Squelch circuit with variable squelch tail and variable hysteres

Details Squelch circuit with variable squelch tail and variable hysteres Squelch circuit with variable squelch tail and variable hysteres

1988-01-11

1991-02-05

Olms, Douglas W.

Telecommunications

Receiver or analog modulated signal frequency converter

Squelch

455220, 455225, H04B 110

Patent

active

049912275

ABSTRACT:
The period of time that the squelch circuit maintains a radio receiver's audio amplifier active after the signal strength of a received carrier fades below a threshold level is inversely proportional to the strength of the received carrier immediately before the fade. This threshold level is also variable. If, immediately before the fade, the received signal strength exceeds a predetermined trigger level (Vtrg) the threshold level is raised. A buffer amplifier (102) charges a capacitor (C) to a voltage which is proportional to the strength of a received carrier. If the carrier fades below a threshold (Vth2 or Vth3, the buffer is disabled and the capacitor slowly discharges through a resistor (R). When the voltage at the capacitor crosses a reference voltage (Vref1) the output of a comparator (104) switches, thereby deactivating the receiver's audio amplifier. If the voltage at the buffer output is within a specified range, a feedback control circuit (112) switches a variable hysteresis feedback circuit (110) such that the negative-going threshold of the comparator (106) is switched from one threshold level (Vth2) to another (Vth3).

REFERENCES:
patent: 4132953 (1979-01-01), Martin, III
patent: 4388731 (1983-06-01), King
patent: 4637066 (1987-01-01), Kennedy et al.
"Syntor X Instruction Manual"; Motorola, Inc.; Published 10/2/81; Chapter `Receiver`, Sections 2.4 and 2.5 plus schematic diagram.
"Nucleus I.C."; Ed Clark, John Fuerst and Jim Kimball; unpublished, but describes a product on sale in the U.S. in 1979; pp. 88-105 only.

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