Television Tuner and Circuitry for Switching Between Two Frequency Ranges

The invention relates to a television tuner for at least two frequency bands having a joint pre-amplifier and having a switchable and tunable band-pass filter circuit connected to the output of the pre-amplifier for the two frequency bands, preferably for the frequency range from VHF band I up to and including the hyper band. The invention further relates to an input circuit of a television tuner and a television turner having a joint antenna connection for two frequency ranges sweeping the frequencies from VHF band I up to and including the hyper band, a pre-circuit filter, a preamplifier circuit dimensioned for the two frequency ranges, and two independent band-pass filters dimensioned for a low and a high frequency range respectively and a subsequent mixer stage and intermediate frequency circuit.

In a known television tuner (DE-PS 29 29 901) a switchable pre-circuit filter is provided at a joint antenna connection and the output of the former is connected to a joint pre-amplifier. Behind the output of the pre-amplifier is connected a switchable band-pass filter circuit, also rated for two frequency ranges and whose resonant circuits each have two series-connected coils of which one can be short-circuited by DC-controlled band-switching diodes. The joint output of the band-pass filter circuit is connected to a mixer circuit. Accordingly, two switching elements are required for switchover of a combined band-pass filter circuit, and corresponding DC supply paths must be provided for these elements. These switching diodes represent capacitive and ohmic loads that not only adversely affect selection, but also restrict the frequency tuning range, so that tuning within a frequency range covering the VHF band I up to and including the hyper band cannot be swept.

One object of the invention is to provide a television tuner of the type described at the outset, by which a high selection effect over a continuous and wide frequency range can be achieved with a simple construction.

This object is attained in accordance with the invention by connecting a two-stage switching device to the output of the pre-amplifier, this device being connected to the separate inputs of independent band-pass filters.

In a tuner design in accordance with the invention, the two-stage switching device and the two separate band-pass filters permit the construction elements necessary for switching not to be part of the actual resonant circuit of the respective band-pass filter circuit, so that the tuning range and the resonant circuit quality, and therefore the selection factor, are not adversely affected. In addition, each band-pass filter can be optimally dimensioned for the frequency band in accordance with the intended purpose, so that no compromises are necessary when dimensioning. In particular, it is thereby possible to selectively tune the entire frequency range from the VHF band I up to and including the hyperband with only two tunable band-pass filters, with one frequency band ranging from 50 MHz to 150 MHz and the other from 150 MHz to 470 MHz, for example. Preferably, the band-pass filter for the high frequency range is connected to the output of the pre-amplifier via a capacitor on the one side, and the band-pass filter for the low frequency range via the series circuit comprising a coil and a capacitor on the other side, with a single band-switching diode being connected between the connection point of the high-pass coil to the low-pass capacitor and the earth potential. With the band switch open, the band-pass filter is then detuned to such an extent for the high frequency range that selection to the reception frequency practically no longer occurs, while the band-pass filter for the low frequency range is coupled optimally to the pre-stage via the low-pass filter. With the band switch closed, on the other hand, the input of the band-pass filter for the low frequency range is short-circuited as regards high frequency, with coil 40 and capacitor 39 forming a high-pass filter dimensioned for the high frequency range, using which the band-pass filter for the high frequency range is so adjusted to the pre-amplifier that it achieves its optimum selection effect.

Another object of the invention is to provide a tuner of the type described at the outset that requires little space and a minimum of components with the use of modern integrated tuner circuit means, and that affords very good reception with high interference immunity.

This object attained by a resonant circuit with switchable coils forming a combined pre-circuit filter for both frequency ranges, with the output of said filter being connected to an input of a preamplifier, by the output of the preamplifier being connected to a two-stage switching device switching over simultaneously with the pre-circuit filter and to which the band-pass filters are connected separately, and by the outputs of the band-pass filters each being connected to their own mixer stages.

In a tuner construction in accordance with the invention, only a joint frequency-determining capacitor and one switching diode in addition are required for the pre-circuit filter, with the joint output of this pre-circuit filter circuit only requiring one preamplifier for both frequency ranges. This reduces the number of components and thus the space requirement to a minumum. High-frequency switching from the joint pre-stage to the two separated band-pass filters is therefore feasible at low expense, while the also separated mixer stages permit the setting of optimum mixing conditions for the two frequency ranges. However, the desired high interference immunity is achieved with very good reception properties.

A still further object of the invention is to provide an input circuit that requires a minimum of components and tuning work, and at the same time effects power matching of the high-resistance input of the pre-stage connected behind the low-resistance antenna connection.

This object is attained by the coil array in the pre-circuit filter being a matching coil between the antenna connection and the connection point of the second and third coil of the series circuit, the sole range switch being connected in parallel with the second and third coil of the series circuit, and the first coil being frequency-determining for the high frequency range.

