Topband: Blocking Dynamic Range, here we agree

[Robert McGwier]

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