[TenTec] AGC thump

[WA3FIY]

I have been doing some work on this matter for a while and 
have identified two different issues.   Thump and Pop.

As you say Paul, the thump occurs on keying.   As I recall, it 
occurs upon opening the key.   I believe it is the result of a 
small DC offset that occurs in the muting circuit when the rig 
is keyed.   When it is un-keyed, the DC offset gets passed to 
the audio PA stage and is heard as a thump.   Since it is 
between the volume control and the amp, it is independent of 
volume and RF gain settings.   If you use a speaker or 
headphones with good low frequency response, the thump will 
be worse.   This is place where "Hi-Fi" is not so great!     :-)

The Pop is a function of AGC action on receive and occurs when 
a strong signal follows a weak signal or after the AGC hang 
timer has reset.  My tests indicate it is the result of the AGC 
control voltage being delayed while the AGC capacitor is being 
charged.   Briefly, the AGC  circuit is comprised of a full wave 
rectifier in the audio chain following the notch filter.  BTW, 
the notch filter is the only filter inside the AGC loop.   Anyway, 
the full wave rectifier is followed by a 3.3K resistor feeding a 
1uf AGC capacitor.   That in turn feeds an IC and transistor 
buffer which drives the AGC PIN diodes.

Assuming the AGC is reset, when a new signal comes along, the 
AGC capacitor begins to charge.   BUT, it takes a certain 
amount of time for the capacitor to charge, that time being a 
function of the R/C time constant and amount of driving signal.  
In the mean time, the IF amplifier is running at full or nearly 
full gain, driving the 'phones off your head until the AGC 
capacitor becomes charged!    :-)  

In a sense, this R/C circuit resembles an integrator, step 
voltage in [rectified incoming signal] and ramp voltage out [AGC 
control voltage].    Reducing the time constant does reduce the 
pop to a certain extent but it also produces some undesirable 
side effects which ruin the other wise very nice AGC in the 
Corsair.   I won't go into all of them but one especially 
troubling one is that the AGC will capture and hold very short 
noise bursts, causing the desired signal to diminish or 
sometimes disappear until the hang timer resets.

I decided to try a complimentary circuit, the differentiator.   
Without going into too much boring detail, what I have is a 
differentiator that acts in parallel with the circuit described 
above.   It works like this.   Assuming the AGC is reset and a 
signal comes along, the differentiator IMMEDIATELY applies 
AGC control voltage to the PIN diodes reducing the gain and 
virtually eliminating the pop...   In the mean time, the AGC 
capacitor is being charged.   When it reaches the appropriate 
charge for the incoming signal, the regular Corsair AGC circuit 
takes control and the differentiator resets........ready for 
action the next time needed.     This takes place during the 
first few milliseconds to a few tens of milliseconds of incoming 
signal.   The balance of the time the standard Corsair AGC 
circuit is in control.   

I find it to be very smooth and pleasant.   An interesting side 
effect is that subjectively speaking, I think the radio is less 
susceptible to noise and other very short duration spikes.   The 
QSK action is not effected in any way.

If this sounds a little bit like a commercial, I guess maybe it is.   
I am planning to offer the "mod" as a kit from Radio 
Adventures if there is any interest.    The mod involves tack 
soldering three wires to points on the component side of the  
IF/Audio board, and mounting a small circuit board using an 
existing hole.   This makes a quick, easy and fully reversible 
modification.    I have tested this on two different Corsair II's 
with similar results.   However, I would really like to find two 
or three people who would be willing to try it out.   Anyone 
interested can email me direct at:    lee at radioadv.com


73 de Lee, WA3FIY

On 12 Mar 2003 at 8:35, Paul DeWitte K9OT wrote:

> Having read several references to AGC thump, I am not sure what it
> sounds like. My Corsair II on transmit will have a thump on keying. If
> I turn down the AF gain or the RF gain the noise will dissappear. Is
> this what is refered to as AGC thump? My Omni D has none of this.
> Smoothest cw rig that I have run yet. If this is what is referred to
> as AGC thump, do any of the mods that are around get rid of the thump
> and still retain 100% of the breakin? Maybe I have an entirely
> different problem? I can live with it but would be nice to get rid of
> the thump. Thanks 73 Paul K9OT
> 
> 
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-- 
73 de Lee,  WA3FIY
wa3fiy at radioadv.com