Hello Cecil,
On 2 mayo, 23:17, Cecil Moore wrote:
On May 2, 1:41*pm, Wimpie wrote:
Please explain (or someone else), as I don't understand anything of
the above with regards to a PA. You described a single-port device and
now starts talking about a two-port device.
If you recognize the example as a two-port device, you correctly
measure an s11 of 0.3333, (100-50)/(100+50). If you happen to overlook
the second port to which the 200 ohm resistor is attached and treat
the example as a single-port device, you measure an s11 of 0.0000,
(50-50)/(50+50). The example has not changed between those two
measurements so which s11 is correct?
You want to know the output/input impedance of your black box, this
means you should look into the 100 Ohms line with 200 Ohms
termination. We will see 50 Ohms, no matter the reference impedance of
the VNA. If it was a 75 Ohms VNA, it would read S11 = -0.2. What the
200 Ohms resistor sees is not important if you just want to know the
behavior of the single-port black box.
Doesn't that fact give you pause to wonder if you are making essentially the same mistake with the PA
measurements?
I am very sorry Cecil, but I still don't see the point where the
discussed method may go wrong.
The only thing I could think of is that you have in mind a setup where
the input of the PA is port number 1 (and the output is port number
2 ) and you carry out a full-port measurement. For a full class-A,
AB, non-saturated PA, this may give useful results. When S12*S21
1, S22 will equal the output impedance, otherwise you have to do the
match.
In real world many amplifiers do not behave as a linear system (food
for discussion) and then the two-port setup will fail, as during S22
measurement the input port is terminated with 50 Ohms (so there is no
source that provides output).
Therefore carrying out a single-port measurement with a slightly off-
carrier frequency (to create non-coherence) under required output
conditions, will result in a meaningful output impedance. As
mentioned before, doing a (slow) manual load pull measurement may give
different results because of bias and supply voltage variations.
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK
With kind regards,
Wim
PA3DJS
www.tetech.nl