Jim,
In my 6 month experience with the RX340, I have never had a need
to lower the RF gain control. On SSB and CW signals, the AGC
of the RX340, when set to the slow setting, is perfect. If there are
no signals at all on a quiet band, I want all the gain I can get to hear the
weakest possible signal. As soon as any signal is heard, the AGC
automatically adjust the gain to just the correct level. Whether this
gain adjustment is done in DSP or analog, the end effect is it is
same. This receiver's AGC works as well as any I have ever seen
and better than most. I see no reason to use manual RF gain.
One comment that I have heard a lot that really bugs me is people
making the comment about what a quiet receiver a particular rig
might be. My experience has been any receiver that is quiet is
deficient in total gain. Any radio should be capable of producing
lots of white noise on a dead band if it has ample gain. It's only
when it can't produce white noise that I suspect it is a problem.
Carl Moreschi N4PY
Franklinton, NC
----- Original Message -----
From: "Jim Reid" <jimr.reid@verizon.net>
To: "Carl Moreschi" <n4py@earthlink.net>; "tentec" <tentec@contesting.com>
Cc: <kh7t@arrl.net>
Sent: Sunday, February 17, 2002 9:30 PM
Subject: Re: [TenTec] Re: RX-340 and Noise Reduction
> Hi,
>
> Yesterday I wrote:
>
> "I suspect the Orion will similarly have a rich set of DSP filters.
> And probably nary a crystal filter."
>
> Wrong! John Buck, KH7T pointed out that the 1st IF of both
> the Pegasus and the RX-340 is at 45.455 mHz. Something
> has to be done to eliminate the "birdies" which would be
> generated by the 1st mixer spurious products and the 2nd
> mixer image at 910 kHz offset. So I went looking in the
> 340 manual again. Sure enough, there are two crystal
> filters: a pair of 2 pole crystal filters do the job in the 340,
> and probably also in the Pegasus. (Note: the Pegasus
> manual is completely unlike any previous Ten Tec
> radio manual I have had -- absolutely NO circuit
> details, diagrams, etc. at all!? Wonder why. The 340
> manual is chock full of details: pc card layouts, alignment
> and elaborate test instructions, but nary a word about
> any of it in the Pegasus book. Is there a separate
> Pegasus Service Book available??). I probably wouldn't
> touch a thing anyway; don't like surface mount component
> fiddling, hi.
>
> Therefore, I suspect the Orion will also have a 45.455 mHz
> IF (note the 455 part which is the 2nd IF frequency), and
> will have the same needed pair of crystal filters, one on
> the input, the other on the output of some gain stages within
> the 455 kHz IF strip. These filters are 16 kHz wide in the 340;
> may be only 8 kHz in the Pegasus.
>
> The "main" rcvr within the Orion is said to be only amateur
> bands, so it probably only needs to be 8 kHz wide; however,
> the "sub" or dual rcvr of he Orion will be general coverage,
> wonder if that one might have a wider IF, as in the 340 for
> more "hi-fi" international broadcast reception, synchronized
> AM?
>
> Will sure be interesting to see what comes within the Orion,
> won't it?
>
> A bit more, also from KH7T which might explain the high gain
> within these DSP core rigs. A quote from one of John's posts
> to me (trust he doesn't mind my giving it out, hi):
>
> "My limited DSP work would suggest that the gain of the amplifiers will
> be adjusted to keep the DSP digital sample memories as near full scale
> as you can without any overflow. Therefore the gain will be greatly
> increased when there are no strong signals in the passband. So you have
> to manually limit the gain to prevent the effect that we have been
> discussing. That is ok because you cannot use stuff below the noise
floor.
>
> Now if you were using additional DSP to dig out correlated stuff below
> the noise level then you might use more gain and your receiver[the 340]
> would be superb for this purpose. Note Ed Hare's comments in
> QRP-L about trying for dsp enhanced 10 micro watt trans continental
> work.
>
> John KH7T"
>
> And that high noise when " the gain will be greatly
> increased when there are no strong signals in the passband.
> So you have to manually limit the gain to prevent the effect that
> we have been discussing."
>
> My experience as noted in previous posts exactly! So turn
> down the RF gain of your Pegasus/RX-340, you won't miss
> a signal so long as you can still just here the antenna noise
> contribution when switching the antenna ON/OFF as you
> cut the gain. Found I was cutting the gain of the IF strip
> by 65 dB last evening on 40 meters. Result was a nearly dead
> quiet rcvr, but signals just appeared from the speaker when
> tuned across, very interesting effect.
>
> 73, Jim KH7M
>
>
>
>
>
>
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