[hpsdr] Boat Anchors..PreAmps

[Larry Gadallah]

I was reading Adam, VA7OJ's excellent reports on his NPR testing of
various receivers including Mercury (see
http://www.ab4oj.com/sdr/hpsdr/hpsdr_npr_va7grr.pdf) and I was
intrigued by this comment:

"The theoretical maximum NPR for a 16-bit ADC is 85.4 dB, as compared
to 74.01 dB for a 14-bit ADC.
Thus, the HPSDR receiver exhibits a significant deviation from the
theoretical maximum value (much
more so than the Perseus, whose highest NPR value in my August 2011
test was 75 dB at 5340 kHz.)

The RF preamplifier in the Mercury receiver is in the RF signal path
at all times.  A 20 dB pad is switched
in at the preamp input for the “Preamp out” function. Apparently, the
noise loading provokes sufficient
IMD in the preamp to degrade the NPR as much as 12 dB below the
theoretical maximum value. (The
optimum noise loading value decreases by 19 dB with the attenuator
out, which reflects the inserted 20
dB attenuation pretty closely. )"

I'm not sure I grok the meaning of "noise loading" in that comment,
but I'm assuming that he is implying that the attenuator contributes
enough noise to trigger some nonlinear operation of the preamp? This
leads to the question: Is the current design of leaving the preamp in
the circuit at all times and switching in/out an attenuator preventing
Mercury from approaching the theoretical NPR capabilities of the
LTC2208? Would it be better to completely switch the preamp in/out of
circuit as needed?

Cheers,

On 12 April 2011 22:33, Lester Veenstra <lester at veenstras.com> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Quite correct, can be anywhere before the A/D. At RF or at baseband.  I suspect that for most of our applications, manual (end user controlled) will suffice. But having the FPGA calculate the number of clips would be useful (until reset) for the user to access.
>
>
>
> Lester B Veenstra  MØYCM K1YCM

-- 
Larry Gadallah, VE6VQ/W7                          lgadallah AT gmail DOT com
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