This discussion of semantics is missing the point. The
point of this discussion was at what frequency/IF stage the DSP
filter operates. The Orion/Main RX has 3 IF's at 9 MHz, 450 kHz
and 14 kHz. My point was that DSP's can be made to operate at
"audio frequencies" fairly easily, because ADC's, DSP's and DAC's
are economical at audio frequencies (sub-20 kHz) but NOT at RF
frequencies. I think we are picking at bones because the DSP
clearly operates on the 14 kHz IF, NOT at 450 kHz or 9 MHz.
It is impractical (i.e. very expensive) or impossible to get
commercial 24 bit (high resolution necessary for dynamic range)
ADC's and DAC's that would operate at 450 kHz...much less at 9 MHz
where the analog roofing filter does the heavy lifting. Just to
cite an example, we had an 8-bit 20 MHz ADC at HP in the early 90's
that sold for around $8000. To do DSP at 9 MHz would require an ADC
with 24 bit resolution at an 18 MHz sampling rate (twice the IF
frequency for no information loss)...and Lord only knows how many
GFLOPS in the DSP itself.
I don't reallly care if we call 14 kHz Audio, IF or RF...it
remains 14 kHz, and is done at the 3rd IF stage well past filtering
necessary for good interfering signal rejection is done by the first
IF roofing filter at 9 MHz. The roofing filter is primarily why we
see IMDDR3 of 101 dB...NOT DSP filtering at 14 kHz. For an example
of a purely analog system that even exceeds Orion's performance, check
out W8JI's "heavy modified" R4-C using a 600 Hz Sherwood roofing
filter below, and note the difference with the stock R4-C's (which
had wide roofing filters) at 48 and 68 dB IMDDR3 respectively:
http://www.w8ji.com/receiver_tests.htm
The medium mod has IMDDR3 of 92 dB and the heavy mod has 118 dB
and that's at 2 kHz spacing...not 5 kHz. Of course Orion might
also do better if the optional roofing filters are used.
73, Bill W4ZV
|