On 9/23/11 10:04 AM, Jose Camara wrote:
I think you are right, often the internal, free running osc will give you
better results. You can use the GPS or rubidium to calibrate the internal one
just before you need some more accurate absolute frequency measurements on the
SA.
It will depend on what measurement you are making, and whether phase noise or
frequency accuracy is more important. For day to day use, the external ref will
work, except when perhaps you need to look at very close skirts, where maybe
the internal alone can give you lower noise. In most cases, you don't really
need either (checking a filter, EMI, radio output, etc.) but a lot of thing in
this list is because we can, not because we need. :-)
Get a real clean, low phase noise 3rd signal, measure it using the internal and
external osc, look at the skirts. They might even be the same, if the limit is
elsewhere in the SA signal chain.
One other thing is that some spectrum analyzers aren't really designed
for low noise performance. Since the noise floor is often pretty high,
the design of the whole RF chain (e.g. spur levels and such) might have
assumed that lots of things would be hidden in the grass. If the
analyzer is of the recent "bring a band of RF down to an IF, sample and
FFT it for fine resolution" architecture, such things as the number of
bits in the ADC and the "cleanliness" of the sampling clock might have
been chosen based upon doing 1024 point transforms being displayed with
100dB dynamic range (10dB/div and 10 divisions).
(not to mention the spectrum analyzer actually generating spurious
signals. I ran across that one last year and thought I had an
interference source, but, no, went back and checked the spec sheet and
it said spurious are <-80dBc, and sure enough, there it was at -82 dBc.
And stories about the first LO coming back out through the input are
legion.)
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