[Elecraft] Sherwood's receiver performance table updated

[Robert Vargas-KP4Y]

Joe, I have an MSEE, so I perfectly understand the design aspects and math behind a direct-sampling SDR. That's why I stated previously that this architecture has some wrinkles that will eventually be ironed out as more capable and affordable ( this is a key factor to be competitive in the  amateur radio market) ADC and DSP chips become available in the future. Also, just that we are clear, I'm familiar with the way how the ADC overload issue that you are describing would manifest in a direct-sampling SDR. However, I'm yet to experience this issue during actual operating conditions. 

BTW, I don't see the need for preselector filter as a deficiency in a direct-sampling architecture. That would be sort of equivalent to seeing the need for roofing filters in the K3 to put out impressive blocking dynamic range numbers as a deficiency.

Finally, I have seen great reviews about the Flex-6300, and I'm actually impressed by the fact that it has not preselector filters. However, as satisfied Elecraft customer, I'm confident they will be able to produce a much better implementation of direct-sampling radio in the no so distant future.

73,
Robert-KP4Y/W4

Sent from my iPhone

> On Apr 26, 2016, at 11:35 PM, Joe Subich, W4TV <lists at subich.com> wrote:
> 
> 
>> On 4/26/2016 10:15 PM, Robert Vargas-KP4Y via Elecraft wrote:
>> Just that you know, K9CT has one of the finest contesting station in
> > the US. He has switched to direct-sampling SDRs (Flex-6700). Just ask
> > him if this architecture is 5 years away from being competitive.
> 
> While I do not know for sure, I suspect K9CT is using bandpass filters
> and stubs to prevent inter-station interference in his multi-multi
> operation just as he did when he used conventional transceivers.  With
> properly designed bandpass filters, stubs and the preselectors in his
> 6700s he's not likely to have a severe overload issue unless he happens
> to be close to some high power AM stations (on 160 meters) and even
> then indications are that a good highpass filter and 10 to 15 dB of
> attenuation will resolve the problems with a 16 bit ADC.
> 
> I have not heard any reports of large multi-multi stations switching to
> direct sampling transceivers in areas of Europe where the 40 meter SW
> broadcast was a problem for many decades.
> 
> The point is that direct conversion receivers need either good filters
> *ahead of the ADC*, attenuation or both to prevent problems.  When one
> uses a Flex 6300 *which has no preselector*,  Flex 6500 on multiple
> bands (which bypasses the preselector), an ELAD DUO which has no
> preselector, etc. one must make a choice between sensitivity and
> dynamic range.  The ARRL lab tests show that, Sherwood's tests show
> that, Adam Farson's noise power ratio testing shows that, and other
> reviewers hint at the problem but they simply don't understand direct
> sampling well enough to recognize its limits when they see them.
> 
> One needs to understand the nature of direct sampling receivers and the
> way they react to overload to recognize the behavior but the test data
> is unequivocal.  16 bit ADCs are probably 4 to 6 bits "short" of being
> able to handle worst case signal levels while simultaneously providing
> maximum sensitivity (MDS of -135 to -140 dBm in 500 Hz) and fully broad
> band performance (multiple slice receivers on multiple amateur bands).
> 
> 73,
> 
>  ... Joe, W4TV
> 
> 
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