From: "Tom Holden" on Tues 31 May 2005 22:08

wrote in message
roups.com...
From: Richard Hosking on Tues 31 May 2005 20:05

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?

If that tells you what you want to know, it is practical.

However, the phase information from that Lissajous display is
rather gross. If, with a closed-loop condition, there is
marginal stability, then a better handle on phase response
would be necessary...or just reducing the AGC control-line
magnitude (which would offer less AGC action).

I'll have to presume the Eico scope doesn't have a slow sweep
rate. If that scope has a DC input on both horizontal and
vertical, then the cobbled-together low-frequency source could
be built with a ramp output that would act as the horizontal
sweep; the display would then be just one cycle but that would
indicate the phase difference. Suggestion for source: Exar
XR-8038 DIP which has both square-wave and sine outputs.

A "bounce-less" switch circuit can be put together out of two
NAND gates connected as an R-S flip-flop, an SPDT switch
grounding/earthing one input on each NAND gate. That simulates
a very extreme "attack" situation to check the response of the
AGC control-line circuit. It's a bit much to infer anything
of numerical value out of that, though, since the amount of
analysis of the waveform out of the AGC control-line is lengthy
and probably more time than it's worth.

I'll have to remind all that a reasonably-calibrated RF signal
source is also necessary. That will yield both the open-loop
gain and the closed-loop gain...which can then be applied to
a standard negative-feedback amplifier formula. Even with a
"cheap" RF signal source, an RF output voltage meter circuit
(even if a 1N34 diode rectifier is used, good to ~ 30 MHz) will
provide a maximum RF output level. Resistor Tee or Pi pads
built on DPDT switches (cheap slide switches work out best due
to least internal inductance) external to the RF generator are
effective although not to the wideband accuracy of the
waveguide-below-cutoff type used in older commercial RF
generators. A sequence of 1, 2, 3, 5, 10, 20, 40 etc db pads
would do well enough. If needs be to make the pads the most
accurate, a spoiler pad of around 10 db at the start of this
chain of pads would insure a good source impedance. While
not of greatest metrology quality, those would be better than
nothing at all.

Note on the above: The RF signal generator meter would
determine the signal level into the attenuator chain. The
chain's output would then be that value minus the total db
of the attenuators switched-in. Making the attenuator-
switch mountings in-line in an outboard long metal box
having 1:2 ratio of width to height will reduce most of the
RF feed-around across switched-in attenuators; if that is
1 x 2 inches it is roughly high C-Band waveguide size and a
maximum of 30 MHz RF input would certainly be below cutoff
frequency of that "waveguide." Attenuation through that
long metal box would be a linear relationship of db v.
length. I did just that with an old Heathkit RF generator
(meter calibration set against lab equipment) and outboard
switched attenuators...until I lucked-out and obtained a
pair of HP 355 step attenuators (wideband to 500 MHz,
easier to use).