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

[Joe Subich, W4TV]

> This is not correct.  You can't just compare the number of 
> bits without taking into consideration the ADC sampling rate 
> and signal bandwidth.  Please see the concept of "signal 
> processing gain" or "process gain". 

The concept of processing gain is completely different than 
blocking dynamic range.  The maximum large signal capability 
of any DAC has an absolute limit based on the largest integer 
the DAC can resolve and the DAC reference voltage. When more 
than one signal is present within the "window" (DC to maximum 
frequency) of the DAC the DAC needs to be able to handle the 
sum of the instantaneous peak (vector maximum) voltages not 
their average levels.  

No amount of decimation and processing will reduce the 
instantaneous peak voltage that the ADC must handle to 
prevent overload.  The clipping (overload) level results 
in distortion (IMD) or imposes an operating condition that
must be prevented by the application of hardware AGC (gain 
reduction) ahead of the ADC (blocking).  Decimation and 
processing gain are only of value as long as he hardware 
is operating within its linear range. 

73, 

   ... Joe, W4TV 
 


> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Philip 
> Covington
> Sent: Monday, December 01, 2008 3:21 PM
> To: Bill W4ZV
> Cc: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 & ADAT ADT-200A by HB9CBU
> 
> 
> On Mon, Dec 1, 2008 at 1:29 PM, Bill W4ZV 
> <btippett at alum.mit.edu> wrote:
> >
> >
> >
> > Alan Bloom wrote:
> >>
> >> On Mon, 2008-12-01 at 06:35, ab2tc wrote:
> >>> Hi,
> >>>
> >>> I, too, would be very interested in seeing how "entire HF 
> spectrum 
> >>> digitized at once" receivers would fare in the real world. I am 
> >>> *very* skeptical. Although the Perseus (with that architecture) 
> >>> receiver fares relatively well
> >>> in the recent ARRL test,
> >>
> >> It compares well for third-order intermodulation 
> distortion, but not 
> >> for blocking dynamic range.  The A/D converter is very 
> linear within 
> >> its range, so you can get some fantastic TOI (third-order 
> intercept) 
> >> numbers if you measure with signals near the top of the 
> ADC range.  
> >> However the ADC full-scale signal level is less than a 
> conventional 
> >> mixer can handle.
> >>
> >
> > Correct.  I believe the Perseus only has a 14-bit ADC so it's more 
> > limited in dynamic range than the 24-bit ADCs commonly used 
> by other 
> > SDR rigs.  Its dynamic range may be OK for IMD but today's 
> SDRs cannot 
> > approach the BDR of rigs like the K3 until even higher 
> resolution ADCs 
> > become available.
> >
> > I believe the best BDR I've seen for any SDR is the Flex 
> 5000, which 
> > has just over 120 dB...the K3 is better by about 20 dB.
> >
> > 73,  Bill  W4ZV
> >
> > P.S.  How many hits for "1000 " or "5000 " in the contest 
> soapbox so 
> > far? ZERO.  (Which says something about SDRs in use by real 
> > contesters).
> 
> This is not correct.  You can't just compare the number of 
> bits without taking into consideration the ADC sampling rate 
> and signal bandwidth.  Please see the concept of "signal 
> processing gain" or "process gain".  A SDR such as the 
> Perseus with a 14 bit ADC gains dynamic range by the process 
> of decimation/filtering.  If a 14 bit ADC is sampling at 80 
> MSPS and you are decimating to common filter bandwidths used 
> for CW signals, ex. 500 Hz, there is an increase of dynamic 
> range by 10 log(80e6/500) = 52 dB.  The 14 bit ADC used in 
> Perseus is rated at ~77 dB SNR at 15 MHz.  With decimation 
> the dynamic range is increased to approx. 77 dB + 52 dB = 129 dB.
> 
> Another example is the QS1R's 16 bit ADC rated at ~77 dB SNR 
> at 30 MHz.  The ADC samples at 125 MSPS.  10 log(125e6/500) = 
> 54 dB, so dynamic range after decimation is approx. 77 dB + 
> 54 dB = 131 dB.
> 
> Many of the better 24 bit audio ADCs used in SDRs are capable 
> of 120 - 124 dB of dynamic range at *best*.  I believe the 
> K3's PCM1804 24 bit ADC is rated at 112 dB SNR.  The AK5394A 
> 24 bit ADC used in the Flex-5000 series radios has a rated 
> SNR of 123 dB.  For ADCs the SNR is pretty much the same as 
> dynamic range.
> 
> 73 Phil N8VB
> -- 
> Phil Covington
> Software Radio Laboratory LLC
> Columbus, Ohio
> http://www.srl-llc.com _______________________________________________
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