GB618462A - Improvements in angle modulation receivers - Google Patents

Abstract

618,462. Frequency and phase modulation. RADIO CORPORATION OF AMERICA. July 4, 1946, No. 19985. Convention date, July 4, 1945. [Class 40 (v)] A circuit for receiving frequency or phase modulated signals comprises a frequency multiplier, means for selecting a particular harmonic and applying this to a demodulator, the arrangement being such that if the signal is below a predetermined amplitude substantially no harmonics are produced and therefore no signal is applied to the demodulator. Fig. 1 shows part of a superheterodyne receiver for frequency or phase modulated waves, the intermediate frequency signals being applied via I.F. transformer 1, to a pentode amplifying valve 3. This valve has a sharp cut-off characteristic, and elements R<1> and C<1> provide a short time constant automatic grid bias circuit. The output from valve 3 is applied to a discriminator circuit comprising tuned circuits P and S and a double diode valve 14. The circuits P and S are tuned to the second harmonic of the intermediate frequency, and the width of their band-pass characteristic is twice that of the I.F. circuits, so that these circuits together with diodes 14 act, at the second harmonic frequency, as a demodulator circuit of known type. Owing to the automatic bias action of R<1>, C<1>, the second harmonic output is produced by valve 3 only if the amplitude of the input signal is large enough to extend partly into the cut-off region of this valve. It is shown that the second harmonic output of valve 3 is substantially constant for values of input signal exceeding the predetermined minimum. Automatic volume control voltage is supplied by resistor R<1>, and the circuit elements associated with the screen grid of valve 3 may be adjusted to provide the desired cut-off voltage or output characteristic. It is stated that the circuit avoids the need for amplitude limiting, or noise squelch circuits. Specifications 489,094 and 547,276 are referred to.