[Elecraft] Comparison: K3 and SDR 5000A

[Joe Subich, W4TV]

> 2a) In a quadrature sampling detector (or integrating sampling 
> detector) architecture, used by the SDR-1000/5000 series, the 
> ADC is exposed to all the signals in the detector passband, 
> typically more than 200 kHz wide even if the displayed passband 
> is less.

This is key in relation to IMDDR (mixing and spurious generation). 
Any signal handling circuits must be able to handle the total 
peak power of all signals within that 20) KHz window ... not just 
the average power.  If the amplifiers/mixers cannot handle the 
"sum of the peaks," there is a real chance that signals well off 
frequency will generate IMD products on the operating frequency 
and the IMDDR measured by a two tone process (Sherwood, ARRL, etc.)
will be significantly optimistic with a receiver design like the 
SDR-1000/FLEX-5000 as it only measures third order (2F1-F2, 2F2-F1) 
products and fails to consider fifth and higher order products 
that result from three, four and more strong interfering signals. 

This is a major issue on 160 meters where there may be several 
strong AM broadcast signals just below the band or on 40 meters 
(particularly in Europe) with multiple strong broadcasters IN 
the band.  It's also an issue on 80/75 meters where there can 
be multiple VERY STRONG "local" signals within 50 - 100 KHz 
of the operating frequency.  

With a "roofing filter" design, mixers and amplifiers (other 
than a preamplifier) need to contend with strong signals only 
within +/- 2.5 KHz (2.7 x 1.6 / 2).  The number of potentially 
interfering signals are reduced significantly with the roofing 
filter design. 

73, 

   ... Joe, W4TV