Input Circuit Comprising at Least Two Input Paths

1. Field of the Invention

The invention relates to an input circuit comprising at least two input paths which are alternately connectable to a common output and have a common switch to ground, and further comprising switches for the input paths, in particular, for combination tuners with input circuits for UHF and VHF and a common preamplifier component.

2. Description of the Prior Art

In a known input circuit of this kind (German Auslegeschrift No. 25 03 785) a combination tuner is provided with three separate input paths located at a common input and alternately connectable, via individual, series-connected switches, to a common output. The input paths constitute selection members or filters for the UHF, the high VHF band and the low VHF band ranges. The common output is connected to a control electrode of a common preliminary stage transistor. The danger inherent in this design is that the fully functioning selection members, due to their desired high selection characteristics when the switching path is open, couple unacceptably high signals into the usable signal of the actuated selection member through the capacitance of the switches.

The object underlying the invention is to take measures in an input circuit for a combined UHF and VHF tuner including at least two alternatively actuatable input paths, one of which is a UHF selection circuit and another of which is a VHF selection circuit, connected between a common antenna input terminal and a common output amplifier, by means of which measures full efficiency of the actuated input path is attained, while the non-actuated input path is strongly damped and out-of-tune.

This object is attained in accordance with the invention by an input circuit for a combined UHF and VHF tuner having a common antenna terminal for a UHF selection filter and for a VHF selection filter and a common amplifier connected subsequent to the two selection filters, wherein: the output of the UHF selection filter is connected to the input of the amplifier; the UHF selection filter includes an impedance having one end connected with the input of the amplifier and whose other end can be connected to ground for high frequency via a switch; the output of the VHF selection filter is connected via the impedance to the input the amplifier and is connected to the switch and can be connected to ground for high frequency via the switch; and the switch is blocked (open) for VHF selection and conducting (closed) for UHF selection.

According to one embodiment of the invention, the first input path constituted by the UHF selection filter is a high pass filter for the UHF range having two shunt inductances, a further switch is connected in the series path between the two shunt inductances, the shunt inductance at the output side of the UHF selection filter constitutes the impedance, the output of the second input path, i.e., the VHF selection filter, is connected to the connection of the impedance on the side on which it is connected to the first mentioned switch, and both switches assume the same switched state.

In this embodiment according to the invention, the first input path is made fully operative when current is flowing through the switches and the signal from the second input path is short circuited by the first mentioned switch. This switch effects a necessary ground connection for the first input path. Accordingly, the selection member for UHF is made fully operative, while the selection member for VHF is put out-of-tune by the short circuit and, in addition, strongly damped so that no disturbing VHF signals are coupled into the UHF selection member via the ground connection through which current is flowing.

During VHF operation, on the other hand, when no current is flowing through the switches, the selection member for UHF is firstly interrupted and secondly put out-of-tune by the actuated VHF selection member so that selection in the UHF band no longer occurs. According to a modification of the invention, instead of using a high pass filter for the UHF range, the first input path may be constituted in the form of a half-wave length resonant circuit for the UHF range with a capacitance diode at the base of the circuit and an alignment inductance branching off from the capacitance diode for alignment at the upper frequency band end and connectable to ground via the first mentioned switch, and with the second input path connected between the alignment inductance and this switch.

In this embodiment with a UHF selection circuit of variable frequency, one switch in the path from the alignment coil required for the upper frequency range to ground is adequate since such an extensive detuning occurs when no current flows through the switch and by way of the VHF range filtering components then connected to the UHF selection circuit that UHF signals are suppressed. The components of the UHF resonant circuit are then simultaneously components of the VHF selection members.

FIG. 1 is a schematic circuit showing a preliminary stage circuit, according to one embodiment of the invention, with non-tunable selection members for the UHF and VHF ranges.

FIG. 2 is a schematic circuit showing a preliminary stage circuit, according to a further embodiment of the invention, with a variable frequency UHF input circuit and non-variable frequency VHF selection members.

