Thanks for the great short, practical explanation...it helped remind me, in
simple language, of the wonderful method used in the Orion to decipher
signals.
73,
Greg, N6GK
"Looking forward to the Orion III, which hopefully will also have 6, 2, and
440, making it the only radio one would EVER need (after all, why should my
little cheap IC706 Mk2G have this and not my Orion?)! C'mon TT, capture the
market!"
>
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. TenTec
radios use a special purpose collection of micro and DSP chips IN the
radio where the needed compilers are not necessarily available on the
consumer computer market.
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. The ultimate radio performance depends on the dynamic range of
the RF and mixer and tremendously on that of the A/D converter. And for
the PC many compilers are available allowing diversity in programming
language as well as operating system.
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. 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. 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. 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
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