[time-nuts] Re: Fixing PN degradation via ADEV measurement

[Bob kb8tq]

Hi

The “typical” approach to doing this is to multiply up to something around
100 MHz with a relatively narrowband PLL. You can get ( or build ) VCXO’s
in this range with phase noise (at 10KHz offset) in the 160’s to (maybe) 170’s. 
This is much better performance than a 5 or 10 MHz reference can deliver
when multiplied to that frequency. 

The “cross over” between multiplied reference noise and a fairly good VCXO
should typically be in the 50 to 200 Hz range. Since it is highly dependent on 
the exact parts chosen, there is no “one size fits all” answer.

Bob

> On Jun 18, 2022, at 12:49 AM, Karen Tadevosyan via time-nuts <time-nuts at lists.febo.com> wrote:
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> Hello group,
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> 
> 
> I am developing a hamradio ground satellite station for the first geostationary satellite QO-100 (uplink 2.4 GHz / downlink 10 GHz).
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> Narrowband digital communication requires high frequency stability so I use a 10 MHz OCXO with an absolute Allan deviation (ADEV) of about 5 mHz as a single reference oscillator.
> 
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> To form a reference signal of 25 MHz for the 9750 MHz synthesizer in LNB I use a PLL (to multiply 10 MHz OCXO frequency by 2.5 times).
> 
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> When measuring the PN of a shaped reference signal at 25 MHz, measured with SA R&S FSQ8 achieved an instrument limit of -130 dBc/Hz @ 10 kHz. According to my calculations, the 25 MHz output PN should be in the range of -150 dBc/Hz. However, when operating via satellite, the nature of the received signal is more "noisy and smeared" with a lower S/N ratio (compared to operation from another low-noise generator at 25 MHz without PLL) which, in my opinion, indicates a worse PN value than expected -150 dBc/Hz.
> 
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> At the same time I noticed such a feature - when measuring the absolute ADEV value of 25 MHz output signal the device does not show the calculated value of 5 MHz x 2.5 = 12.5 mHz but a value of approximately 60 - 70 mHz.
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> Additional tests at 10 GHz through the radio end-to-end confirmed a real decrease in S/N ratio of about 10-15 dB and I found the source of the problem.
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> Question: it turns out that the Pendulum CNT-91 frequency meter-analyzer (which I measure the Allan deviation) allow to record significantly lower PN values ​​than the R&S FSQ8?
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> That is, with PN borderline values ​​​​between -130 and - 150 dBc / Hz (in our particular case), the Allan deviation measurement allow to accurately fix the PN degradation where a not the worst SA R&S FSQ8 can not help.
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> I would like to hear the opinion of experts in this matter.
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> 
> Karen, ra3apw
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> 
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