average fractional frequency instability

Feb 11, 2008 2 Replies
E

Hi,



I need a high quality RF signal source, for example from OCXO or RF signal generator. But I need to know, what is a fractional frequency instability measured during 1s period. One can find information about SSB phase noise spectra in datasheets, but how to convert this numbers to frequency instability? In RF signal generator I can set requested frequency on a display. But how stable generated signal is? Can I expect that generated frequency will change about 1e-8 or maybe 1e-12 during 1s? Of course, this depends on the internal reference and phase loops inside the box, but such a data should be published in datasheets. From SSB phase noise spectra one can obtain time jitter, but it is possible to convert this value to frequency deviation?



thanks,



E.C.


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T

The information you want would be included in the "Allan Deviation" plot. Google that term and "Allan Variance". 1 second is getting a little long-in-the-tooth but the phase noise is very definitely related.

A really good stable crystal oscillator, e.g. HP10811A, will often have an Allan Deviation of 1E-12 to 1E-11 at a tau of 1 second. Lesser crystal oscillators might be 1E-9 to 1E-6 in the same tau.

Tim.

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E

But I can't find AVAR plots for frequency synthetizers. I've made a simple calculation. From Agilent E8663B datasheet we know L(f) [dBc/Hz] for a given frequency. We can calculate S_phi(f) [rad^2/Hz] using relation

S_phi(f) = 2*10^( L(f)/10 )

Allan variance is related with S_phi(f) by equation (12-27) from

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(page 204). For data taken from Agilent's specification and for carrier frequency 1GHz, I have AVAR(t=1s)=2e-13s, but this value doesn't agree (it's too low) with AVAR for example HP10811A
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osc/). Maybe I made mistake during calculations, but is this way of obtaining AVAR is correct?

thanks, E.C.

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