At 06:49 AM 12/13/2005, Gerald Youngblood wrote:
Ross,
Thanks for bringing some reality back into the discussion. The simple fact
is that the locations of the worst case spurs don't really matter. Those
frequencies are rarely if ever used. Also, most spurs are well below the
atmospheric noise floor on HF so you don't hear them in normal operation. I
own a late model Japanese radio (I paid more than the SDR-1000) that you can
hear spurs everywhere if you remove the antenna. With the antenna on, most
of them can't be heard.
I think it's important to distinguish between spurs you can hear
("birdies") and spurs in the LO that result in poor adjacent signal rejection.
The former are covered up by the noise, as you mention, and realistically,
are no different than any other interfering signal.
The latter, however, reduce the useful dynamic range of the system and
adversely affect selectivity.
A telling test would be to look at spurs that appear when you have a
moderately strong input signal from an otherwise clean source (so the
spurious signal isn't covered up by the noise sidebands of the signal)
James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875
