[Elecraft] SDR - K3 - and GUI Interface

[lyle johnson]

>  Brian Lloyd and others have claimed that if you had an SDR radio with knobs 
>and switches you would have a K3.  Yes, to a certain extent that is 
>true...but the K3 and SDR radio as developed by Flex Radio are two different 
>radios because of the technology used.

The FLex architecture uses input lowp[ass filters to suppress harmonic 
respones from the mixer (QSD or ISD depending on the model).  The QSD or ISD 
implements a bandpass filter function of a few hundred kHz width.  The output 
is passed to the ADC and then on to the DSP (done in the PC).  This is 
*effectively* the same as using a "roofing filter" of a few hundred kHz width. 
 You need an incredibly good ADC to cope with all that.

The K3 uses roofing filters from 13 kHz or so down to 200 hz or so.  The ADC 
is thus exposed to far fewer signals than the Flex.  Both radios use similar 
performance ADCs.

When you want to listen to a weak signal and a huge signal (capable of 
over-ranging the ADC) is nearby, the filtering (and linearity) before the ADC 
is going to make the difference.  You either filter the signal before the ADC 
(to maintain gain to hear the weak signal), or you reduce the gain of the 
signal chain before the ADC (to protect the ADC from over-ranging).  In the 
first case, you have a better chance of hearing the weak signal than in the 
second case.

73,

Lyle KK7P