[Elecraft] Analog vs. Digital Front Ends

[Jim Brown]

On Tue,9/15/2015 1:52 PM, Tony Estep wrote:
> On Tue, Sep 15, 2015 at 3:36 PM, Joe Subich, W4TV <lists at subich.com> wrote:
>
>> ....noise power ratio testing hides the fact by not providing MDS values under each test condition *and* fails to indicate that even with *no preamplifier* the total noise signal is more than 10 dB *less*...
> =========
> I take this to mean that the test results are not comparable across radios, and the SDRs are 10db worse than they show up on the comparison chart. If this is true, it ain't good.

You've got the concept right -- the only difference is that 10 dB is not 
a "hard" number -- it could be anything from 5 to 30 dB, depending on 
how much test signal strength has been turned down, or preamp gain 
removed, to get the input of the digital system out of clip!

Here are some comments by an engineer i trust.

73, Jim K9YC

=   =   =   =   =

- According to the ARRL, the Flex 6700 has 14 dB worse preamp-off BDR than the K3.

- If you turn the Flex's preamp ON, the BDR is probably 24 dB (or more) worse than the K3's. But if you turn the preamp on the 6700 OFF, its 2-kHz IMDDR3 is 8 dB worse than the K3's. You can't win

This all stems from the poor NF of their ADC and its low input voltage range. The MDS of the Flex is only -118 dBm (according to Sherwood) with the preamp off. (A K3's MDS with preamp off is -134 dB.) So you have to turn the Flex's preamp ON to get good MDS. The 2-kHz IMDDR is then good, but the BDR is terrible. Sherwood was so baffled by the weird responses he saw that he simply put "A/D Limit" in the BDR column for the Flex. He's still thinking about how to test it.

ARRL measured Flex's BDR, but only using the conventional method. To properly assess the damage caused by an ADC at the front end, you have to inject MANY signals into its input to give them a chance to combine in phase and hit the ADC's clipping threshold. And ADCs do NOT gracefully degrade.