so as to bring the receiver into tune and hold it in tune accurately with the received signal. It is therefore desirable to disable the A.F.C. when changing the channel to which the receiver is tuned.

In the example of the invention under consideration, four particular channels are selected and the corresponding digital signals are stored in the programmable read only memory 11, together with the frequencies allocated to all of the channels.

In Figure 2, the four channels are indicated by the points A, B, C and D on the curve relating frequency to voltage applied to the varactor diodes. Because there is a specific relationship between the channel numbers and the signal frequency, it is clear that the selected points-are associated with specific channel numbers. Since the actual voltages required to be applied to the varactor diodes to tune the resonant circuits in the tuner 16 to the frequencies corresponding to the channel numbers have to be determined by experiment, the corresponding digital signals are determined by the results of the experiments performed either by the manufacturer or the dealer in preparing the receiver for sale.When the four digital signals corresponding to the four selected channels have been determined, this information is recorded in the programmable read only memory 11, which forms the basis of the information from which linear interpolation, for example, can be used to calculate the digital signal corresponding to any given channel number.

A more complex interpolation can alternatively be used, for example quadratic or parabolic interpolation; for such interpolation it would be desirable to choose the points A, B, C and D so that they are equally spaced with respect to frequency. If fa, fh and fc represent the frequencies at the points A, B and C respectively, then fb=T(fa+fc). If va, vb and v, represent the corresponding voltages at A, B and C, then it can be shown that for a point P on a parabola through A, B and C corresponding to a frequency fp, the voltage required is::

In this expression only f,fa and (fpfb)2 cannot be pre-calculated, so that the calculation can rapidly be performed by the microprocessor when the value of fp is applied to it.

In the operation of the example of the invention shown in Figure 1, the number of channel required is entered by pressing the keys sequentially on the keyboard 1. Signals representing the selected channel number are applied by means of the connections 2 to the microprocessor 3, where they are used to select the frequency corresponding to the channel number from the PROM 11 which frequency is then used in a calculation of a digital signal by interolating among or extrapolating beyond the four values corresponding to the points A, B, C and D, shown in Figure 2. The microprocessor 3 produces steady signals on connections 13, 14 and 15 to select the correct band.The microprocessor 3 also generates output signals which are applied via the output connections 10 to cause the display digits 7 and 8 to produce a digital display of the selected channel; the channel selected in the example shown is 35. When the calculation of the digital signal is completed, it is applied to the counter in the microprocessor 3 and used to generate a pulse width modulated signal on the conductor 18 which is smoothed by the low pass filter 19 to produce the corresponding analogue voltage which is applied to the varactor diodes in the tuner 16.

The microprocessor 3 is programmed to perform a sequence of operations cyclically, the sequence being variable either by the use of test and branch instructions to by-pass groups of instructions which are not required in the particular cycle or by means of interrupts for performing extra instructions when needed, or possibly by both branches and interrupts. The connections 4 are scanned during the sequence and the connections 2 tested for input signals which would indicate that a key of the keyboard 1 is being pressed. The input signals on the connections 2 are checked to ascertain whether they represent a valid input and to remove the effects of contact bounce of the key of the keyboard 1. The input signal, if valid, is decoded and entered in an input register to await a further input.

When two digits representing a channel number have been received, the data in the input register is transferred to a main memory for further processing.

In synchronism with the scanning of the lower two of the connections 4 outputs are generated on the connections 10 which cause the display digits 7 and 8 to display the channel number stored by the microprocessor 3.

When the channel number has been stored in the main memory of the microprocessor 3 it causes an address signal to be applied to the PROM 11 via the connections 10 simultaneously with a "chip select" signal on the connection 12. In response to these inputs the PROM 11 produces output signals which are applied to the microprocessor 3 via the conductors 4 and which represent the frequency corresponding to the channel number. An indication of the band to be selected is also derived from the PROM 11, and also a representation of the voltage to be generated if the channel selected is one of the four reference channels, A, B, C and D.

