[hpsdr] I wonder how HPSDR will affect this?

[Jim Miller]

----- Original Message ----- 
From: "rightstuff4u2" <n0fp at highmarks.com>
To: <softrock40 at yahoogroups.com>
Sent: Tuesday, February 13, 2007 10:37 AM
Subject: [softrock40] Topband reflector posting on SDR


This came through the Topband reflector today.  It represents some
interesting measurements of SDR performance by a respected
engineer...Tom-W8JI.  I am cross posting this without permission,
which should be OK...

*********************************************************

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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