On Apr 26, 4:49*pm, Wimpie wrote:
Other method is injecting a slightly off-carrier frequency signal into
the amplifier (this emulates a constant small VSWR shown to the PA
(wtih 50 Ohms load), but with continuous varying phase). Because of
the difference in frequency, one can measure the forward (towards the
PA) and reverse (reflected by PA) signal with a two channel VSA.
This will give you the PA's complex output impedance.

Unfortunately, the impedance encountered by the off-carrier frequency
signal is probably not the same as the impedance encountered by the
carrier frequency so the results don't correlate and are not very
useful. The carrier frequency has interference components that the off-
carrier signal doesn't encounter.
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK