You should be able to change the 270-ohm resistor to find a more suitable
attack time. You don't want attack *too fast.* Then, the AGC starts
becoming overly aggressive with every micro-burst of electrical impulse
noise. This is a huge problem with some DSP-based transceivers like the
Icom '7700 and '7800. Rob Sherwood has discussed the effect in his
presentations. I had both Icoms here and the AGC response to noise was very
frustrating.
Paul, W9AC
----- Original Message -----
From: "Barry N1EU" <barry.n1eu@gmail.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Tuesday, December 13, 2011 9:04 AM
Subject: Re: [TenTec] Omni6/Omni6+ AGC pop on cw leading edge
Paul, the Omni 6 circuit is exactly as you describe, with a 270 ohm
charge capacitor and a 330K discharge capacitor. It uses a 3.3uF
electrolytic capacitor in fast mode which is paralleled with a 10uF
capacitor in slow mode.
Tnx/73,
Barry N1EU
On Tue, Dec 13, 2011 at 8:58 AM, Paul Christensen <w9ac@arrl.net> wrote:
Barry,
Possibly leakage with age. I heard the recording, but it sounds like the
AGC attack time is a bit too long. Most conventional AGC circuits sample
the I.F. with a small signal diode followed by a series resistor to
charge a
parallel cap. The attack time is set by the time constant of the R and C
components. What about the release time? A second resistor is added in
parallel with C, producing a value that decays the discharge but does not
alter the charge time. It's the diode that prevents discharge through the
attack time resistor.
It's been a long time since I've seen the Omni Six AGC, but look for
components around the 3.3 uF cap that show this charge/discharge
relationship. My guess is that if C replacement doesn't do the trick,
then
you will be able to find a value of series R to shorten the attack time.
Paul, W9AC
----- Original Message -----
From: "Barry N1EU" <barry.n1eu@gmail.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Tuesday, December 13, 2011 8:35 AM
Subject: Re: [TenTec] Omni6/Omni6+ AGC pop on cw leading edge
Is it possible that I'm seeing AGC popping because the capacitors are
aging?
I'm thinking of replacing the 14-year old 3.3uF electrolytic AGC
timing capacitor with a new tantalum capacitor of equal or slightly
smaller value. Comments?
Tnx/73, Barry N1EU
On Sun, Dec 11, 2011 at 4:00 PM, Barry N1EU <barry.n1eu@gmail.com>
wrote:
I notice up to a +10dB pop on the leading edge of cw elements with my
Omni 6+. Please see a recorded waveform at
http://n1eu.com/omni6agc.gif
and the accompanying 20sec recording is at
http://n1eu.com/omni6agc.wav
It's clear that when there's been no signal for a second or so, the
big 10dB pop is much more likely. Otherwise there's a "little" pop of
3-5dB. I'm not seeing much in the archives but has anyone tried to
tame the "pop" via AGC time constants etc? Can anyone contribute a
little technical analysis?
I see little difference in the popping with fast vs slow AGC.
Thanks & 73,
Barry N1EU
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