[Elecraft] K3 & ADAT ADT-200A by HB9CBU

[Joe Subich, W4TV]

> Moreover, that is not to say that the QSD guys can't put a "roofing"  
> filter between their quadrature mixer and the A/D converters (just  
> that they are not doing it today) to get even better BDR than 
> they are getting today.
> 
> This QSD "roofing filter" would just be a matched low-IMD LPF 
> pair for the I and Q channels.  A 500 Hz "roofing filter" would 
> reduce unwanted large signals from clipping the A/D converter 
> and can raise the 2 kc separation BDR of the 500 Hz receiving 
> system to something that is just a function of the dynamic range 
> of the QSD itself.

Unfortunately, as soon as they put matched 3 KHz or 5 KHz LPFs 
between the quadrature detector and the A/D converters they 
lose panoramic operation and have to go back to a knob or some 
kind of scroll tuning instead of "click on the blip."  That is 
unless they do two sets of detectors ... a low dynamic range 
ADC with no filters for panoramic tuning and a high performance 
ADC with "roofing filters" (perhaps even selected by mode) for 
high dynamic range single signal use.  Hardware AGC, if needed, 
could be derived from the high performance decoder. 

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Kok Chen
> Sent: Thursday, December 04, 2008 5:21 PM
> To: Elecraft Reflector
> Subject: Re: [Elecraft] K3 & ADAT ADT-200A by HB9CBU
> 
> 
> 
> On Dec 4, 2008, at 1:06 PM, Bill Tippett wrote:
> 
> > This looks like a much better system than Flex's QSD.
> 
> For single signal use, I agree completely Bill.  By the time you've  
> matched the sampling rate of a direct SDR to a signal with 500 Hz  
> bandwidth, you have adequate dynamic range. (And not need the  
> expensive Asahi codec that is used in the Flex 5000, to boot.)
> 
> However, the QSD based SDRs still has a dynamic range advantage when  
> you want to demodulate (think CW skimmer for RTTY) multiple 
> signals in  
> a wider bandwidth, say 50 kHz.
> 
> With added complexity, you can of course implement multi-rate filter  
> banks with direct SDR and stitch the result back together to 
> create a  
> wider bandpass.  Toby had earlier suggested multiple direct 
> SDRs, and  
> polyphase and multi-rate decimation is just one way to do it (i.e.,  
> use the same RF chip, the rest is "just software" HI).  We can leap  
> frog one another until the cows come home.
> 
> Moreover, that is not to say that the QSD guys can't put a "roofing"  
> filter between their quadrature mixer and the A/D converters (just  
> that they are not doing it today) to get even better BDR than 
> they are  
> getting today.
> 
> This QSD "roofing filter" would just be a matched low-IMD LPF 
> pair for  
> the I and Q channels.  A 500 Hz "roofing filter" would reduce 
> unwanted  
> large signals from clipping the A/D converter and can raise the 2 kc  
> separation BDR of the 500 Hz receiving system to something that is  
> just a function of the dynamic range of the QSD itself.
> 
> It is funny to imagine a "roofing filter" that is centered at 
> DC, but  
> its function is no different from the crystal roofing filters in  
> superhets.  Instead of preventing out of band signals from reaching  
> the I.F. amplifiers, you prevent them from reaching the A/D  
> converter :-).
> 
> 73
> Chen, W7AY
> 
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