Fm Squelch Schema



Fig, 72 Coi?/wcf/ofi of /npi/f of rto? digital COR to the squelch circuit of the repeater receiver. //?<? circuit shown is a portion from a Motorola G-strip receiver; 77?f co/wecWo/j point wilt be similar in othet types of tube receivers.

11 T y V 6 3 v

/ I Simple 5 uo// regulated power supply.

commcrcial FM receivers the grid of the first stage following the noise rectifier has this characteristic. A simplified sketch of the squelch circuit of a typical tube-type receiver and 1 he correct point of connection to the COR are shown in Fig, 12.

Once the shielded cable connection to the proper point in the receiver is made, determine if the squelch control threshold setting has been significantly altered. 11 so, the 10 and 22 megohm resistors must be substituted with ones of higher resistance, Some experimentation may be required to find the correct resistance values to match a particular receiver.

NcnL, inject a carrier signal into the receiver and adjust the 5K Ohm trimmer in the COR input circuit unti! the green LED lights at the desired level of receiver quieting. If consistent operation of the input circuit cannot be obtained for any setting of the trimmer, the ratio of the two input resistors will have lo he varied, remembering not to let iheii sum be less than the value that will ieave the squelch threshold setting unaffected. The voltage at the gate of 01 compared with the typical gate voltage noted on Fig, G should be used to determine which way the ratio must be altered. 1 he value of the resistor between the gate and ground must be increased il the amplitude of the g^te voltage is significantly less negative than -1.0 volts during carrier reception. Conversely , ihis resistor must be decreased iif the negative excursions of the gate voltage are significantly more negative than -1.0 volts.

Shema Simple Amater Radio

aooa coil

aooa coil

If a signal generator is not available, the input adjustment may be made by varying the squelch control to simulate carrier reception. Opening the squelch control until audio noise is heard roughly corresponds to the reception of carrier, thus causing the input circuit to switch tJon" when the 5k Ohm trimmer has been suitably adjusted.

In all cases of difficulty, the typical voltages given in Fig. 6 should be used to help diagnose the problem.

This completes the settings and adjustments of the COR.

Optional Circuits

As was previously mentioned, the digital COR requires a well regulated five volt power supply. For those who don't have a persona! preference, the circuit of Fig. 13 is suggested as the simplest and probably the cheapest available.

The digital COR was originally designed to control a solid state switch that did the actual transmitter keying. For those who prefer to use a relay to switch on the transmitter, the circuit of Fig. 14 is offered. The diode should be attached directly across the relay coil to prevent inductive transients from damaging the semiconductors.

Finally, Fig. 15 shows a simple tone burst circuit that can be used for an audible indication that the three minute timer has been reset.

Conclusion n r

The digital COR described in this article has been used successfully with both G.E. Progress Line receiver strips and Motorola Sensicon G-strip receivers. The COR is currently in use on the WR0AGZ repeater in Sioux City, Iowa. ■


Signetics Lineaf Data Book, 1972, page 177.

7 Joseph M. Sieim WAIOMS,'JIC Repeater Logic System," 73( Sept. 1973, page 27. X G. Allen W2FPP, R. Sobus K2QLE, "Repeater Control with Simple Timers," Ham Radio, Sept., 1972( page 4G. * Clifford Klinert WB6BIH, "Repeater Keying Line Control," 73, Feb. 1973, page 60.

Parts List

Q1 - Radio Shack 276-112 N-Channel JFET Q2 - HEP 52 Q3, Q5 - HEP 726 04,07,011 HEP 719 Q6, OS, Q9, Q10 — HEP 736 D1-D5 — small signal germanium diodes

1C1 - SN7400 Quad NAMD gate IC2-JC5 - NE555 Timer IC LED1 — yeNow light emitting diode

LED2 — green light emitting diode

LED3 - red light emit Ting diode AH .01 uF caps are 25 volt disc ceramics

AN fi*ed resistors are Vt Watt, 10% toL, resistance as marked on schema lie

SW1 - DPDT, center-oH switch

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