[TenTec] Carl, Pmni, 756 etc

[N4LQ@igLou.com (Steve Ellington)]

The PSK crowd really gets upset when I tell them I'm using a 500hz filter.
They say stuff like...."You need to keep the filters open wide so you can
see all the stations on your waterfall." Weird Science!

Steve
N4LQ

----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "tentec reflector" <tentec@contesting.com>; "Carl Moreschi"
<n4py@earthlink.net>
Sent: Sunday, March 17, 2002 2:06 AM
Subject: Re: [TenTec] Carl, Pmni, 756 etc


> Hi Carl,
>
> > Let me describe the main receive characteristics of the rx340.  I do
> > not consider the 10 khz dynamic range specs to be meaningless.  If you
> > have ever run in field day with lots of transmitters in the near
> > field, one of the most challenging things for a receiver to do is
> > operate on SSB with another transmitter on CW on the same band or
> > viceversa.  Most receivers fail miserably at this.
>
> That actually is one of the least challenging applications, and has
> little to do with how the receiver works with close signal spacing.
>
> Only the stages in front of the roofing filter are involved in wide-split
> overload. If we look at manufacturer and old ARRL tests and
> reviews, we see they all like to measure outside the roofing filter
> passband with 10-20kHz spacing or wider. Almost anything looks
> good at wide spacing.
>
> > That said, if your are inside the roofing filter, you are obviously
> > limited to the dynamic range of the DSP chip.  This is 40 db.  So for
> > signals that are closer than 10 khz to the desired signal, 40 db is
> > all the dynamic range you have.  When an undesired signal is within 10
> > khz of the desired signal and stronger than 40 db above the desired
> > signal, the analog AGC will come up some to keep the DSP processor
> > from overloading.  This is done as best as it can be by the rx340
> > hardware and I find I rarely get into this situation.
>
> I don't know what the selectivity ahead of the DSP is.
>
> If the blocking or IM DR of the DSP system is 40 dB and if the
> receiver **can't** use a 500Hz or narrower roofing filter on CW or a
> 2.5kHz roofing filter on SSB, the receiver would offer poor
> performance in many real-world situations.
>
> It might be fine for casual SSB use (wide filters) in urban or some
> suburban locations, but stay away from weak-signal CW on the low
> bands, especially if there are multiple signals in the passband of
> the roofing filter!
>
> As a matter of fact, that DR is about the performance of a typical
> PSK system. You should hear the PSK folks whine and bellyache
> about overload when a high power (100 watt transmitter) signal
> comes on a kHz from them!
> 73, Tom W8JI
> W8JI@contesting.com
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