Re: Adjustment of L34 on a K2

[W0YG, Charlie Summers]

I received a lot of answers but Rick, KE0X, has what seems to me to be the best explanation on how to set L34.  I just tried it and it seems to work.  I want to revisit setting the AGC now but I can see exactly what Rich is talking about.  Here is what he recommends:

Here is a technique I used to adjust L34 - While performing the "I.F.
Amplifier Alignment" section of "Alignment and Test Part II," I  used
Spectrogram (just about any program that displays the received signals
should work).  I had meant to put this hint on  the reflector long ago and
keep forgetting.

Follow the instructions for "I.F. Amplifier Alignment."  When you get to the
last step plug the PC sound card into the Speaker Jack (not Headphone as it
attenuates some high frequencies - but it probably don't make that much
difference).  You will notice some noise spread across the spectrum
displayed. Make sure the XFIL is set for FL-1 OP-1.  Now continue to adjust
L34 for a compromise of the lowest displayed noise and the highest level of
the tone (signal).  You will find a point where the noise is almost non
existent.

Even though I had adjusted this to what I thought was the best by "ear,"
after using this method and then re-adjusting the AGC threshold,  I was
amazed at the difference in sensitivity and audio level (I think the noise
de-senses the AGC - perhaps the experts can explain this).  Maybe this is
one of the problems/reasons some people complain about "low output volume."

Thanks Rich!  I received a lot of direct replies and apparently more than just me are/were confused by this adjustment.

73,

Charlie, W0YG..>>
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[Ron D'Eau Claire-2]

As at least one person noted, the AGC makes it difficult to adjust L34, or
make any adjustment in which you are peaking a signal. The AGC is
automatically "twiddling the RF gain control up and down" constantly as you
work. It's not that noise "desenses" the AGC, but that the AGC cannot tell
the difference between signals and noise. The AGC simply monitors the level
of whatever's coming through the I.F. passband and adjusts the receiver gain
as needed for about the same level in the phones or speaker at all times.
The "threshold" that one sets is the point at which the AGC starts working.
The AGC leaves the RF gain full up below the threshold level. Below the
threshold if all you have in the passband is noise, the noise level is at
full RF volume. When you encounter a signal or a higher noise level that
exceeds the threshold you've set, the AGC senses the higher level in the I.F
passband and turns the gain down so the audio output won't change much. If
you tune across a signal, it turns the gain down farther, trying to keep the
signal level down to about the same as the noise you hear between signals.
What the operator hears is that the noise level seems to drop down when
tuning in a signal. The AGC doesn't maintain an exactly constant output.
Signals and noise in the audio still vary somewhat, but very little compared
to having the AGC off.

Of course the advantage is that the AGC is turning up the noise level so you
can hear very weak signals without your having to adjust the RF gain by
hand, and if you encounter a really strong signal it will turn down the
level so the signal won't sound as loud as it really is.

The disadvantage is that the bands always sound a lot nosier than they
really are! Every time you tune off of the signal and onto a quiet
frequency, the AGC turns up the gain to fill the phones or speaker with
background noise that is much louder than it'd be if the AGC were off.

Many operators like that "action" with lots of noise between signals or the
noise level jumping up in volume whenever a signal stops sending. Many rely
on an "S-Meter" to indicate signal strengths. The S-meter which simply
reports on a bargraph or meter how the AGC control voltage is adjusting the
RF gain. Even so, many avoid using AGC, preferring to hear the signals as
they really are without the noise being artificially increased on quiet
frequencies.

Whether you prefer to have AGC on or off in normal operation, it's important
to be sure the AGC is always off when making adjustments that involve
listening for changes in signal level or signal to noise ratio in the
receiver output.

Ron AC7AC  


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