A precise vector voltmeter (Princeton Applied Research Model 129, operated in the two-phase lock-in amplifier mode) is used in a microwave power calibrating system to accurately measure the outputs of a network analyzer (Hewlett Packard Model 8410 with Model 8411A harmonic frequency converter and Model 8413A phase-gain indicator). The validity of the measured outputs depends on the accurate representation of the network analyzer output (278 kHz) at a lower frequency (20 kHz) with sufficient signal level for proper operation of the vector voltmeter. A dual-channel mixer was constructed to convert 278 kHz to 20 kHz without loading the output ports of the network analyzer or local oscillators. In addition to being properly matched to the remaining circuitry, the mixers are driven in their linear region.