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[TenTec] RX-340 and Noise Reduction

To: <tentec@contesting.com>
Subject: [TenTec] RX-340 and Noise Reduction
From: wa3fiy@radioadv.com (WA3FIY)
Date: Thu, 14 Feb 2002 15:34:03 -0500
On 14 Feb 2002 at 9:13, Jim Reid wrote:

> Hi again,

> Anyway,  have learned  a bit more about the audio noise
> level from the 340 -- the amplitude is AGC dependent!
> With the 599zx I think I will be able to measure the actual
> audio amplitudes,  I hope also of the noise.  In FAST agc,
> the noise output is "considerably" louder than in SLOW,
> and a bit louder in MEDIUM agc.  Carl,  N4PY has an 
> explanation as follows:
> " At slow agc, the gain will set itself to the peaks of the random
> noise.  At fast agc, the gain is constantly varying at a very fast
> rate giving a higher average gain value and hence more noise."
> Well,  I never noticed this with my FT-1000D which had fast,
> med,  and slow agc.  But the 340 AGC is DSP generated,  not
> just feedback from the product detector to the IF stages.
I don't think this has anything to do with AGC being DSP 
generated.  I can't imagine any other response than what you 
have just described about the '340.  I have the same or similar 
characteristic on many different vintages of RX from various 
manufacturers.   When the AGC is set to "fast", the RX is 
returned to full or near full gain quickly in the absence of 
signal, whereas with "slow" AGC, the gain is more or less 
constant for a second or more after the signal drops, hence 
holding the background noise down.   Just as Carl says.  The 
effect,at least to me, is that in fast AGC, the RX is noisy and 
the demodulated output is somewhat compressed.  At slow 
AGC, the RX is quiet and the audio is pretty clean [not 
compressed].   I use slow AGCevery time I can and go to fast 
kicking and screeming or on digital modes where the computer 
has to listen to the racket, not me.    :-)

> The companion Pegasus in this system does have DSP noise
> reduction,  which is spec'd to be about 15 dB;  it works,  but
> causes distortion to SSB signals.  The FAST agc setting of the
> 340 makes SSB copy very unpleasant;  huge noise between
> every word spoken on low level signals.  I have checked this
> on both SSB and CW modes and across the  IF bandwidth
> selection options available in the 340.  Once the BW is below
> about 300Hz or so,  the impact of the FAST agc to noticeably
> increase the noise amplitude is down.

Whenever I have to use fast AGC, I use the RF gain control to 
reduce the background noise as much as possible.

> What I hope to yield from the zx with the 340 is:



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