[hpsdr] SDR Dynamic Range Considerations

[Tayloe Dan-P26412]

>When you state below that the SDR-1000 does have a roofing filter, are
>you are referring to the first order RC filter from charging the
>capacitors for the sample and hold, and/or the low pass filter in the
>sound card?

It sounds like the SDR receiver front end is now trivial (other than the
LO source) since we need only a receiver pre-selector and the detector
itself fed directly into the balanced inputs of the A/D board.

I did not find the input impedance specs of the AKM5394A A/D converter,
but I assume it is reasonably high compared to the 200 ohms out of the
detector.  If they are high impedance, the detector R/C roll off ought
to be pretty good as opposed to connecting to a normal op-amp, which
does decrease the single pole R/C roll-off effectiveness.

With the front end being so simple, one possibility might be to allow
switching in additional banks of caps in parallel with the basic four
detection caps.  The basic set of four caps could be set to roll off 3
db at +/- 100 KHz, while one or more other sets might be used in
parallel with this as narrower "roofing filters" to narrow the 3 db roll
of to perhaps +/- 11 KHz or even +/- 3 KHz.  It seems like this might be
useful when we don't need to see the entire band at one time and could
result in a more narrow receiver when we are focused on a particular
signal.

With the 145 db blocking dynamic range (~ 140 db is best in class) and
potential 145 db IP3 dynamic range (108 db is best in class) without
narrowing the detector, further narrowing the detection bandwidth when
focused on a particular signal could give us absolutely killer receiver
specs.  We could easily pick up another 20+ db of off frequency signal
rejection.  No mere superhet could then even come close to the resulting
receiver performance. 

We will then need a killer clean LO to match.  A few tricks with an
analog VFO could help do this, especially if we track the frequency with
a counter and continuously correct in software for the VFO drift.  It
could be an interesting combination of old and new in order to maximize
overall receiver performance.

- Dan, N7VE