[Top] [All Lists]

[TenTec] Re: RX-340 and Noise Reduction

To: <tentec@contesting.com>
Subject: [TenTec] Re: RX-340 and Noise Reduction
From: jimr.reid@verizon.net (Jim Reid)
Date: Sun, 17 Feb 2002 11:30:51 -1000

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

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 

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

<Prev in Thread] Current Thread [Next in Thread>