My workbench includes a Hewlett-Packard 8405 A VVM (vector voltmeter) of 1960s vintage. The VVM can convert two inputs at frequencies of 1 MHz to 1 GHz to outputs of 20 kHz, complete with modulation and sideband components. It can read signal levels as low as -70 dBm at full-scale and phase-difference readings over a range of 360° with 1.5° accuracy. With an APC (automatic-phase-control) PLL, it locks to the input-signal frequency in 10 msec. You can then measure level and phase angle in the presence of interference as high as -40 dBm and a signal level of -60 dBm.
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