--- "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
wrote:
> There is a fundamental difference between the TenTec and
the various ham
> SDR that have open source software. The many SDR use PC
audio cards for
> the A/D and then do ALL the computation in the PC external
> to the radio. Sometimes only a certain few audio cards will
work fast
> enough.
The trend is to shift away from consumer sound cards for this
purpose. The HPSDR Janus board and Flex-5000's being cases in
point. FWIW it hasn't been the "speed" of the sound card that
defines their usability for radio IF applications, but
instead their own dynamic range, and noise flatness over
their spec'd range of frequencies.
The big payoff for DSP processing on an external PC is the
user interface possibilities that are opened up. The signal
processing (DSP) part of doing SDR via a generic PC HW is
like smashing ants with a sledgehammer. The extra CPU
horsepower is more likely to be burned up via the OS and the
other UI things that you may attempt. But PC horsepower is
cheap, so who cares unless you need very low power
consumption and/or high portability. I really do hope that
that upcoming panadapter add-on at a minimum has the ability
to provide an I/Q out that can be fed to other external PC
based programs. This will allow 3rd party SDR programs to
become the "eyes" to a primarily HW radio. I'm already see
talk of doing this very thing with the K3. And you can bet
that it will be happening pretty quickly with the K3.
> And then the SDR of the current market are direct
conversion radios with
> RF stage, a LO with quadrature outputs and a couple mixers
> with low pass filters and gain stages having probably no
more than 150
> KHz bandwidth. The software controls the LO in large (maybe
10 to 50 KHz
> steps) and ALL the fine tuning, filtering, and detection is
done in the
> DSP software running in the attached PC. The simple rock
locked radios
> neglect the LO tuning and cover a big chunk of band all
with the DSP
> software in the computer.
This is not true in all cases. In the case of the SDR-1000
the radio itself is fully capable of tuning in very fine
steps. But there are good design reasons to not normally run
it that way. Since it uses a DDS LO there are certain
frequencies when generated will have higher spur content than
others. All DDS's do this. Traditional wisdom has been to
follow a DDS with a PLL of some kind to "knock down" the
spurs. However since these spurs are entirely predictable you
can simply choose to not use the specific frequencies that
are most spur prone. This is precisely what the
PowerSDR/SDR-1000 combo does with "spur reduction" turned on.
As it works out the LO tuning step size is ~3.1 kHz.
> The ultimate radio performance depends on the dynamic range
of
> the RF and mixer and tremendously on that of the A/D
converter.
More often than not it is the A/D alone that is the limiting
element. The QSD I/Q has proven itself to be capable of very
high dynamic range.
> In TenTec radios, the LO is controlled to the finest of
frequency steps,
> the RF is bandpass filtered then at the IF its filtered
more with the
> "roofing" filter, then converted down to a 15 KHz IF where
the direct
> conversion Q and I process is done and then the fine
filtering, very
> finest of tuning (if the roofing filter allows), and
detection is
> accomplished in the DSP.
I have surmised that the reason that TT chose the 2.5 kHz
tuning step for the RX320, Peg, Jupiter, plus a few others
was to minimize phase noise from the MC145170 PLL used. It is
interesting to note that the K2 also uses the MC145170 PLL
that is also tuned in comparatively coarse steps with other
means used for the fine step tuning,
> So the software does a great deal more radio control before
the DSP and with the use of narrow "roofing"
> filters the DSP hasn't all that much to do.
Don't think it changes the "amount of work" for the DSP one
way or another. Processing 24 bits at 192 Ksamples per second
is the same whether the sampled passband is wide or narrow.
What the roofing filter does do is to reduce the odds of the
signal being mangled before it hits the A/D.
However what I think I'm seeing is that the designers of
narrow roofing filter radios are now bumping into is the IMD
generated within the roofing filter itself. I'm increasingly
convinced that this is what the Orion and the K3 have as
their limiting factor for close spaced (<2 kHz) IMD DR
performance.
> That makes most of the dynamic range depend on RF hardware
and allows a smaller dynamic range
> and narrower bandwidth A/D but exposes the radio design to
all of the
> same foibles in dynamic range and close in intermod of
multiple conversion
> analog radios that have been fought for decades.
Ah yes, the folly of numerous IF conversions. It is kinda
ironic that radio started out without any IF stages at all;
and then reached a peak of 4 or so, and has now retreated
back to having just one or two. And now the possibility of
having none is just not that far away.
Duane
N9DG
> But TenTec has known how to produce
> radios with good dynamic range for nearly that long too.
> --
> 73, Jerry, K0CQ,
> All content copyright Dr. Gerald N. Johnson, electrical
> engineer
____________________________________________________________________________________
Fussy? Opinionated? Impossible to please? Perfect. Join Yahoo!'s user panel
and lay it on us. http://surveylink.yahoo.com/gmrs/yahoo_panel_invite.asp?a=7
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|