FI75962B - AVSTAEMNINGSSYSTEM FOER EN FLERBANDIG TELEVISIONSMOTTAGARE. - Google Patents

Description

1 75962

Tuning system for a multiband television receiver

The invention relates to a tuning system for a television receiver, comprising: a first and a second signal path for receiving radio frequency signals arranged in a first and a second frequency range, respectively, each signal path having frequency selective characteristics adjustable by the tuning signal; tuning control means for causing said tuning signal to select one of said radio frequency signals in the first and second frequency bands and for generating a frequency band signal indicating one of the first and second frequency bands including the selected radio frequency signal; and a connecting device connecting the signal from the first and second signal paths to a common signal path; means for transmitting an excitation signal to the first and second signal paths.

Television (TV) tuning systems select the desired signals from a plurality of TV radio frequency (RF) signals at different frequencies obtained from Broadcast or cable TV signal sources to provide an intermediate frequency (IF) signal that provides display and audio information. . In a tuning system using two intermediate frequencies, two period number changes (transfers) are performed to convert the RF signal first into an IF signal and then into a second and final IF signal.

It is not uncommon for signals from different channels to interfere with each other, resulting in a deterioration in TV picture and / or sound quality during final playback. The extent to which such interference occurs depends on the period-selective characteristics of the filters, the operation of the mixer and local oscillator, and the choice of intermediate frequency ranges.

Consider, for example, the selection of a VHF channel 5 having an RF picture carrier of 35.25.25 MHz with a tuning system using two intermediate frequencies, in which the first IF passband surrounds the frequency of the first picture carrier of 415.75 MHz. The frequency of the 493 MHz local oscillator is developed so that the first mixer converts the RF image carrier from 77.25 MHz to 415.75 MHz. If the signal of the UHF channel 31 with the 573.25 MHz RF image carrier 5 is also mixed with the signal of the local oscillator, a difference frequency 80.25 MHz signal may be generated. Since 80.25 MHz is in the period number range of VHF channel 5 (i.e. 76-82 MHz), a condition for interference is created. Similarly, there are UHF RF carriers that can provide the conditions for interference conditions, for each low frequency band VHF channel 2-13, as well as for many cable TV channels.

Conventionally, interference signals can be prevented by placing switches, such as PIN diodes or transistor switches, between the source of the RF signals in the signal path and the mixer, thus preventing interference signals from a potentially unselected frequency range from entering the mixer. However, at UHF frequencies, "leaks" of the signal through or past such switches, when assumed to be non-conductive, such as through stray capacitance, can be strong enough to cause unpleasant interference. The present invention provides a significant attenuation of unwanted signals to ensure that any interference that develops will be negligible and therefore not appreciable in TV picture or sound.

This is achieved by an introductory tuning system having a device coupled to the first signal path that responds to the frequency band signal by changing the tuning signal input to the first signal path when the selected radio frequency signal is included in the second frequency band.

To this end, the tuning system of the present invention comprises first and second signal paths corresponding to the first and second tunable frequency ranges, the frequency selective properties of which are adjusted with respect to the tuning signal generated in the tuning control device. The signals of the first and second signal paths are combined into a common signal path. The tuning control device 3 75962 also outputs a frequency range indicating signal which changes the tuning signal of the first signal path when a channel in the second frequency range is selected.

Fig. 1 is a circuit diagram of a tuning system according to an embodiment of the present invention shown in block diagrams; Fig. 2 graphically illustrates amplitude variation-period-reading characteristics related to the embodiment of Fig. 1; Figures 3 and 4 are circuit diagrams of circuits incorporating an embodiment of the present invention and useful in the tuning system of Figure 1.

In the dual transmission system of Figure 1, television signals are received at UHF antenna input 10, VHF antenna input 30A, and CATV (cabinet-15 li-TV) input 30B is transferred to diplexer 20 (a connection filter, as described below. television signals include channel numbers and locations in the frequency bands according to the following table.

Table 1 Frequency band Channel 20 TV frequency band cross name

Small VHF yl.radio (L-VHF) 54-88 2-6

Central Area Cable TV (MB-CATV) 90-174 A-5 - I

Large VHF yl.radio (H-VHF) 174-216 7-13

Superband cable TV (SB-CATV) 216-412 J-W +17 25 UHF gen.radio radio (UHF) 470-890 14 - 83

Each channel is assigned a bandwidth of about six MHz in the frequency spectrum, and each has an image carrier at a period number 1.25 MHz higher than the lower limit period number of the frequency range assigned to the channel. In the following description, when reference is made to specific period numbers, this period number corresponds to the period number of the selected TV channel picture carrier (PIX).

