I wouldn't want anyone to get the impression mixer noise is
less or non-existent on a SDR!!
All mixers generate noise, and the typical mixer (once we
move away from 6BE6 converter tubes) is so far below noise
floor on 160 the mixer makes virtually no contribution to
noise floor at all.
In diode or other passive mixers, noise figure is almost
always established by conversion loss and the noise figure
of the stage following the mixer. A typical passive diode
mixer and 5dB NF stage following the mixer makes noise from
our antennas firmly set the noise floor of the receiving
system. If not, we only need add a 5dB noise figure amp of
enough gain and a screwdriver blade antenna would set the
noise floor at most locations.
We heard reports (or excuses) that one contester who showed
up regularly in "packet pileups" was using a SD Radio to
monitor the band, so W8XR brought his Flexradio SDR1000 and
Firewire sound system down from Ohio. W8XR set up away from
single operator N5OT (no cheating allowed) and tapped into
my rearmost receiving antenna. This antenna is 3000 feet
away from the transmitting antenna field and allows duplex
operation at frequency spacings as close as a few kilohertz
with FT-1000 MKV transmitters and receivers.
Our goal was to see if there was any substance to claims a
pileup could be spotted on an SDR frequency display.
We found the SDR useless in the presence of the transmitter
despite the Yaesu's and Drakes being very useful. The
reasons are simple to understand.
1.) There isn't any magic, the SDR is nothing more than a
mixer with a system noise figure higher than the noise
figure of our standard receivers. While our other receivers
made -130 to
-135dBm sensitivity at 250Hz bandwidth without preamp, the
SDR was about -106dBm without preamp and -135.9 with preamp.
2.) We measured a best average 20kHz IM3 DR with preamp off
of 85dB. This is actually worse than the Yaesu performance,
and far worse then the heavily modified Drake receivers.
3.) We measured a best average 2kHz IM3 DR with preamp on
of 84.6dB. The IM3 DR was very non-symmetrical above and
below the test oscillators, indicating distortion in
multiple stages. The upper IM DR was only 76.2dB, the lower
was 93.1 dB. This is ballpark for other communications
receivers, but not as good as modified conventional systems
or a stock Ten Tec Orion.
4.) 20kHz blocking dynamic range was 98.7dB, again being
best with the preamp on. This is because the sensitivity and
noise figure was the limiting factor without the preamp.
5.) The display had odd artifacts when subjected to very
strong signals. It generated a large number of strong
spurious "pips" from our transmitter that weren't really
there. If we clicked on them there was no signal.
Our conclusion was in a contest environment the SDR at its
present state of development needs a conventional mixer and
roofing filter. Since it has no narrow pre-mixer selectivity
all signals from the BC band up impinge on the mixer and all
stages following the mixer and rob later stages of dynamic
range. While a normal receiver only has to process filtered
signals through multiple stages, the SDR depends on later
stages processing and sorting hundreds of strong signals.
We found the display totally useless for spotting "packet
pileups". The receiver was totally destroyed at any spacing
by our own transmitter while the Yaesu and Drakes were
largely unaffected at a few kHz or more spacing.
This isn't to say there were some very positive aspects of
the SDR. As an example on an almost empty band the operator
could "click and jump" to signals to see if they were worked
before. No more tuning across empty frequencies. On an
almost empty range of frequencies it is very easy to spot a
new signal popping up and check out who it is. The SDR
shined when the band wasn't loaded with strong signals, but
was not very useful on a crowded band. It totally fell apart
when the local transmitter came on at any signal spacing.
The difference in strong signal density and local noise
floor almost certainly the key to the differences in
opinions. Under conditions of low strong signal density and
high local noise floor, the SDR shines. Decrease local
noise floor and pack in the multiple strong signals (even at
very wide spacing) and the picture changes.
IMO the best combination is exactly what Ten Tec does, a
narrow roofing filter with a good DSP system.
73 Tom
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