US 4,731,876 AGranted

VHF mixer circuit with band dependent switching

AssigneeDeutsche Thomson-Brandt GmbH

InventorsGerhard Maier, Bertram Fischer, Erich Kirschner

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Abstract

A mixer circuit with a conventional very high frequency circuit for the reception of color television bands I and III, which is connected dependent on the band and which is provided with an NPN transistor. The NPN transistor operates via a collector resistor onto a two circuit band filter. Moire interferences in the band I region expanded for CATV are avoided by having the mixer current used to 0.3 to 0.6 mA via a counter coupling resistor disposed between base and hot end of the resistor 4. Thus the self-interference in band I is reduced by a value of about 60 dB. In case of band III operation the corresponding switching voltage is fed via a series connection of a diode 6 and a resistor 5 to the base and thus the base level is lifted up to higher mixing current, since in band III self-interferences cannot occur in the case of the presence of signals.

Claims17

Dependent claims

A mixer circuit for the VHF-television transmission bands I and III comprising

a mixer transistor having a base, an emitter and a collector, where emitter and collector represent intermediate frequency power connections of the mixer transistor and where the collector provides a collector voltage during operation;
an input terminal connected to the base of the mixer transistor for applying an oscillator voltage to the base;
a collector resistor having a first terminal connected to the collector of the mixer transistor;
a two pole intermediate frequency filter connected with a first terminal to a second terminal of the collector resistor;
an output terminal connected to a second terminal of the two pole intermediate frequency filter;
a current limiting resistor connected to the second terminal of the two pole intermediate frequency filter, where the collector voltage is fed to the transistor via the current limiting resistor, the two pole intermediate frequency filter and the collector resistor;
a base resistor having a first terminal connected to the base of the mixer transistor, where a second terminal of the base resistor is connected to ground;
a series circuit of a diode and a resistor having a first terminal connected to the base of the mixer transistor, where a second terminal of the series circuit is connected to ground;
and
a resistor for operating point adjustment disposed between the base for the mixer transistor and the second terminal of the two pole intermediate frequency filter, where the resistor for operating point adjustment together with the base resistor form a voltage divider, which is dimensioned such that self-disturbances in the band I are suppressed while operating in the band I and that while operating in the region of band III the base voltage level of the same mixer transistor is modified by a switching of the voltage resulting for band III via the series circuit of the diode and the resistor such that the mixer current while operating in band III is substantially increased in comparison with the mixer current corresponding to operating in the region of band I.

7 more elements

A method for signal mixing in receivers for the VHF-television transmission bands I and III comprising the steps of

employing a mixer transistor having a base, an emitter and a collector, where the collector provides a collector voltage during operation;
applying an oscillator voltage to the base of the mixer transistor via an input terminal connected to the base of the mixer transistor;
connecting a collector resistor with a first terminal to the collector of the mixer transistor and said collector resistor having a second terminal;
connecting a two pole intermediate frequency filter with a first terminal to the second terminal of the collector resistor;
picking up an intermediate frequency signal at an output terminal connected to a second terminal of the two pole intermediate frequency filter;
connecting a current limiting resistor to the second terminal of the two pole intermediate frequency filter;
feeding the collector voltage to the transistor via the current limiting resistor, the two circuit intermediate frequency filter and the collector resistor;
connecting a base resistor with a first terminal to the base of the mixer transistor and connecting a second terminal of the base resistor to ground;
connecting a series circuit of a diode and a resistor with a first terminal to the base of the mixer transistor, and connecting a second terminal of the series circuit to ground;
disposing a resistor for operating point adjustment between the base of the mixer transistor and the second terminal of the two pole intermediate frequency filter;
providing a voltage divider from the resistor for operating point adjustment and the base resistor to avoid while operating in band I self-disturbances in the band I by appropriate dimensions of the voltage divider;
and
modifying the base level of the said mixer transistor during operation in the region of band III by a switching of the voltage resulting for band III via the series circuit of the diode and the resistor such that the mixer current while operating in band III is substantially increased in comparison with the mixer current corresponding to operating in the region of band I.

10 more elements

Dependencies are inferred from the claim text.