DE2717185C2 - TV channel selector for the frequency ranges VHF and UHF - Google Patents

Description

The invention relates to a television channel selector according to the preamble of the claim.

In a known television channel selector of this type (Funk-Technik, Zz. Volume, No. 5/1977, pages R & E 65 to 69 and 72) the following band filter via a capacitor with an input selector is in the VHF channel selector part after a preliminary stage Field effect transistor connected, which is simultaneously coupled to a VHF oscillator via a further capacitor. The field effect transistor serves as a mixer. In the UHF channel selector part, the UHF intermediate frequency is decoupled from a self-oscillating mixer via a low-pass filter, from which the UHF intermediate frequency is coupled via a switching diode and an additional capacitor connected in series to the above-mentioned input electrode of the field effect transistor, which is selected during UHF operation of the television channel acts as an output amplifier for the UHF intermediate frequency. The input capacitance of the field effect transistor operating as a VHF mixer is of the same order of magnitude as the capacitance values of the capacitors for supplying the VHF input signal, the VHF oscillator signal and the UHF intermediate frequency. This leads to a capacitive voltage division in VHF operation, which causes a gain reduction in the VHF mixer. This loss of gain is particularly noticeable in the VHF band III frequency range after the smallest gain values are reached there. In addition, the relatively powerful VHF oscillator signal in particular can reach the UHF channel selector part via the junction capacitance of the switching diode and from there back to the antenna via the capacitor for coupling the UHF intermediate frequency in VHF operation. This path is relatively low-resistance, especially for the harmonics of the VHF oscillator signal.

A VHF handheld radio for a very narrow frequency range is also known (Funkschau 1976. Hcfl 18, pages 80 to 82). in which a field effect transistor working as a mixer is supplied with the VHF input signal to one input electrode and the VHF oscillator signal is supplied to a second input electrode. The VHF oscillating signal is coupled in via a series resonance circuit.
Finally, it is known from a VHF tuner (ELO 1977, No. 7, pages 23 to 25) in which the oscillator signal is coupled to the base of a bipolar transistor operating as a mixer via a series resonance circuit

ίο The invention is based on the object at a TV channel selector according to the preamble of the claim to take measures through the one simple circuit design in the VHF band III frequency range, a gain in gain in the VHF mixer is achieved.

This object is achieved according to the invention by the characterizing features in the patent claim.

In a circuit construction of a television channel selector According to the invention, the series resonant circuit is parallel to the input capacitance of the field effect transistor of the VHF mixer. The inductive behavior of this series resonance circuit increases the input capacitance of the field effect transistor is transformed to a lower value. The voltage division is also correspondingly especially reduced with the capacitor, coupled in via the VHF input signal will. However, this reduced voltage division causes a corresponding gain in gain.

At the same time, via the series resonance circuit at UHF operation, the UHF intermediate frequency is fed in, for which the series resonance circuit simulates the usual coupling capacitance. The series resonance circuit acts here also as a low-pass filter, so that in particular VHF-Os / Iator harmonics can only reach the UHF channel selector section at a strongly damped level. This is no special connection diode required for the UHF intermediate frequency and W.2nn the corresponding one Resistance is omitted, via which the control voltage for the switch-on diode is supplied. Through the improved Attenuation of the VHF oscillator harmonics via the Series resonance circuit in the UHF channel selector the oscillator voltage values at the antenna input are further reduced.

The invention is explained in more detail below on the basis of the switching forces of an exemplary embodiment. A television channel selector for the frequency ranges VHF and UHF has in the VHF range z. B. not one Field effect transistor shown in MOS technology as a preamplifier for the VHF input signal and a Field effect transistor 1 in MOS technology as a mixer for generating a VHF intermediate frequency from the preamplified VHF input signal and a VHF oscillator signal, the pre-amplified VHF input signal on line 2 via a capacitor 3 and the VHF oscillator signal on line 4 via a Capacitor 5 can be fed to a first input 6 of field effect transistor 1. In the UHF range the UHF input signal is preamplified by a transistor (not shown) and does not reach you transistor shown as a self-oscillating mixer for generating the UHF Zwischenfrequcnz. These UHF-Zwischenfrequcnz on the line 7 is the input 6 of the l-'cklcf via a reverse resonance circuit

b ^r > fcktlransistors 1 supplied, the field effect transistor 1 in UHF operation as an output amplifier for the UHF ^r -7th intermediate frequency is used.

The backward resonance seen from the first entrance 6

circuit 8 consists of a capacitor 9, a resistor 10, a coil 11 and a capacitor 12 and in VHF operation is parallel to the input capacitance of the field effect transistor 1. The series resonant circuit 8 isl dimensioned so that it is in the VHF frequency range Band Hl, which in any case has a lower gain than Band I, has a gain in gain, because it has an inductive behavior for band llt frequencies shows and thus transforms the input capacitance of the field effect transistor 1 in such a frequency-dependent manner, that in the entire VHF frequency range band 111 the capacitive voltage division ratio between the pre-amplified VHF input signal on capacitor 3 and the VHF oscillator signal on capacitor 5 is favorable being affected.

Furthermore, the series resonant circuit 8 has a capacitive one in the frequency range of the UHF intermediate frequency Behavior and therefore acts as a UHF coupling capacitor, so that there is no need for a UHF connection diode can, since the series resonance circuit 8 is dimensioned so that it acts as a low-pass filter for the oscillator frequency. Consequently VHF oscillator harmonics can only be attenuated strongly get into the UHF part, whereby the oscillator voltage values at the antenna input (not shown) can be further reduced.

1 sheet of drawings

30th

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Claims (1)

Claim: TV channel selector for the frequency ranges VHF and UHF, in which a field effect transistor is provided as a mixer for the VHF range, to which the VHF input signal and the VHF oscillator signal and in UHF operation the UHF intermediate frequency on a common input electrode a capacitor is coupled in, characterized in that the capacitor (12) has a series resonant circuit (8) for coupling the UHF intermediate frequency with the series connection of a coil (11), a resistor (10) and a capacitor (9) connected to ground. which has inductive behavior in the VHF band III frequency range and capacitive behavior in the frequency range of the UHF intermediate frequency and that the UHF intermediate frequency via a fnk to the resistor (10) and the one-sided connected to ground capacitor {%} line ( 7) is fed into the series resonance circuit (8)