The frequency spectrum of the channel frequencies "f" of the various TV frequency ranges in the United States is shown in capital letters in Fig. 2 (a). The level of signals received in the L-VHF region 202, the H-VHF region 206, and the UHF region 210 is represented by a number of levels, indicating that radio wave signals can vary over a wide range of intensities, for example between 10 μV and 100 mV. On the other hand, CATV signals show a much smaller change in signal strength, typically between 1 mV and 6 mV, as shown in MB-CATV area 204 and CB-CATV area 208.

Figure 2 (b) defines the radio frequency (RF) frequency ranges low-band, high-band and UHF-band associated with the respective filters 44, 36 and 14 of Figure 1, which are described below. The frame-10 wave of the first intermediate frequency (IF) signal is selected as 415.75 MHz, which is SB-CATV and UHF-TV

between regions. The picture frequency of the second intermediate frequency IF signal is at the standard period number of 45.75 MHz used in the United States. The bandwidth of the desired TV signal at each IF period is about 6 MHz and has a center frequency of 414 MHz for the first IF frequency and 44 MHz for the second IF frequency.

It is to be understood that since the present invention is described as the various radio and cable frequency bands currently used in the United States, the present invention is not limited to this arrangement. For example, signals from the over-the-air Broadband 20 frequency band could actually be fed from a kaa-game TV signal source and, in addition, could be in TV and cellular ranges used elsewhere, such as in Europe or Japan.

According to Figure 1, when the selected TV channel is in the UHF area-25a, it is connected from the UHF antenna 10 to the first input terminal of the UHF area periodic selective tunable filter 14 / diplexer 20. The filter 14 receives a tuning voltage VT at the interface 14C so that it is primarily tuned to pass through, at the input 14A and the output 30 14B, signals at period numbers corresponding to the period number of the selected TV channel.

The UHF filter 14 exhibits a tunable attenuation having a period number selective property that is dependent on the tuning voltage VT. It attenuates signals-35 and whose cycle numbers are below the cycle number of the selected channel and attenuates relatively more with cycle numbers that are above the cycle number of the selected channel. Filter 5 75962 14 provides an amplitude peak response around the period number of the selected channel (i.e., the peak of the passband has a minimum attenuation). The bandwidth of this peak is about 25 MHz when tuned near UHF channel 14 and increases somewhat to about 40 MHz when tuned near UHF channel 83 for the particular embodiment of filter 14 described later with respect to Figure 3.

The IF trap 12 reduces the level of each signal at which the period numbers are close to the first-10 IF period numbers obtained from the UHF antenna 10. This is desirable because the first IF period of 415.75 MHz is close to the lower cutoff frequency of the UHF. These unwanted signals could either be generated externally from, for example, radio or radar signals, or be emitted or derived from the first IF block. Trap 12 attenuates unwanted signals near the first IF period number that could otherwise propagate to the IF circuits. For this purpose, trap 12 attenuates signals between 411 and 417 MHz.

The UHF amplifier 16 transfers signals between the output 14B of the filter 14 20 and the first input of the diplexer 20. Amplifier 16 amplifies about 14-15 dB in the UHF period range and has an input and output impedance of about 50 ohms to match the impedance of filter 14 and diplexer 20. Amplifier 15 operates only 25 when a channel in the UHF band is selected because its operating voltage, range switch voltage VB 3 (about 18 volts) occurs only when channels in the UHF band are selected, as shown at level 260 in Figure 2 (f). Because amplifier 16 has no supply voltage when non-UHF range signals are selected to be received, it provides attenuation to the path between antenna 10 and diplexer 20 at such times.

TV signals in the VHF and CATV bands (54-402 MHz) are divided into low and high tuning bands because they exceed the 7: 1 frequency adjustment range. Tuning in a range greater than 3: 1 is impractical due to the limited voltage range of adjustable capacitance diodes. Accordingly, the tuning apparatus of the VHF and CATV ranges of the figure is divided into tuning in the low and high tuning frequency ranges, which are separated in the MB-CATV range by the cycle number, i. about 150 MHz, as shown in Figure 2 (B). As a result, both high and low tuning frequency ranges contain signals with a frequency range smaller than 3: 1.

