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AMPLITUDE-MODULATED FM

IP.com Disclosure Number: IPCOM000005417D
Original Publication Date: 1980-Aug-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 1 page(s) / 45K

Publishing Venue

Motorola

Related People

Jerome Breitenbach: AUTHOR

Abstract

It was desired to limit the bandwidth of an FM signal with little loss in performance and with minimal additional circuitry. The solution was to impose AM modulation on the FM signal in such a way as to make the resulting RF spectrum more bandlimited than had no AM been added at all. This would require no changes in the PM receiver as the receiver would strip off the AM by limiting the received signal, and hence would only detect the FM component as before. The scheme would also not affect transmitter design as the highly efficient class-C power amplifier could still be used, with the AM being added on at the last stage as is commonly done in transmitters used in the Citizens' Band.

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MOTOROLA Technical Disclosure Bulletin Vol. 1 No. 1 August 1990

AMPLITUDE-MODULATED FM

By Jerome Breitenbach

   It was desired to limit the bandwidth of an FM signal with little loss in performance and with minimal additional circuitry. The solution was to impose AM modulation on the FM signal in such a way as to make the resulting RF spectrum more bandlimited than had no AM been added at all. This would require no changes in the PM receiver as the receiver would strip off the AM by limiting the received signal, and hence would only detect the FM component as before. The scheme would also not affect transmitter design as the highly efficient class-C power amplifier could still be used, with the AM being added on at the last stage as is commonly done in transmitters used in the Citizens' Band.

  The central problem with the method was how to generate the required AM that would bring about the desired bandlimiting. This was done with the circuit of the figure, which is a Kahn amplifier preceded by a filter. The signal was passed through the bandpass filter which removed the unwanted sideband energy and, by doing so, produced an AM component on the signal. This AM was detected at the final stage of the class-C power amplifier. Measurements on such a system showed a reduction in the RF bandwidth with a reduction in the passband of the bandpass filter. There was a noticeable loss in fidelity of the signal.

RF

MESS&x YODU~TOR

CLASS c FINAL NAPLIFIER STAGE

BANDPASS FILTER

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