Voltmeter

Fig• 5. Using a tunable filter ahead of an RF voltmeter convenience, an optional DPDT slide switch can he added to bypass the 11 iter Mount the whole thing in a small (3" x 2" x 1.5") aluminum box, using BNC connectors for input and output.

Calibration

You can mark frequency calibrations on the tunable filter dial by using a calibrated signal generator Feed a known frequency into the fitter and connect its output to the RF voltmeter. Tune the filler for maximum output and mark the frequency on the dial. Continue with other known frequency signals until the entire range has been calibrated. A dipper with a two-turn coupling link wound over its coil and connected to the input BNC jack can substitute for the signal generator if necessary.

Ne602 Unbalance
Fig. 6, Basic tunable filter circuit. For 7-20 MHz, use 38 turns if2(> wire on T68-10 core* Links, 5 turns H26. For 22-60 MHz. use 10 turns wire on T68-10 core. Links, 2 turns #22.

Other circuits

In addition to the MCI496, other double balanced mixers are the TL442CN, formerly known as SN76514, and the NE602. Single balanced and unbalanced mixers are also widely used— the only difference is whether or not the carrier or modulating signal is rejected ai all by the mixer circuit itself In a single balanced mixer such as the CA3028, one will be rejected.

With an unbalanced mixer such as the dual-gate 40673, neither input is rejected.

Both will appear at full strength at the output along with the sum and difference signals.

It's up to the filter following the mixer to rcjecl all but the desired sum or difference frequency. As a minimum, at least 40 dB of rejection should be used.

Signal levels at the inputs and output should he checked to ensure that only the desired mixing product is passed on to the following stages in a ham transmitter or receiver.

A sensitive wideband RF voltmeter and out hoard tunable filter are essential to perform the checks, but can be easily constructed at low cost. With these on hand a ham can verify or troubleshoot his mixer circuits with confidence.

References

1. tvdBm Meter" John Pivnichny N2DCH Electronics Now, Nov. 1995. A complete kit including case is available from Unicorn Electronics. Valley Plaza Drive, Johnson City NY 13790, (607) 798-0260,

2. "A Sensitive RF Voltmeter/' John Pivnichny N2DCH, Ham Radio, Jul) 1989,

3. "An RF Voltmeter/' Ian Braithwaite G4COL, Ham Radio. Nov. 1987.

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