Where I grew up in Alabama with a Methodist preacher father, we were too
poor to have red clay dirt roads so that would have been the lap of
luxury. ;-).
Blocking dynamic range is an often touted figure of merit and it is an
indicator of something and in our argument, it does matter. IP3 or DR3
are both good figures of merit to describe the analog front-end of a
receiver because they are both related to the fundamental transfer
equation of the receive system. Two tone tests for the SDR-1000 show it
performs better with near in signals than anything I have ever seen SO
LONG AS THE A/D is not saturated. However, A/D converters used in
digital radios are not well described by the familiar formulas. If
multiple strong signals are allowed to enter the A/D converter, the
relevant figure of merit is the A/D saturation limit. Below this limit,
A/D converters are typically very linear – they produce practically no
IM3 all the way up to the saturation limit where they fail completely. A
digital receiver will have IP3 and DR3 values that look very good, but
in real life they can not be compared to “good old analog” receivers
that will continue to function with input voltages high above the range
indicated by DR3. For example, an analog receiver that is subjected to
10 signals at the DR3 level will produce IM3 for all combinations of
these signals, but these spurious responses will be near the noise
floor. A digital receiver will see the 10 signals sometimes add in
amplitude to give a peak level that is 20 dB higher, so it may become
heavily saturated and useless. At the present state of the art, digital
receivers need roofing filters to limit the number of signals entering
the A/D converter, but inside the roofing filter bandwidth they remain
vulnerable to saturation effects when the band is crowded with very
strong signals. Test procedures must take realistic account of this and
they typically do not. The SDR-1000 DOES have a roofing filter. The
sampling detector is a mixer and roofing filter in one circuit. The
wider that roofing filter is, the greater the likelihood that you will
have N signals add up and saturate the A/D.
I think perhaps this is what Tom was attempting to say or SHOULD HAVE
SAID and if so, we agree that the 100 dB BDR versus 150 dB BDR for the
FT1000D is a thing. My disagreement with the published numbers is that I
have never measured one of these radios with numbers that were as poor
as given here EVER. He attributed things to mixer noise, etc. and we
disagree with the mechanism. The measurements done by me or witnessed by
me were done in professional laboratories with state of the art
equipment and I am biased to believe what I am absolutely certain have
been repeated by world class professionals.
It is my OPINION that it is much more important to have good response to
close in signals on crowded bands in typical operations than to have a +
(few) dBm signal block your front end and AT THE SAME TIME have a 0 or
even NEGATIVE IP3 well inside the roofing filter such as the typical
analog receivers have, even the $13000 ones. To put it mildly, the
situation is complicated and more so than has been discussed here. There
is not one radio I know outside of the SDR-1000 that will perform as
well as it does in most conditions inside the roofing filter of the
OTHER radio. The SDR-1000 would definitely have its noise floor raised
in a super strong signal environment such as nearby transmitter in a
contest station but that is NOT caused by "mixer noise". It is caused by
clipping the A/D on peaks of the envelope of the I/Q signal being
presented to the A/D pair. So here Tom and I disagree. I have excised
the signals from the SDR-1000 mixer and measured them. The A/D after it
is without a single exception I can think of the primary limit on its
performance. The 26 dB of gain following that mixer needed to overcome
the noise figure of the early A/D's on most sound cards is a serious
factor here because of the occasional clip it causes.
To say that the system would be improved by better A/D's is clearly
obvious. I am personally anxious to test the next generation of this
mixer in the new radio Flex is bringing out and in the HPSDR circuits
that have been proposed because the A/D in them or proposed to be in
them has a MUCH better noise figure than any I have used and the need
for a 26 dB gain after the mixer being goine should lead directly to an
improvement in the BDR. HPSDR is about to release its Janus board which
is essentially a sound card replacement dedicated to SDR. We should be
able to bring out balanced signals from the SDR-1000 and bypass the
INA's which do the 26 dB gain and see a big improvement of the overall
system.
At that time, Tom and I can have the argument again with better toys.
Let's settle this by saying we agree there is an issue. Tom believes it
is in the mixer and it is unsurmountable. I do not. I believed it has
next to nothing to do with the mixer but is in the BDR limits imposed by
the A/D systems. The Janus board and some measurements should easily
tell us who is right.
Bob
N4HY
--
AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
"Taking fun as simply fun and earnestness in earnest shows
how thoroughly thou none of the two discernest." - Piet Hine
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