The pre-circuit filter here preferably comprises the series circuit of three frequency-determining coils to which a joint frequency-determining capacitor is connected in parallel. The first coil here is connected to earth potential and the third coil to the input of the preamplifier, with only one range switch being parallel to the series circuit comprising the second and third coils. In addition, a matching coil is connected to the connection point between the second and third coils that is connected on the other side to the antenna connection. The first coil is here frequency-determining for the high frequency range, while the three coils in series together define the low frequency range. The joint resonant circuit capacitor is here preferably alterable so that a selection of certain channels is feasible within the frequency ranges. The preamplifier is preferably connected to the band-pass filters in such a way that the band-pass filter for the high frequency range is connected via a capacitor, and the band-pass filter for the low frequency range via the series circuit comprising a coil and low-pass capacitor, with a single band switch being provided between the connection point of the coil to the capacitor and the earth potential. With the band switch opened, the band-pass filter for the high frequency range is detuned so far that selection practically no longer occurs, while the band-pass filter for the low frequency range is optically coupled to the pre-stage via the low-pass filter. With the band switch closed, on the other hand, the input of the band-pass filter for the low frequency range is short-circuited at high frequency, with the coil and the capacitor forming a high-pass filter dimensioned for the high frequency range, using which the band-pass filter for the high frequency range is so matched to the preamplifier than an optimum selection effect is achieved.

In an input circuit in accordance with the invention, only three frequency-determining coils are needed, which are so dimensioned that, in conjunction with the sole range switch, the entire frequency band to be swept is divided into two frequency ranges. In particular, the capacitive load from these coils is very low, so that a large frequency tuning range is obtained that only requires a division into two frequency-bands. In the low frequency range all three frequency-determining coils are effective, while in the high frequency range only the first coil is frequency-determining, whereas the other two coils form a parallel circuit connected in series to the antenna connection and serve to match the power. The power match in the low frequency range is achieved with the matching coil connected in series to the antenna connection and with the matching coil's connection to the connection point between the second and third coil of the series circuit. Turning work to match the inductance values can be carried out for the low frequency range at the second or third coil and for the high frequency range at the first coil. Here, a change in the inductance of the first coil in the low frequency range has practically no effect, since its inductance is low in comparison with the overall inductance of the series circuit. A change in the matching coil has no effect on the set frequency range. The matching coil for the power match is determined in the high frequency range. The input circuit is preferably part of an overall tuner circuit, with the output of the pre-circuit filter being connected to an input of a joint pre-amplifier whose output is connected to a two-stage changeover device switched simultaneously with the pre-circuit filter and to which are connected independent band-pass filters whose outputs are each connected to their own mixer stage with separate intermediate frequency selection.

The invention will now be described in greater detail, with reference to circuit diagrams of an embodiment.

FIG. 1 shows a block diagram of a tuner constructed in accordance with the invention,

FIG. 2 is a schematic diagram of a pre-circuit filter, according to the invention showing the connection of the components the low frequency range,

FIG. 3 shows a schematic switching circuit array for connection of band-pass filters to the pre-stage, and

FIG. 4 shows the circuit array according to FIG. 3 supplemented with DC paths.

A television tuner in accordance with FIG. 1 is provided with a joint antenna connection 1, from which a first range switch 2 leads to a tuner branch for the frequencies of the VHF band I up to and including the hyperband, and a second range switch 3 to a UHF tuner branch. The UHF tuner branch comprises a pre-circuit filter 4 having a subsequent pre-amplifier 5, and band-pass filter 6 adjacent thereto having a subsequent mixer stage 7 to which is allocated an independent oscillator 8.

The UHF mixer stage 7 is coupled to an intermediate frequency circuit 9 which is followed by an intermediate frequency amplifier 10 having an intermediate frequency output 11.

By contrast, a pre-circuit filter 12 is connected to the first range switch 2 and can be switched over to a low and directly adjacent higher frequency range. This pre-circuit filter 12 combined for the two frequency ranges is connected by its joint filter output 13 to the input 14 of a joint pre-amplifier 15. Output 16 of pre-amplifier 15 is connected to a two-stage switching device 17, to one of whose outputs is connected input 18 of a band-pass filter 19 dimensioned for the low frequency range, and to whose other output is connected input 20 of a band-pass filter 21 dimensioned for the high frequency range. Switch-over of the pre-circuit filter and of the switching device 17 is effected simultaneously via a switch connection 22 indicated by a dashed line, depending on the selected frequency range. Outputs 23 and 24 of band-pass filters 19 and 31 respectively are each connected to an independent mixer stage 25 or 26. Mixer stages 25 and 26 are each provided with their own oscillator 27 or 28 respectively. In addition, the mixer stages 25 and 26 are connected independently of one another to the joint intermediate frequency circuit 9. The mixer stages and oscillators are part of an integrated tuner circuit 29, for example of type TDA 5330 T or TUA 2007. The separated band-pass filters 19 and 21 afford very high reception sensitivity with high interference immunity, since they can be optimally dimensioned for the appropriate frequency range and require no switching diodes that can affect the tuning range. For this reason there are no special switching voltages necessary for switching diodes. The independent mixer stage provided in each case also contributes to high selection and interference immunity, so that the usual compromises in mixer stage wiring can be dispensed with.