The TV signals of the VHF and CATV period ranges are transmitted to the diplexer 20 in Figure 1 as follows. Switch S1A may be connected to position BC-A to transmit signals from VHF-10 antenna 30A to input 32 of filter 32, or may be connected to position CA-A to transfer CATV signals from input terminal 3OB to filter 32. Filter 32 is a wideband filter that attenuates signals with period numbers below about 40 MHz, which is slightly less than the lowest period number received (i.e., VHF channel 2, 54-60 MHz). Filter 32 passes signals in both the low (54-150 MHz) and high (150-402 MHz) frequency bands. If the selected channel is in the high frequency band, then VB 2 is applied to switches 34 and 38 to make them conductive (closed), thus connecting a high frequency bandpass filter 36 between the filter 32 and the VHF amplifier 40. If the selected channel is in the low frequency band, then VB 1 is applied to switches 42 and 46 to make them conductive to switch the low frequency band 44 between filter 32 and filter VHF amplifier 40.

The high-frequency band tunable filter 36 shows a tunable high-frequency band having a period number selective property that is dependent on the tuning voltage VT. It attenuates more than 30 signals whose period numbers are below the selected high frequency band and channel period number than signals whose period numbers are above the selected channel period number. It has a minimum attenuation in the peak emission range, which includes the period number of the selected channel. As a result, the filter 36 not only selects an RF carrier for the selected channel, but also tends to suppress signals at lower period numbers, in particular 7 75962 signals in the low frequency band, the low frequency tunable filter 44 represents a tunable low frequency band. a frequency band having a period number selective feature that is dependent on the TV in the same manner as previously described for the UHF filter 14 except that the frequency range of the amplitude peak corresponding to the passband is allowed to increase to a much greater extent than the upper cutoff frequency of the selected channels. 10 select the RF carrier of the selected channel but also tends to repel signals at higher cycle numbers, especially those in the high-frequency band and the first IF frequency.

The VHF amplifier 40 in Figure 1 transmits the signals from the filter 15 or 40 to the second input of the diplexer 20. The operating voltage VB 12 is applied to the VHF amplifier 40 whenever a channel is selected from the low, high tuning band, but VB 12 is not applied when a channel is selected from the UHF band VHF and the CATV signal paths are disconnected from the diplexer 20, 20 when the UHF channel is selected. The operating voltage VB 12 is formed from the range switch voltages VB 1 and VB 2 or by a diode current circuit comprising diodes D 12 and D 14.

The diplexer 20 of Figure 1 receives RF signals from the UHF band signal slot at the first input terminal and 25 receives RF signals from the VHF and CATV signal path at the second input terminal and transmits the received RF signals to its common signal path output terminal. Diplexer 20 may well be a passive element, a signal combiner.

Mixer 50 receives RF signals from the output of diplexer 20 and local oscillator frequency signals from amplifier 52 and converts the RF signals to an IF signal having an image carrier at a first IF frequency of 415.75 MHz.

The tunable voltage-controlled local oscillator matching (VCO) 56 generates a troll oscillator frequency signal for each of the three tunable regions 875962. The VCO frequency, which is dependent on the reference voltage VT so that it follows the tuning of the filter charged by the bandpass filters 14,36 and 44. The VCO cycle cell ranges are in the following table.

5 Table 2

Frequency range Channel number Local oscillator frequency _ _ frequency (MHz) __

Low frequency range 2-6 (L-VHF) 471-499 A-5 - E (MB-VATV) 507-561 10 High frequency range F - I (MB-VATV) 567-585 7-13 (H-VHF) 591-627 J - W + 17 (SB-CATC) 633-813 UHF frequency range 14 - 83 (UHF) 887-1301

Amplifier 52 amplifies the signal from VCO 56 so that the RF signals from diplexer 20 can also be relatively higher in intensity without causing further distortion in mixer 50.

The first IF signal from mixer 50 is then amplified by a tuned IF amplifier 60. Amplifier 60 20 comprises a two-part input filter tuned to a period IF number of 415.75 MHz and having a bandwidth of about 12 MHz and a three-part output filter which is also tuned to 415.75 MHz and having a bandwidth of about 10 MHz.

25 The center frequency of the passbands of these filters is in fact about 414 MHz. The amplified IF signal from amplifier 60 is then mixed with the frequency signal 370 MHz of the second local oscillator 64 to generate a second IF output 68.

