[hpsdr] Blocking Dynamic Range of uWSDR Receivers

[Chris Bartram]

Grant

> Has anybody got any detailed lab. measurements of (for example) an
> SDR-1000 with Janus?  I believe that there are some lab measurements of
> the SDR1K w/Delta-44; I wonder if something similar has been done for
> HPSDR.
>
> A question arose at Microwave Update about the Blocking Dynamic Range of
> the uWSDR project, which uses the same ADC as on Janus albeit with a
> different RF front-end.  I'm not a great fan of this parameter; I think
> spurious-free dynamic range is more meaningful but as it was mentioned I
> thought I would ask the question here - I think it would be limited by
> the ADC, rather than the RF, and I haven't simulated BDR for the uWSDR.

It's quite important to define what you mean by blocking dynamic range. It 
could be wrt gain compression in the analogue signal path, reciprocal mixing 
or the ADC going out of range... Each will have different characteristics.

The BDR of most practical microwave SDR quadrature receiver systems of the 
kind we're developing in the uWSDR group will be frequency dependent, defined 
by the phase noise of the LO, and the bandwidth of any post-downconverter 
filtering. The ADC BDR is only really significant within the bandwidth of the 
post-downconverter filtering. 

In many amateur VHF/UHF contest situations - which can often be much tougher 
to deal with than at HF, as the noise floor is very much lower above 50MHz 
and all of the blocking measurements tend to shifted downwards - gain 
compression blocking can actually be the most significant parameter, as there 
is often only one really strong major interferer to worry about, (unless you 
live in IO91, JO33 etc. or the North American QRM Alley equivalents!)  So, IM 
often isn't a first order problem, but blocking can be.  But, it's a horses 
for courses situation. In practice it's important to design for excellent 
performance in all its aspects, and not to favour one measure over another.

The same is also true of conventional, transverter based receiver systems, and 
of many transceivers, of course. In practice transverters - unless they are 
_very_ carefully set-up - tend to lead to quite poor dynamic range using any 
of the usual measures. 

At lower frequencies, the preliminary analyses of my slowly developing 
50/144/432/1296 receiver front-end design for the uWSDR group tends to 
suggest, as we discussed on the uWSDR group a few weeks ago, that the 
performance outside the post downconverter filter should be rather better 
than most, if not all, current commercial amateur radio vhf/uhf transceivers.  
It should also a few dB better than, for instance, an FT225 with muTek front 
end, which after 25years is still likely to be better than anything currently 
available on the amateur market. I may be biased when I say that though!

Vy 73

Chris
GW4DGU
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