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Re: [TenTec] First DSP IF Corsair II?

To: <tentec@contesting.com>
Subject: Re: [TenTec] First DSP IF Corsair II?
From: "Tommy" <aldermant@alltel.net>
Reply-to: tentec@contesting.com
Date: Sat, 4 Dec 2004 17:53:15 -0500
List-post: <mailto:tentec@contesting.com>
Really NEAT!! Ham radio is alive and well at Duane's QTH. Congrats on the 'playing'...could be very interesting!

Tom - W4BQF

----- Original Message ----- From: "Duane Grotophorst" <n9dg@yahoo.com>
To: <tentec@contesting.com>
Sent: Saturday, December 04, 2004 5:02 PM
Subject: [TenTec] First DSP IF Corsair II?

I don't know if any one else has done this before but
I have created a DSP IF Corsair II, well sort of. The
experimental lash-up that I'm playing with took all of
15 minutes to setup and get going. All that I did was
connect the antenna input of the Flex Radio SDR-1000
through a capacitor/tap to connector #73 of the Xtal
Filter Board (81252). This is the RX output side of
the 9 MHz 1st IF filter of the Corsair II. This first
IF filter then essentially becomes a 2.4 kHz roofing
filter for my little experiment, sort of like an Orion
:).  The SDR-1000 was then tuned to 8.999800 MHz to
hear the IF signal coming through the Corsair II I 9
MHz IF filter. Tuning around the bands is accomplished
with the Corsair II's VFO, the SDR-1000 is run in USB
mode for all bands except 17M which is opposite, this
is necessary for the same reason you must use SB-R to
operate SSB on that band normally. With the SDR-1000
tuned to 8.999800 MHz the frequency display of the
Corsair II is exactly on when the audio pitch of the
RX signal is correct in both receivers. Since I'm only
tapping the 9Mhz signal off of the RX path of the
Corsair II I can actually listen to both the Corsair
II and SDR-1000 simultaneously; so making performance
comparisons is easy.

Some impressions and observations:

1. Much quieter and cleaner recovered audio.
2. Incredibly sharp "IF" filters in the SDR-1000, they
can be set to as little as 10 Hz and you can still
copy CW FB at this bandwidth, and no ringing.
3. Automatic notch filter, the Corsair II did not have
that before, the SDR-1000 now provides it.
4.  The SDR-1000's DSP processing does add ~100 - 200
ms of delay to the audio (pretty much excludes ever
using QSK CW with this configuration).
5. The Corsair II alone does have a slight edge in
absolute sensitivity from the tests I've done so far
(but I haven't calibrated the SDR-1000's settings very
carefully yet).

The SDR-1000 is no slouch for dynamic range all by
itself but coupling it with the RX RF filtering and
low phase noise 1st LO and 2.4 KHz IF filter of the
Corsair II does make for an extremely pleasant
listening experience. By tapping into the first IF
output of the Corsair II I avoid all of the mixer
noise contributions by the additional mixers needed
for the Corsair II's PBT functions. It also bypasses
the Corsair II's product detector and all of its
low-level audio stages and AF filtering (all of which
are rather noisy by today's standards).

Since the Corsair II is essentially running normally
it provides most of the AGC action to prevent
overloading the SDR-1000. Even so with 2-3 roughly
equal and strong CW signals inside the Corsair II's
2.4 IF filter the SDR-1000 still does not suffer any
blocking effects that I can tell from my tuning about
the bands so far (I even tested it a bit during the
160M contest last night). You can easily pick out
pretty weak signals between two strong signals <200 Hz
away, and you also get a panadapter display for a 2.4
KHz slice of the spectrum that is defined by the
Corsair II's 1st IF filter (so I can tell when those
strong signals are there).

All in all a fun little experiment that did not take
long to set up, I will play with it a bit more to see
if it is truly useful from a practical standpoint or
not. Either way there are many other fun things to try
with SDR-1000 that I haven't gotten around to messing
with yet. This experiment has also confirmed my
suspicions about where most of the internal RX noise
of a Corsair II actually comes from.


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