The tuning control 70 responds to the channel selection and includes the tuning voltage VT and the band switch voltages VB 1, VB 2, and VB 3. The tuning voltage VT, shown in Figure 2 (c), typically ranges from about 1.5 volts low level, shown at dashed line 220, and about 24 volts. between the higher level 35, indicated by dotted line 222. When the selected channel is in the low tuning band, the VT tends toward 9,75962 low value at point 224 when VHF channel 2 is selected and tends toward the high value at point 226 when MB-CATV channel E is selected. When the selected channel is high; in the tuning band, the VT tends toward the low value at 5,228 when the MB-CATV channel F is selected and tends toward the high value at the point 230 when the CB-CATV channel W + 17 is selected. Similarly, the VT tends toward the low value at the point 232 , when UHF channel 14 is selected and towards a high value at point 234 when UHF channel 83 is selected.

10 The band switch signals VB 1, VB 2 and VB 3 are at a high level of about 18 volts, as shown in Figures 2 (d), 2 (e) and 2 (f) by characteristic curves 240, 250 and 260, only when the channel is in the range , to which they belong, are selected and zero in V when selecting a channel 15 that is outside this particular range. U.C. in Patent Application No. 271,742, entitled "A phase-locked loop tuning system including a prescaler conditioned to oscillate at an out-of-band frequency" and filed by D. J. Carlson et al. June 20, 1981, assigned to the same assignee as the present invention and referred to herein for the purpose of describing the tuning control device, a device suitable for developing the tuning and bandwidth potentials of the type developed by the control device 70. The UHF bandpass filter control 15 receives a bandpass signal VB 12 that is at a high level whenever the selected channel is in a high or low band. In this case, the selected low band RF signal passes through either filter 36 or 44. The UHF RF-sig-30 signals should then not pass to the diplexer 20 and are attenuated by the UHF amplifier 16, which is in a voltage-free state, because the signal VB 3 in the UHF frequency range is at a low level. However, strong UHF RF signals can "leak" past or through amplifier 16 due to unavoidable stray-capacitive switching pathways that are inherent in its physical fit.

75962 These unwanted UHF RF signals may be mixed with the signal from the VCO 56 in the mixer 50 to become interference signals at the IF frequency described above.

Such unwanted UHF RF signals are not initially (i.e. without filter control 15) attenuated by a filter 14 which is tuned to follow the tuning of the bandpass filters 36 and 44 because all three filters are tuned to the same tuning voltage VT. To reduce the interference signals to levels not detectable by the TV viewer 10, the filter control 15 responds to the band signal VB 12 to change the tuning voltage VT of the bandpass filter 14 via the connection 14D. The filter 14 is set aside in such a way that it attenuates the UHF signals passing through it to the UHF amplifier 16.

As will be explained below, the side tuning of the filter 14 by causing its tuning circuits to receive the converted tuning voltage so that it tunes to a period number that differs significantly from the period number of the potentially interfering channel.

20 Figure 3 is a UHF-band filter 14, which uses two excitation frequency that is a sub-pass filter having a "top side of the inductive coupling" is provided between the inlet 14 A and the outlet 14 B, by connecting the inductance L 402, L 406, 25 L 408, L 410 and L 414. Capacitor C 408 acts as a DC blocking capacitor with low AC impedance at UHF frequencies. Inductors L 404 and L 406 are tap-type inductors with which the impedance at input 14 A is adjusted to about 50 ohms. Similarly, inductors 30 L 410 and L 412 are tap inductors with which the impedance at output 14 B is adjusted to about 50 ohms. The input and output inductors L 402 and L 414 help maintain a substantially constant bandwidth over the wide tuning range of the filter 14. A capacitor 35 C 404, which may have a weighting of the capacitance associated with the conductors of the circuit board, is connected in parallel with the inductor L 408 il 75 9 6 2. C 404 and L 408 are selected to resonate at about 1000 MHz. The variable frequency tuning is provided by adjustable capacitance diodes (varactors) CD 42 and CD 44, which are connected to the ground from the ends of the tuning silicon L 408 to C 404, respectively, via the switching capacitors C 402 and C 406. C 402: 11a and C 406: 11a have a very low impedance at the frequencies of the UHF television signals passed through the filter 14 The anodes of CD 42 and CD 44 are DC-connected to ground potential by inductors 10 L 404, L 406, L 408, L 410 and L 412.

The excitation voltage VT is applied to terminal 14 C to change the capacitance of diodes CD 42 and CD 44 via insulating resistors R 402, R 404 and R 406. The VT can vary between about 1.5 and 24 volts on UHF channels 14-83, as shown in Brochure-15 in Figure 2 (c). Since the capacitance diodes CD 42 and CD 44 conduct a negligible DC current; so that essentially none of the R 402, R 404 and R 406 results in a decrease in the DC voltage, so that in fact the full value of the excitation voltage VT is applied to the cathodes of the DC 42 and CD 44.

