To: <amps@contesting.com>
> Date: Thu, 28 May 1998 06:37:26 -0700
> From: Shawn Tayler <stayler@powernet.net>
> Subject: Re: [AMPS] Triode Amp Out/In isolation
> >How did you measure the S12 or S21?
>
> The measurement was made with an IFR-1200S service monitor in Tracking
> Generator Mode. Had a 10dB
> pad on the cable going to the input of the assembly, but no attn on the cable
> connected to the output connector
> of the amp, I am going to redo the test with an attn there just for SP.
OK. But keep in mind you can NOT measure phase at these points to
determine feedback phase around the tube unless you have a way to
allow for phase shift in the tank components and leads in the PA.
Phase would have to be determined right at the anode and cathode.
>The connector of the amp was connected
> to the Generator output of the TG, level about -20dBm. The input of the amp
> had a 10dB pad on it then a cable
> to the input of the SA/TG. The tune point of the amp was clearly visible on
> the scope and running all the
> controls moved and affected the isolation but the peak in feedback, so to
> speak, was always the
> combination tune point of the input and output, and never exceeded the -14dB.
Here's a possible measurement rub. You measured feedthrough with no
load at the anode and cathode, while the operating tube provides such
a load. When I measure feedthrough power in a cold system, I measure
with the cathode and anode sourced and terminated in typical
operating impedances.
Consider the circuit, where feedthrough capacitance is between
source and a load impedances and the load impedance and series
feedthrough capacitance form a divider. The tank, if you crank things
for maximum feedthrough, will partially (almost never completely
however) match the source impedance presented by the feedthrough
capacitance (I'm assuming it is capacitive because of the tube and
circuit you are using...that's a safe bet) to the load. With no
loading impedances, you have much less of a divider and feedthrough
will measure artificially high.
It's a safe bet the PA has nowhere near the feedthrough power
measurements indicate, although it certainly has some.
Can you more properly terminate the tube and measure again?
BTW, if you use a mixer to measure phase, I believe you need to
insure equal levels to each mixer input port. You can also use a
variable delay line section at VHF to measure phase, and by looking
only for a minimum or maximum level in combined output equal levels
become much less important.
73, Tom W8JI
w8ji.tom@MCIONE.com
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