GB549342A - Improvements in carrier-signal frequency-detector systems - Google Patents

Abstract

549,342. Frequency modulation. HAZELTINE CORPORATION. July 22, 1941, No. 9246. Convention date, Aug. 12, 1940. [Class 40 (v)] A detector for a frequencymodulated system comprises a pair of networks arranged to give minimum output at two different frequencies f1, f2 which are situated symmetrically on opposite sides of the mean frequency fo, and to give peak outputs at frequencies beyond the normal operating range f1, f11. A linear relation is obtained between frequency variation and detected output if such a pair of networks is followed by square law detectors with differential output. Alternatively the pair of networks may feed a succeeding similar pair followed by linear detectors. The IF output from a stage 14 is kept constant by an AVC system 20 and is fed in parallel to limiting or linear amplifiers 21, 22. Their output is fed to the respective networks 27, 28 each comprising a series-resonant branch, a condenser, and an inductance all in parallel. The network 27 has its series-resonant branch tuned to the frequency f1, Fig. 2a, .and gives peak response at frequencies beyond f1, f11 one peak being more marked than the other as shown in the full line curve d. The other network 28 gives a response wave e shown . in dotted lines. Owing to the linearity of the curves d, e between the limits f1, f11, the output from the squarelaw detectors 37, 38 follows the parabolic curves g, h of Fig. 2b, and their differential combination yields a linear output as shown in Fig. 2c. The detectors 37, 38 are shown as biased pentodes with their output -differentially combined by -a push-pull transformer 40. In the alternative arrangement shown in Fig. 3, the output from the first pair of networks 52 is fed through coupling amplifiers 53 to a similar second pair 54, giving IF outputs following the curves g, h of Fig.2b. These are fed to linear rectifiers 46, 47 connected with opposite polarity to load resistances 48, 49. The output is taken between the earthed junction of resistances 48, 49, and the junction of resistances 50, 51 connected across the former.