K3 and very strong signals

[Dean Straw]

Back in the "good old days" in the late 1960s at National Radio Company, my
first employer after I graduated from college, we had several quick-check
tests for AGC performance. Each used an AM-modulated signal generator
(usually the venerable HP-606A -- remember those lovely, heavy old beasts?).
A "quick-check" to characterize how well the AGC worked might take five
minutes.

1. AGC hold-level dynamic-range test: The HP-606A was set up for 50% AM
modulation at 400 Hz. The receiver being tested was set up for SSB reception
(using a 2.7 or 2.4 kHz SSB filter) and the receiver was carefully tuned so
that we could hear the tones resulting from both the carrier and one
sideband coming through the SSB filter.

Then the generator's output level was increased in 10-dB steps while the
audio was monitored by loudspeaker. When the input level just started to
exceed the dynamic range of the AGC system, the sound of the audio became
first "mushy" sounding (i.e., the audio output IMD started rising quickly).
In extreme cases the 400-Hz sideband actually disappeared as the system went
into hard clipping mode (think FM capture ratio).

By the way, how the audio output sounded during the quasi "two-tone test" in
SSB mode was a quick check on how good the receiver sounded without having
to actually do a full-fledged on-channel IMD test using a spectrum analyzer.
It turns out that the human ear is able to detect harmonic distortion and
IMD pretty well.

Most receivers started to "fold" at input levels approaching a volt of RF,
and the point where the S Meter first started to rise (what we used to call
AGC threshold level) was usually about 1 microvolt. Thus, the effective AGC
range was 120 dB, from 1 uV to 1 V.

This AGC hold-level dynamic-range test was also done using the AM mode in
the receiver. Of course, we used good old diode envelope detectors in those
days, not DSP algorithms!

2. If we were getting fancy, we might put a scope or an audio voltmeter at
the receiver's output to monitor how much the audio level rose from 1 uV to
1 V of RF input.

3. Another very quick AGC test was to get a quick feel for how stable the
AGC loop was. We would turn off the modulation on the generator and tune it
so that the frequency was at the edge of the passband of the SSB filter.
Then we'd crank up the RF level and check to see if the AGC loop remained
stable without going into massive "whoop-whoop" type motorboating while the
generator frequency was slowly varied in and out of the passband. This sort
of servo-loop instability could occur because the group-delay of the SSB
filter quickly changes near the passband edges for filters with steep
skirts.

4. Then we'd do a quick check of AGC-attack characteristics. At a level of
about 60 dB above the AGC threshold point, we'd pull the BNC connector at
the output of the HP-606A signal generator and then quickly re-insert it,
while listening to the audio output. The size and duration of the initial
"thump" where the audio would rise before the AGC could act to reduce the
gain was a tradeoff between the attack time and the loop stability measured
in test 3. above.

You can learn a lot about a receiver's AGC by spending five minutes with a
signal generator. You may ask why I haven't done these tests myself at home.
My K3 is out at N6RO's multi-multi station about 50 miles east of my home in
San Francisco, but maybe Wayne could do these quick-and-dirty tests there at
his lab.

73, Dean, N6BV

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