[time-nuts] Any thoughts on best rubidium?
EWKehren at aol.com
EWKehren at aol.com
Sun Sep 25 09:47:11 UTC 2011
If you want low noise in a spectrum analyzer it all comes down to the
signal quality into the first mixer. Every thing else with today's technology
is down hill.
Bert Kehren
In a message dated 9/25/2011 5:32:31 A.M. Eastern Daylight Time,
Robert at delien.nl writes:
> 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.
True, it's one of the many selection criterions for selecting the
instrument that meets your needs.
I've been looking a the luggable HP series 859x and 856x, preferring the
latter because they have a PLL YIG whereas the fist uses a free-running
oscillator. But these machines are old, 80's and 90's, pricey, and not really
THAT good. Add decent range (up to 9GHz to see recent 5.8GHz devices) and a
tracking generator and before you know it, you'll be paying $6k or more for
a 20 year old instrument.
> 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).
Most modern instruments do that, at least to some degree. My R&S goes down
to a RBW of 10Hz by just mixing. Additionally RBWs of 5, 3, 2 and 1Hz are
achieve by additional FFT. This instrument dates from 2001, but I don't
think more recent instruments can achieve a mixing-only RBW of 5Hz or below.
> (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.)
Gee, I wish I had consulted this group BEFORE buying my instrument. I'm
happy with it and I don't regret anything, but you could have added a lot
more arguments in favor or against…
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