K3WWP's CW - Zero Beat Circuit

Randy Henderson's (WI5W) article in March 1992 QST (Do You Know Where Your Signal Is?) eloquently described a long-standing shortcoming in contemporary transceiver design.

As Randy pointed out, many transceivers set the CW transmit off-set equal to the set's sidetone frequency. But it was left to the operator how to match incoming signals to the sidetone for one-frequency QSOs -- an awkward and time-consuming procedure.

If yours is one of these (check your manual), a simple visual indicating circuit can be assembled to take advantage of the offset/sidetone relationship. A 567 decoder chip set to the transceiver sidetone frequency drives an LED. Audio from the transceiver speaker line is applied to the 567 input. The LED lights when an incoming signal is tuned to match this frequency. The RIT and XIT must be OFF for initial tuning.

The few components required are shown in the accompanying diagram, and also listed below with RADIO SHACK part numbers. R2 sets the decoder frequency. A 35mm film container was used as the housing for the indicator.

If your transceiver is one of the few which doesn't match offset to sidetone, the circuit is still applicable. However, the offset must first be determined, then R2 adjusted accordingly.

A more detailed discussion of this use of the 567 appeared in the December 1990 "73" magazine.

R1 - value depends on DC voltage available. For 13.5V use a 470-ohm 1/4w, for 9V use a 180-ohm 1/4w.
R2 - 10K-ohm potentiometer RS #271-282
R3 - 470-ohm 1/4w resistor RS #271-1317
C1 - 1.0 Mfd tantalum RS #272-1434
C2,C4,C5,C6 - 0.1 Mfd tantalum RS #272-1432
C3 - 0.47 Mfd tantalum RS #272-1433

1 6.2V zener diode
1 8-pin IC socket RS #276-1995
1 Yellow LED RS #276-021

Except for the LM567 IC, all parts are currently available at RADIO SHACK. The 567 is a common, inexpensive IC and available from any outlet stocking a general supply of IC's.

Use tantalum capacitors for stability. The two 0.1 Mfd in parallel at C5 and C6 are to provide a total of 0.2 Mfd since RADIO SHACK does not stock 0.2.

Audio input comes from the transceiver speaker line.

The DC source is usually from an accessory voltage jack on the transceiver. The value of the series resistor will depend on voltage available.

Adjust the variable resistor using transceiver sidetone, decreasing level to the minimum value that will light the LED.

Be sure to provide a ground return for the DC and Audio inputs.