...under perpetual construction.
E4406A VSA + 2 x HP8657B + HP6632B + GPIB card + homemade control SW in Python
The good old (and relatively cheap) HP 8657B signal generators do not have an extremely accurate RF output level; they are specified for ±1 dB from -127 dBm to 3.5 dBm and can output nominally up to 17 dBm with unspecified accuracy.
As usual for HP specs, they are normally much better than that but a little characterization and SW correction can greatly improve their accuracy.
The following pictures show the RF output power measured by the E4406A at the output of the combiner: first picture show the raw, uncorrected output:
Mean losses through the combiner are about 4 dB for this path and there is about 0.5 dB of maximum deviation due to the generator output uncertainty.
After SW correction the maximum deviation is about ±0.06 dB; cannot be much better than that since the 8657B output level resolution is 0.1 dB;.