[TenTec] Omni6/Omni6+ AGC pop on cw leading edge

[Paul Christensen]

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 at gmail.com>
To: "Discussion of Ten-Tec Equipment" <tentec at 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 at 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 at gmail.com>
>> To: "Discussion of Ten-Tec Equipment" <tentec at 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 at 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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