[time-nuts] Fixing PN degradation via ADEV measurement

[Karen Tadevosyan]

Hello group,

 

I am developing a hamradio ground satellite station for the first geostationary satellite QO-100 (uplink 2.4 GHz / downlink 10 GHz).

 

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.

 

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).

 

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.

 

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.

 

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.

 

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?

 

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.

 

I would like to hear the opinion of experts in this matter.

 

Karen, ra3apw