As shown in FIG. 1, a preliminary stage according to the invention for a combined UHF-VHF tuner basically includes a metal oxide semiconductor field-effect output transistor 1 or input, a UHF selection member 2 and a VHF selection member 3. A low-pass filter 4, having an upper limit frequency which lies between the UHF range and the VHF high-band range, is connected in series with the input of the VHF selection member 3. The inputs of the selection member 2 and of the selection member 3 via the low-pass filter 4 are connected to a common antenna connection 5. The outputs of the selection members 3 and 4 are commonly connected to a gate 6 of the metal oxide semiconductor field-effect transistor 1, whose second gate 7, for high frequency, is practically connected to ground. The amplified high frequency signal is then passed on via the drain electrode 8 and an isolating capacitor 9 to a band-pass filter which is not illustrated.

In accordance with FIG. 1, the UHF selection member 2 consists of a transforming high-pass filter which enables the characteristic impedance at the antenna input 5 and the input resistance at the gate 6 of the metal oxide semiconductor field-effect transistor 1 to be matched. The high-pass filter comprises a capacitor 10, an inductance 11, a further capacitor 12 and two further inductances 13 and 14, all of which are connected in series. Inserted between the inductance 11 and the capacitor 12 is a switching diode 15 which is preferably a PIN diode. At the point of connection of the switching diode 15 and the inductance 11, a shunt inductance 16 is connected to ground, while the control current is fed to the other connection of the switching diode 15 via a resistor 17. Connected to the connection point of the inductance 13 and the inductance 14 leading to the gate 6, is an additional shunt inductance 18 which is connected via a DC-isolating capacitor 19 to a switching diode 20 acting as a switch to ground. The control current for the switching diode 20 is supplied via a resistor which is connected to the same control current line as the resistor 17. Accordingly, when current flows through the diodes 15, 20, the UHF selection member 2 is actuated.

The transition of the VHF selection member 3 to the UHF selection member 2 occurs at the connection point of the second shunt inductance 18 and the DC-isolating capacitor 19. When current flows through the switching diode 20, this connection point is, in high frequency terms, at ground potential, so that any selected VHF signals are connected to ground. During UHF operation, the VHF signals are, therefore, short circuited. When the diodes 15, 20 are blocked, the UHF selection member 2 is, on the other hand, blocked or out-of-tune, while the VHF selection member 3 then guides the selected VHF frequency band via the shunt inductance 18 and the inductance 14 to the gate 6 of the metal oxide semiconductor field-effect transistor 1 for amplification.

The selection member 3 includes a low-pass filter which consists of two series inductances 21 and 22 and a shunt capacitor 23 and is short circuited via a third switching diode 24 during operation in the VHF high-band range as a result of a control signal supplied to the switching diode 24. With the low pass filter 21, 22, 23 short circuited via the diode 24, the selection circuit 3 forms a high pass filter whose lower limit frequency lies above the VHF low-band range. For selection of the VHF low-band range, the switching diode 24 is, on the other hand, blocked, so that the entire selection member 3 including the active inductances 21 and 22 and the shunt capacitor 23 operates as a bandpass filter which blocks both the frequencies of the VHF high-band range and the lower video intermediate frequency.

In accordance with FIG. 2, the input circuit comprises as the selection member 2 for the UHF range a variable frequency tuning circuit in half-wave length technique with a tuning diode 25 connected to ground from which an alignment coil 26 for the upper frequency range is connected to ground during UHF operation via DC-isolating capacitors 27 and the switching diode 20 acting as a switch.

Accordingly, during UHF operation, the selection member 2 acts as a half-wave length resonant circuit which can be tuned by the tuning diode 25. During VHF operation, on the other hand, the switching diode 20 is blocked. In this embodiment separate selection members or band-pass filters 3 are provided for the VHF low-band range and for the VHF high-band range.

The selection members 3 designed for the VHF low band range and the VHF high-band range, respectively, have their inputs commonly connected via the low-pass filter 4 to the antenna 5 and their outputs commonly connected to the connection point betwen the DC-isolating capacitors 27. Consequently during UHF operation when current flows through the switching diode 20, these selection members 3 are again short circuited to ground at the connection remote from the antenna i.e., at their common output.

When the switching diode 20 is blocked, on the other hand, the respectively selected VHF frequency band is fed via the alignment coil 26 to the impedance 18 and directed from there to the gate 6 of the metal oxide semiconductor field-effect transistor 1.

During VHF operation, one of the selection members 3 can be connected, in high frequency terms, to ground, via switching diodes 28 and 29, respectively, so that only the frequencies of the second frequency ranges are selected and directed to the gate 6 for further amplification.