Mark-The symptom that you describe in your e-mail is probably related to the
analog AGC loop in front of the DSP. The A/D converter's dynamic range is
insufficient to cover the full range necessary for an HF communications
receiver, so Ten-Tec added an analog AGC loop in the second IF (which only
has an effect at high signal levels) to provide the overload protection
needed for the A/D. Gain distribution in a receiver with DSP at the IF
differs slightly from an analog design, because the level into the A/D
converter must be high enough to preserve the desired noise characteristics
of the receiver (by minimizing the effects of quantization noise in the ADC)
while at the same time preventing an A/D overload condition that can result
in severe intermodulation distortion.
As Steve mentioned, the front-end filters are relatively wide (about 12 kHz
at the 3 dB points), so any signal within this window above S9+20 to S9+30
or so has the potential to "pump" the analog AGC, which causes a clicking
noise. I noticed this same effect when I reviewed the Pegasus for
Communications Quarterly (although the article actually appeared in last
year's July and August CQ, but that's a long story).
Switching in the attenuator and/or adjusting the RF Gain will solve this
problem in most cases, although that solution may not always be suitable.
Virtually all DSP-based receivers have an analog AGC in front of the A/D
converter. There are some very complex methods that can be employed to
minimize or eliminate the pumping that you noticed, but that's beyond the
price point that the Pegasus was designed to meet.
Of course, it's always possible that the station causing you a problem was
splattering as well.
Scott
N7NB
----- Original Message -----
From: "Mark Erbaugh" <mark@microenh.com>
To: <tentec@contesting.com>
Sent: Sunday, January 28, 2001 19:17
Subject: [TenTec] Interference on Pegasus
>
> Tonight, during the TenTec user's net on 14.267, I noticed some strong
> intermitent crackling interference while listening on my Pegasus. This was
> accompanied with a strong (20+ over 9) S-meter indication.
>
> Upon investigation, this interference coincided exactly with some
> transmissions from WA5VVT who was net control of the Collins net on
14.263.
> It didn't happen every time he transmitted, but when it did happen, it
> corresponded exactly with his syllables. I was listening to 14.263 on
> another receiver.
>
> According to qrz.com, that station is in AR, and I am in Ohio, so it's not
a
> case of a strong local neighbor. I did not seem to notice the interference
> when others were transmitting on 14.263.
>
> While WA5VVT's signal did sound clipped, I did not notice the same degree
of
> interference when listening to 14.267 with my Kenwood TS940. Narrowing the
> filter on the Pegasus didn't seem to help.
>
> Does this indicate a problem with my Pegasus? Did any of the other Pegasi
on
> the net experience the same interference?
>
> 73,
> Mark, N8ME
>
>
> --
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> Problems: owner-tentec@contesting.com
>
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