An incoming signal voltage of frequency fe, whose amplitude is to be measured, is additively heterodyned in a first mixer with a locally generated variable test frequency ft to provide a first beat frequency fi1 fe + ft clearing a first filter with a pass band including that beat frequency. In a second mixer, preceding a second filter with a narrow pass band centered on a second beat frequency fi2, the first beat frequency fi1 is subtractively heterodyned with a locally generated ancillary frequency fc of magnitude fi1 - fi2 to provide the frequency fi2 which passes the second filter and reaches an amplitude indicator through an adjustable amplifier or attenuator. In a calibrating position, a fixed reference frequency fo fi2 from a local oscillator is substituted for the input frequency fe whereas the test frequency ft is replaced by the ancillary frequency fc in the first mixer.