20 The filter control circuit changes the voltages applied to DC 42 and CD 44 to tune to the side of filter 14 when the selected channel is not in the UHF band as follows. The voltage VB 12 indicating the range is then at a level of about +18 volts for supplying a bias voltage 25 to the base of the switching transistor TS via resistors R 410 and 412. The TS is thus made to make a conductive connection between the cathode of CD 44 and the ground potential, which includes the TS collector-emitter path and terminal 14 D. As a result, resistors R 406 and TS 30 now act as voltage dividers for VT between terminal 14 C and ground. , generating substantially the same altered excitation voltage for the respective cathodes of CD 42 and CD 44.

Since the voltage of both CD 42 and CD 44 is substantially reduced, each has a much larger capacitance, thus substantially reducing the resonant frequencies of the respective circuits 12 to 75962 to which they are connected. Since the voltages of these decrease by approximately the same amount, their respective resonant circuits tune to a lower frequency, thus causing the peak of the head-5 range of the filter 14 to change to a period near or below the lowest cut-off frequency of the UHF TV range.

Fig. 4 shows a modification of Fig. 3 in which the filter control circuit changes the voltage applied to both the CD 42 and the CD 44 to tune to the si-10 of the filter 14 when the selected channel is not in the UHF range as follows. The voltage VB 12 indicating the range is then at a high level of about +18 volts, which biases the base of the switching transistor TS via resistors R 410 and R 412. The TS is thus made to make a conductive connection between the loss of CD 44 and the ground potential, which includes the collector-emitter path of TS, the connection 14 E and the resistor R 414. As a result, the resistors R 406, R 404 and R 414 now act as voltage dividers VT between the connection 14 C and ground, thus providing different modified excitation voltages for the respective cathodes of CD 42 and CD 44. Since the voltage of both CD 42 and CD 44 is reduced, each has a higher capacitance, thus reducing the resonant frequencies of the respective circuits to which they are connected. As these voltages decrease by different amounts of glass, their respective resonant circuits are tuned to different cycle numbers, thus causing the filter 14 to act as a broadband attenuator.

The VB 12 may further be introduced into the CD 42 via a series resistor R 420, a diode D 420 and a connection 14 F 30 to increase the tuned voltage across the CD 42, thereby tuning the circuit to which it belongs to a resonant frequency higher than the frequency associated with the value of VTs. . Therefore, both resonant circuits are tuned to the side in different directions due to the change of their respective tuning voltages in different directions. So. The CD 42 portion is tuned to the upper 13 75962 frequency while the CD 44 portion is tuned to the lower frequency. This transformation is advantageous in that it provides the greatest difference in the number of cycles in which the resonant circuits, including CD 42 and CD 44, are tuned. It will be appreciated that the present invention eliminates sources of potential interference other than those previously described herein. For example, when the VCO 56 generates a component signal at the second harmonic period of the desired local oscillator for the VHF to TV channel, the signal 10 from the selected UHF channel cannot move to the frequency range of the first IF signal by the mixer 50 in response to the second harmonic signal. One such case is when VHF channel 4 is selected. The second harmonic frequency (966 MHz) 15 of the frequency (483 MHz) of the local oscillator 56 can transmit the signal to the UHF channel 27 (549.25 MHz PIX) to 416.74 MHz, which is too close to 415.75 MHz for the first IF signal and is not acceptable. Modifications of the described embodiments are considered to be within the scope of this invention, which is limited only by the following claims. For example, it is also sufficient that the transistor TS in Fig. 3 or the resistor R in Fig. 4 is connected so that the excitation voltage, when changed, increases in magnitude so that the peak of the pass region of the filter 14 shifts to a much higher frequency. In addition, it is sufficient to connect the resistor to the collector circuit of the switching transistor TS, which leads to the junction of the resistors R 402 and R 404, as shown in Fig. 3 by the resistor R 414 (shown in broken lines). When the dashed line of the filter 14 shows radio frequency protection, the embodiments described herein preferably reduce the number of feed-through capacitors (not shown) to provide connections through the protection.

