[time-nuts] Any thoughts on best rubidium?

[Robert Deliën]

> 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…