John Larkin wrote:
On Wed, 19 Sep 2007 00:17:53 GMT, Joerg
wrote:
John Larkin wrote:
I was playing with our new spectrum analyzer and decided to just hang
a 12" cliplead off the input to see what was floating around.
See pics. The biggest signal was about -26 dBm, an FM station I guess.
Just about every slot on the FM band is full.
Looks like a luxury analyzer. Aeroflex? I thought you could buy those
only with a GSA account or for government contract work.
It's the new 3 GHz Aeroflex; anybody can buy it. With all the options
(tracking gen, preamp, high resolution) it was $13K or something. It
has a few firmware bugs, but I think they're sincere about fixing
them. Except for a few glitches, it's a lovely box, with a beautiful
display.
Hmm, when I bought a PC-controlled receiver for EMI debug purposes
lately they told me I can only have a version where the cell bands have
been disabled. That's because this thing could decode stuff which I
really need to identify sources or gauge their phase noise. Only gov
folks can buy the full range version. Importing a model for foreign
markets without such restrictions would not have been legal. They show
up on Ebays at times but I guess instead of the Fedex truck another
vehicle would ...
I was disappointed in the harmonic distortion, -65 dB typically, but
it looks like that's actually not real bad compared to other units. We
have some RF signal generators that have THD numbers like 1.5%!
For wideband outputs it's probably the usual trade-off between
dissipation in the final amp and linearity. Or maybe the engineers
didn't care.
To test some amps we're doing, I guess we'll have to buy a few sets of
matched lowpass/highpass filters, so we can pump in a clean sinewave,
then and pull out the fundamental before we feed it to the spectrum
analyzer. What a nuisance. Maybe we should find/make a couple of
tunable notch filters.
How does one measure the THD of an amplifier, down around -70 dbC, in
the 1-30 MHz range maybe?
We used to do that the old-fashioned way with bolometer heads. One
measurement at full BW, then the harmonics via a steep enough highpass
where whatever of the fundamental pipes through is subtracted. I always
keep a sizeable stash of toroids to whip up a filter.
Why don't spectrum analyzers do log sweeps?
I've used a few that did. And my network analyzer does. For spectrum
analyzers it is rarely needed though. I guess with a modern machine like
the Aeroflex you could pipe it through USB and then let Excel re-scale
it. Don't know about your analyzer but most modern gear unfortunately
does not come with SW that allows direct live import into Excel. I do
not understand why but Excel certainly does not provide an easy path
through a virtual COM port (wrestling with that right now). I have no
idea why Senor Gates has restricted the license for mscomm.ocx so you
can only use it when you also have VB6 installed on the PC banging head
on table.
--
Regards, Joerg
http://www.analogconsultants.com