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From: Richard Hosking on Tues 31 May 2005 20:05

I have struggled with this in the past
It is a function of the behaviour of the servo loop at low freq and I
dont have the theory to analyse it properly.

The "theory" part should be evident to anyone who has made
a negative-feedback amplifier, single transistor to op-amp.
Getting to know op-amp responses both open-loop and closed-
loop (with negative feedback) can be helpful. Note that
op-amp designers actually build in open-loop phase shifts
at high frequencies to avoid oscillation with feedback.

The only difficult part is in MEASURING the PHASE at low
frequencies in the 0.1 to 10 Hz range...especially that of
the AGC control-line (feedback) circuitry. If not, some
dog-work on analyzing the magnitude and phase response of
that circuit will show that. [ability to handle complex
quantities is preferred there]

Lacking a calibrated RF source and much other critical equipment, I do have
a 45 year old Eico scope that once belonged to the famous Bach pianist Glenn
Gould, and could cobble together a variable dc source and a low freq
oscillator. To observe phase response of the open loop system, I'm thinking
that the loop could be broken between the AGC detector and the AGC time
constant/buffer. Drive the latter and the X input of the scope with the dc
supply and superposed low freq signal, feed the receiver with steady state
RF carrier and take the output of the AGC detector to the scope's Y input.
The variation of the input to the AGC system will cause variation in the
receiver gain and the output of the AGC detector. If in phase, the scope
would show a line with positive slope; if antiphase, a line with negative
slope; if in-between, an ellipse or some open shape subject to time
constants and non-linearities. This arrangement would leave the receiver's
RF gain control intact and its effect on time constant and phase observable;
it appears to modify the discharge resistance seen by a 1uF cap at the RF
and 1st Mixer in addition to pulling down the AGC voltage applied to them.

Does that seem to be a practical approach, Len?

Tom