Claims (13)

1. A tuner system for a television receiver, comprising first and second signal paths (12, 14, 16; 32, 34, 36, 38, 40, 42, 44, 46) for receiving radio frequency signals located in the first and second, respectively; second frequency bands, each signal path having frequency selective characteristics which can be varied in response to a tuning signal (VT); a tuning control means (70) for developing the tuning signal (VT) so as to select the lower frequency of the radio frequency signals within the first and second frequency bands and to form a band signal (VB1, VB2, VB 3, VB 12), which indicates neither of the first and second frequency bands which includes the selected radio frequency signal; a combination means (20) for combining signals from the first and second signal paths out onto a common signal path (50, 60); and means (14 C, R 406) for applying the tuning signal (VT) to the first and second signal paths; ne-20 is characterized by a means (15, 14 D) which is coupled to the first signal path (12, 14, 16) and responds to the band signal (VB 12) which modifies the tuning signal (VT) applied to the the first signal path (12, 14, 16), the selected radio frequency signal is included in the second frequency band. 2. The tuning system according to claim 1, characterized in that a filter means (14) in the first signal path (12, 14, 16) contains a variable capacitance diode (CD 42, CD 44), pk to which the tuning control signal (VT) is applied; the means (14 C) for applying the signal includes a resistor (R 406), through which the tuning control signal (VT) is applied to the capacitance diode (CD 42, CD 44); wherein the modifier includes a device (14D) coupled to a circuit junction 35 (R402, R404, 14D) between the terminus (R406) and the capacitance diode (CD42, CD44) for changing the potential therein in response to the frequency band signal (VB) . 3. A tuning system according to claim 2, characterized in that the device (15) for changing the potential includes a switching means (TS) for selectively establishing a conductive connection to a potent source (ground V) in response to the frequency band signal. (VB 12). 4. A tuning system according to claim 3, characterized in that the coupling means (TS) include a transistor (TS), whose output electrode (TS collector) is coupled to the circuit point (14D), whose common electrode (TS emitter). ) is coupled to the potential source (ground V), and having an inductor (TS base) on which the band signal (VB 12) is applied to conduct the transistor between its output electrode and the common electrode. A tuning system according to claim 1, characterized in that the filter means in the first signal path (12, 14, 16) includes a filter (14) with first and second resonant circuits (L406, L408, L410, CD42, CD44), wherein respective first and second resonant circuits include a first and a second variable capacitance diode (CD 42, CD 44) to which the tuning control signal is applied; the device (14 C, R 406) for applying the signal includes at least one resistor (R 406), through which the tuning signal (VT) is applied to the first and second capacitance diodes (CD 42, CD 44); and the modification arrangement (15, 14D) includes a device coupled to a circuit junction (R402, R404, 14D) between the resistor (R406) and the first and second capacitance diodes (CD 42, CD 44) for changing the potential therein . 6. A tuning system according to claim 5, characterized in that the modifying devices (15, 14D) change the respective potentials of the first and second capacitance diodes (CD 42, CD 44) in the same direction. A tuning system according to claim 1, characterized in that the first signal path (12, 14, 16) includes first and second resonant circuits (L 406, 20 7 5 9 6 2 L 408, L 410, C 404, CD 42, CD 44), each of which includes first and second variable capacitance diodes (CD 42, CD 44), to which the tuning signal is applied; the means (14 C, R 406) for applying the signal includes at least one resistor (406), via which the tuning signal (VT) is applied to the first capacitance diode (CD 42, CD 44); and the modifier (15, 14D) includes: a first device (15, 14E) coupled to a circuit point (R 404, CD 44) between the resistor (R406) and the first capacitance diode (CD 44) for changing the the potential therein; and a second device (D 420, R 420, 14 F) responsive to the band signal (VB 12) for changing the tuning signal (VT) applied to the second capacitance diode (CD 42), in a direction opposite to the direction of the change in potential in said circuit point. 8. A tuning system according to claim 7, characterized in that the second device (D 420, R 420, 14 F) for changing the tuning signal (VT) which is applied to the second capacitance diode (CD 42) includes a diode (D 420) and a resistor (R420) for coupling the band signal (VB 12) to the second capacitance diode (CD 42). 9. A tuning system according to claim 1, characterized in that the first signal path (12, 14, 16) includes first and second resonant circuits (L406, L408, L410, C404, CD42, CD44), which include the first and second, respectively. other variable capacitance diodes (CD42, CD44) to which the tuning signal (VT) is applied; the device (24C, R406) for applying the signal includes a first resistor (R404), via which the tuning signal is applied to the first capacitance diode (CD44) and a second resistor (R402), via which the tuning signal is applied to the second capacitance diode (CD42); and the modifier (15, 14D, 14E) includes a first device (14E) coupled to a circuit node (F.404,