The pre-circuit and pre-amplifier circuit is shown in FIG. 2 without allowance for the first range switch 2. The pre-circuit filter accordingly comprises a series circuit of three frequency-determining coils 30, 31 and 32, to which a joint frequency-determining capacitor 33 preferably in the form of a controllable-voltage capacitance diode is connected in parallel. The first coil 30 is connected to ground on the one side, and the third coil 32 via a separating capacitor 34 to input 14 of pre-amplifier 15. Parallel to the series circuit, comprising the second and third coils 31 and 32, is a range switch 35 designed in particular as a DC-controllable switching diode. A matching coil 37 leads to the antenna connection 1 from the connecting point 36 between the second and the third coils 31 and 32. In the switching position shown in FIG. 2, the pre-circuit filter 12 is dimensioned for the low frequency range which extends from approx. 50 MHz to 150 MHz. Within this frequency range the matching coil 37 is largely ineffective at low frequencies in particular. Basic matching is achieved here by the division ratio of the inductance values of coils 30, 31, and 32. The capacitance variation of capacitor 33 thus permits tuning of the low frequency range in which all three coils 30, 31, and 32 are frequency-determining. Since the frequency variation range in the low frequency range is somewhat lower than in the high frequency range, which extends from about 150 MHz to 470 MHz, a correction capacitor 38 can be connected parallel to the range switch 35, this capacitor limiting to the necessary extent the effective variation range of capacitor 33 when range switch (35) is open. Range switch (35) therefore serves at the same time to alter the tuning ratio.

If range switch 35 is closed, than only coil 30 is connected directly parallel to capacitor 33 and so determines the high frequency range. With range switch 35 closed, the correction capacitor 38 too is short-circuited and so capacitor 33 alone determined the frequency with its entire variation range. In addition, when range switch 35 is closed the series circuit comprising coils 31 and 32 also becomes a parallel circuit which is connected in series to matching coil 37. Coils 31, 32, and 37 then act jointly to match the pre-circuit filter circuit to the wave impedance of the antenna line, the inductance of matching coil 37 then being changeable in order to tune the match. This change has practically no effect in the lower frequency range. Also, any change in the inductance of coil 30 to be made to tune the resonant circuit affects the resonaqnce frequency of the entire resonat cricuit only negligibly in the lower frequency range, since the inductance of coil 30 is low compared to the total inductance of coils 31 and 32. This pre-circuit connection also requires only a joint pre-amplifier 15, so that just one range switch 35 permits a frequency range switchover, a change in the tuning ratio and a changeover of the adjustment in the frequency ranges. For this minimum number of components, therefore, only a small space is required in the tuner housing, without adverse effects on selection, tuning range or interference immunity.

At output 16 of pre-amplifier 15 is connected the switching device 17 for band-pass filters 19 and 21, with the switching device in accordance with FIG. 3 having a capacitor 39 connected from output 16 to input 20 of band-pass filter 21 for the high frequency range. A coil 40 passes from output 16 via the parallel connection of a high-frequency choke 41 and a band switch 42 to a current source 46, or via a low-pass capacitor 43 to input 18 of band-pass filter 19 for the low frequency range. Band switch 42 is, like the high-frequency choke 41, connected to ground via a blocking capacitor 44 for high frequency. Band switch 42, is preferably a switching diode and is controlled together with range switch 35. With band switch 42 closed, capcitor 39 forms with coil 40 a high-pass filter for the high frequency range, with connection point 45 of coil 40 and capacitor 43 being connected via band switch 42 to ground.

The band-pass filter for the low frequency range is therefore high-frequency, input-side and short-circuited. The high-pass filter 39, 40 therefore permits an optimum adjustment between pre-amplifier 15 and band-pass filter 21.

With non-conduction, i.e. open band switch 42, by contrast, coil 40 with capacitors 43 and 39 forms a low-pass filter for the low frequency range, with capacitor 39 is ground via the input impedance of band-pass filter 21, with the connected band-pass filter 19 detuning band-pass filter 21 for the high frequency range coupled via capacitor 39 to such an extent that selection of certain frequencies is no longer possible.

With high-frequency choke 41, there is a DC path to output 16 of amplifier 15 when band switch 42 is opened, so that via this path the amplifier element present in preamplifier 15 can be supplied with DC current without affecting the low-pass filter.

In FIG. 4 a circuit array in accordance with FIG. 3 is shown, in which a switching diode 42 is used as the band switch. Here, a simple diode 47 is provided in series with the high-frequency choke 41 with otherwise identical circuit structure, via which diode the DC current is supplied to pre-amplifier 15 when the low frequency range is to be activated. In this case, switching diode 42 is without current and therefore blocked, and the range switch therefore open. If, on the other hand, switching diode 42 is connected to DC source 46, the current flowing through it effects through-switching, thereby closing the band swuitch. The circuit array is then switched to the high frequency range once connection point 45 is connected to ground via switching diode 42 and the allocated blocking capacitor 44.1. In switching device 17, switching diode or band switch 42 is neither in a frequency-determining resonant circuit nor in the high frequency path from pre-amplifier 15 to band-pass filters 19, 21, so that it does not exercise adverse effects on selection and the tunable band width.