If the channel is not one of the four reference channels, the microprocessor performs tests to ascertain which two of the reference channels are nearest to the selected channel, and representations of the frequencies and voltages for these reference channels are derived from the PROM 11. The microprocessor performs the interpolation for calculating the voltage for tuning the tuner 16 to the selected channel, and stores a digital signal representing that voltage in the memory.

The digital signal is transferred into the counter (decrementer) of the microprocessor which then counts clock pulses down to zero, when the complement of the digital signal is stored in the counter and again clock pulses are counted down to zero, and the process starts again with the digital signal being transferred to the counter again. A latch is set by alternate zeros and reset by intervening zeros, so that there is obtained from the latch a sequence of pulses width modulated by the digital signal. The width modulated pulses are output from the microprocessor on the conductor 18 and are used as described above for producing the tuning voltage. The operation of the counter takes place simultaneously with the data processing performed by the main part of the microprocessor 3.

The band selection signals on the connections 13, 14 and 15 are derived from latches in the microprocessor and therefore are continuous signals which can be used directly to operate the band selection switches in the tuner 16.

The "A.F.C. disable" signal is produced on the connection 21 whenever the digital signal representing the tuning voltage is changed and is maintained for a sufficient period of time for the new tuning voltage to become established.

The microprocessing means may, as indicated above, comprise a commercially available microcomputer or it may be constructed from a number of integrated circuit components including, for example, a separate random access memory and a separate read only memory with a simple form of microprocessor.

The programmable read only memory 11 may also be included within the microprocessor or it may be a separate unit and it may be an EAROM.

Although the invention has been described with reference to its application to tuning television signal channels in the UHF bands 4 and 5, it is equally applicable for tuning in other wavebands and to radio signals as well as television signals. It is desirable that the receiver to which the invention is applied includes automatic frequency control to effect the final pulling into tune of the receiver, but provided that the bandwidth of the receiver is adequate and the accuracy of tuning effected is sufficient the automatic frequency control may be omitted.

WHAT WE CLAIM IS: 1. A tuning circuit arrangement having a resonant circuit including a varactor diode the capacitance of which is variable under the influence of an applied d.c.

voltage to vary the resonant frequency of the circuit, means for generating the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The microprocessor 3 is programmed to perform a sequence of operations cyclically, the sequence being variable either by the use of test and branch instructions to by-pass groups of instructions which are not required in the particular cycle or by means of interrupts for performing extra instructions when needed, or possibly by both branches and interrupts. The connections 4 are scanned during the sequence and the connections 2 tested for input signals which would indicate that a key of the keyboard 1 is being pressed. The input signals on the connections 2 are checked to ascertain whether they represent a valid input and to remove the effects of contact bounce of the key of the keyboard 1. The input signal, if valid, is decoded and entered in an input register to await a further input. When two digits representing a channel number have been received, the data in the input register is transferred to a main memory for further processing. In synchronism with the scanning of the lower two of the connections 4 outputs are generated on the connections 10 which cause the display digits 7 and 8 to display the channel number stored by the microprocessor 3. When the channel number has been stored in the main memory of the microprocessor 3 it causes an address signal to be applied to the PROM 11 via the connections 10 simultaneously with a "chip select" signal on the connection 12. In response to these inputs the PROM 11 produces output signals which are applied to the microprocessor 3 via the conductors 4 and which represent the frequency corresponding to the channel number. An indication of the band to be selected is also derived from the PROM 11, and also a representation of the voltage to be generated if the channel selected is one of the four reference channels, A, B, C and D. If the channel is not one of the four reference channels, the microprocessor performs tests to ascertain which two of the reference channels are nearest to the selected channel, and representations of the frequencies and voltages for these reference channels are derived from the PROM 11. The microprocessor performs the interpolation for calculating the voltage for tuning the tuner 16 to the selected channel, and stores a digital signal representing that voltage in the memory. The digital signal is transferred into the counter (decrementer) of the microprocessor which then counts clock pulses down to zero, when the complement of the digital signal is stored in the counter and again clock pulses are counted down to zero, and the process starts again with the digital signal being transferred to the counter again. A latch is set by alternate zeros and reset by intervening zeros, so that there is obtained from the latch a sequence of pulses width modulated by the digital signal. The width modulated pulses are output from the microprocessor on the conductor 18 and are used as described above for producing the tuning voltage. The operation of the counter takes place simultaneously with the data processing performed by the main part of the microprocessor 3. The band selection signals on the connections 13, 14 and 15 are derived from latches in the microprocessor and therefore are continuous signals which can be used directly to operate the band selection switches in the tuner 16. The "A.F.C. disable" signal is produced on the connection 21 whenever the digital signal representing the tuning voltage is changed and is maintained for a sufficient period of time for the new tuning voltage to become established. The microprocessing means may, as indicated above, comprise a commercially available microcomputer or it may be constructed from a number of integrated circuit components including, for example, a separate random access memory and a separate read only memory with a simple form of microprocessor. The programmable read only memory 11 may also be included within the microprocessor or it may be a separate unit and it may be an EAROM. Although the invention has been described with reference to its application to tuning television signal channels in the UHF bands 4 and 5, it is equally applicable for tuning in other wavebands and to radio signals as well as television signals. It is desirable that the receiver to which the invention is applied includes automatic frequency control to effect the final pulling into tune of the receiver, but provided that the bandwidth of the receiver is adequate and the accuracy of tuning effected is sufficient the automatic frequency control may be omitted. WHAT WE CLAIM IS:

