[Elecraft] K3 vs. HPSDR Mercury receiver

[Joe Subich, W4TV]

> Next generation transceivers will not use crystal filters 
> and mixers, they are already appearing and the performance 
> is outstanding.

If the next generation of transceivers does in fact abandon 
narrow filtering ahead of the ADC it will be a sorry step 
backward.  While some of the designs like the Flex-5000 and 
Peresus have excellent specs for the most part when tested 
against two tone situations, they are still substandard in 
dealing with multiple very strong in-band signals as would 
occur on 160/80/75 meters in a contest or on 40 meters with 
the multiple megapower broadcasters. 

To maintain linearity, an ADC must be able to withstand the 
instantaneous peak signal (vector sum - not average) from 
all the signals within its passband.  For some DSR designs 
that maintain bandwidths from several hundred KHz to 10s 
of MHz that peak signal handling capability requirement 
can be significant and represents a required dynamic range 
orders of magnitude greater than even the best current 
technology.  It is the multi-tone interference that is best 
handled by narrow filtering ahead of the ADC (e.g., the K3 
design).  

73, 

   ... Joe, W4TV 
 






> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Dave G4AON
> Sent: Saturday, November 08, 2008 6:38 AM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 vs. HPSDR Mercury receiver
> 
> 
> The explanation from the article is as follows:
> 
> "When noise pulses/spikes pass through a crystal
> filter, the phase response of the filter
> changes, depending on the noise frequency.
> However, when noise pulses/spikes pass
> through an ADC with a linear response, the
> phase response stays the same, because the
> ADC treats them in a linear manner."
> 
> While the K3 is "SDR", it also includes crystal roofing filters which 
> the Mercury does not. My Perseus SDR (which also operates with a high 
> speed ADC for direct decoding - no crystals or mixers) does deal with 
> noisy signals better than my K3.
> 
> Next generation transceivers will not use crystal filters and mixers, 
> they are already appearing and the performance is outstanding.
> 
> 73 Dave, G4AON
> K3/100 #80
> -----------------------------------------
>  >/ SDR where the authors compare a K3 with a HPSDR Mercury 
> receiver and />/ explain why the SDR sounds better than the 
> K3 (noise through a crystal />/ filter causing phase 
> changes). / The K3 is also an SDR!
> 
> -- 
> David Woolley
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