[time-nuts] Any thoughts on best rubidium?
brooke at pacific.net
Fri Sep 23 17:54:52 UTC 2011
Do you know how the HP/Agilent 4395A stacks up as a SA? I really like
the true RMS power detection and the 1 Hz RBW (not video).
Jim Lux wrote:
> 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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