1. A tuning circuit arrangement having a resonant circuit including a varactor diode the capacitance of which is variable under the influence of an applied d.c.

voltage to vary the resonant frequency of the circuit, means for generating the

applied voltage in response to pulse signals, input means entering an input value, and microprocessing means responsive to the input value for generating the pulse signals, wherein the microprocessing means includes storage means for storing a plurality of digital signals corresponding respectively to a plurality of predetermined input values and a processing unit for interpolating among or extrapolating beyond the stored digital signals in response to the input value from the input means to produce the pulse signals corresponding to the input value from the input means.

2. An arrangement according to claim 1 wherein the processing unit is programmed to interpolate linearly between the stored digital signals.

3. An arrangement according to claim 1 wherein the processing unit is programmed to perform quadratic interpolation among the digital signals.

4. An arrangement according to claim 1, 2 or 3 in which the storage means is a programmable read only memory.

5. An arrangement according to any preceding claim in which the input means is a keyboard having a set of input conductors and a set of output conductors and the processing means is arranged to apply scanning signals to the input conductors and to receive signals from the output conductors corresponding to a key of the keyboard which is pressed.

6. An arrangement according to claim 5 including means for displaying the data entered by means of the keyboard, the scanning signals from the processing unit also being applied to the display means to effect selection of different digits thereof.

7. An arrangement according to claim 6 wherein the display means comprise seven-segment numerical display digits, the processing unit being arranged to produce seven output signals synchronously with the scanning signals to cause the selection and energization of the appropriate segments of the display digits.

8. An arrangement according to claim 7 wherein the storage means is addressed by means of the output signals produced by the processing unit and used to energize the -numerical display digits, a separate chip select signal being applied to the storage means to enable it to respond to only those signals which represent addresses in the storage means.

9. An arrangement according to any preceding claim wherein the processing unit includes a counter to which a number is applied representing the voltage to be generated so as to tune the resonant circuit to a frequency corresponding to the input value and clock pulses are arranged to be applied to cause the counter to count to a reference value, the unit also including means responsive to the state of the counter to generate a recurrent pulse signal having a duty ratio corresponding to the voltage to be generated, the pulse signal being applied to a low pass filter to effect smoothing of the signals so as to produce the voltage to be applied to the varactor diode.

10. An arrangement according to any preceding claim in which the varactor diode circuit includes an automatic frequency control signal input and switch means for disabling the automatic frequency control, the processing unit being arranged to generate a signal for disabling the automatic frequency control when the voltage applied to the varactor diode is changed.

11. An arrangement according to any preceding claim wherein the processing unit includes latch means for storing band selection signals derived from the storage means, which band selection signals are applied to operate switches associated with the resonant circuit to change the frequency band to which the circuit is tuned.

12. A tuning circuit arrangement substantially as described herein with reference to the accompanying drawings.