GB620271A - Improvements in or relating to mixing circuits - Google Patents

Abstract

620,271. Valve circuits. PHILIPS LAMPS, Ltd. Jan. 14, 1947, No. 1260. Convention date, Nov. 12, 1943. [Class 40 (v)] In a U.H.F. mixing circuit in which an improved signal to noise ratio and reduced damping of the input circuit is obtained hy applying the incoming signals in push-pull and the local oscillations in phase to a pair of valves, damping of the local oscillator by the inductance of the lead connecting the cathodes of the mixing and local oscillator valves in common to earth, is reduced by connecting between the common cathode point and earth an impedance constituted by the impedances of the two halves of the push-pull circuit and of the two input circuits connected in parallel and tuned so that a maximum voltage at the local oscillator frequency is set up between the input electrodes of the mixing valves. The figure shows an arrangement in which signals from dipole aerials D are applied to a pair of diodes via Lecher lines L1, which are tuned to the frequency of the incoming signals by short-circuiting bridge K. The local oscillator comprises a triode valve coupled to the mixer at the common cathode point P. The I.F. is taken off from the Lecher lines and tuned by circuit L4, C4. The damping of the local oscillator is reduced by tuning the impedance Z1, in parallel with the impedance between point P and earth via the mixer valves, to the frequency of the local oscillations. The local oscillator voltage appearing across the input electrodes of the mixing valves is further increased by adjustment of the variable impedance Z2. To reduce radiation of the' local oscillator frequency, the aerial is connected to a voltage node for the local oscillations whilst correct matching of the aerial to the mixer is maintained by adjusting impedance Z2. The impedance Z4 which is usually capacitive is then adjusted to give maximum voltage at the local oscillator frequency across the mixing diodes, the inductors Z3 being of high value and having negligible influence on the tuning of the push-pull circuit. The aerial may alternatively be connected to a voltage node point by including a circuit resonant at the local oscillator frequency between the points of connection of the aerial and earth.