On 27 abr, 03:44, Cecil Moore wrote:
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

Helo Cecil,

Depending on the frequency resolution of your VSA, the frequency of
the injected signal can be well within 1 kHz of the carrier, so LC
filters in the PA will not distort the measurement. In case of a 100W
PA and injection of about 100 mW, the difference in wanted signal and
signal to be rejected is 30 dB (not that large).

If you don't have a VSA, you can still do it (with some more
calculations) by using a diode detector, this will give you the
difference frequencies directly and you can observe the phase
differences on an oscilloscope.

There can be a difference between a very slow load variation (for
example manually changing loads and noticing current and voltage
[including phase]) and the VSA method. This is because of decoupling
capacitors in the power supply or bias circuits (for example RC
combination in the grid circuitry to limit the grid current). If you
vary the load about 10..100 times/s, bias and supply voltages don't
have the time to settle to their steady state.

If the VSA method is basically wrong, I would love to hear why.

With kind regards,


Wim
PA3DJS
www.tetech.nl