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METHOD OF COMPENSATING FOR LOW FREQUENCY AUDIO DISTORTION IN DIRECT FM MODULATION OF A SYNTHESIZER

IP.com Disclosure Number: IPCOM000008636D
Original Publication Date: 1998-Mar-01
Included in the Prior Art Database: 2002-Jun-28
Document File: 1 page(s) / 67K

Publishing Venue

Motorola

Related People

James K. Gehrke: AUTHOR [+2]

Abstract

Two-way communication transmitters using frequency modulation have a need to support a modulation signal bandwidth of 7 Hz to 3k Hz to allow for sub-audio signaling and voice modulation. The modulation level accuracy must be very high at low frequencies (7 to IO0 Hz) if the transmitters are used in a simulcast contiguration which has two or more transmitters operating on the same frequency. Without this high level of accuracy, the recovered audio quality would be poor in a simulcast overlap area. This overlap area is where the same frequency transmitters have nearly equal signal strength.

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MOTOROLA Technical Developments

METHOD OF COMPENSATING FOR LOW FREQUENCY AUDIO DISTORTION IN DIRECT FM MODULATION OF A SYNTHESIZER

by James K. Gehrke and Robert Sarocka

BACKGROUND

  Two-way communication transmitters using frequency modulation have a need to support a modulation signal bandwidth of 7 Hz to 3k Hz to allow for sub-audio signaling and voice modulation. The modulation level accuracy must be very high at low frequencies (7 to IO0 Hz) if the transmitters are used in a simulcast contiguration which has two or more transmitters operating on the same frequency. Without this high level of accuracy, the recovered audio quality would be poor in a simulcast overlap area. This overlap area is where the same frequency transmitters have nearly equal signal strength.

PROBLEM

  Current design practice is to use direct FM modulation of the transmitting carrier frequency by modulating the frequency synthesizer, A problem arises when trying to modulate the voltage controlled oscillator in the frequency synthesizer at sub-audio frequencies. The frequency control feedback loop corrects out or cancels modulation at frequencies below the natural frequency of the closed loop. It is common practice to use frequency synthesizer IC's with dual-port modulation circuits to prevent the loop from causing distortion to low frequency audio modulation such as DPL signals. This dual port modulation is only available on Motorola custom synthesizer IC's, which typically

have a higher...