[time-nuts] RFDO - Experience and questions

[Gilles Clement]

Hi all, 

I have been working for a "while"… on a reference oscillator disciplined by an RF signal (162khz), Cs stabilized, but phase modulated with time coding information. The phase modulation is quite significant : +/-1rad 10Hz triangular shape, leading to a frequency shift of +/- 20PI (+/-6,3Hz). Extracting the base signal was quite an interesting challenge… I tried many options (and even a bit more…) but it was - and is still - fun...

Long story short, the "best" combination I came up with is the following: 
1- local oscillator built around:  CD4060 Collpits + 5184Khz crystal + pulled by varactor diode + divide by 32 
2- Phase detection with 4046 (XOR) with pp ouput voltage limited to 1 volt (input to 10bits ADC)
3- Filtering and VCO controlling with a PIC16F876, programmed in assembly language, implementing a 3rd order loop (much better than second order). Poles at 1E-5 and 1E-3, Zero at 1E-4 => 1E-10 attenuation at 10Hz (if my maths are correct). 
4- 13 bits 20Khz PWM control of the VCO (K0=10-3 Hz/volt) with strong filtering (two RCs in cascade)
5- Crystal is temp controlled (NTC resistor) as well as the whole circuits placed in a foam isolated box.   
6- ADEV measurements perfomed with HP53132A counter (high stability option 10 ocxo  HP10811) frequency input to TIMELAB through HPIB connection (I know it's not the best setup but simple and provides tool for comparison between trials)

Results to date : 
	. « Left side » of the ADEV 0.5s: pretty flat around 1E-9 up to tau =10s 
	. « right side » of ADEV 0.5s plots: reaching 3*1E-11 at tau = 3000s

Comment: 
6,3Hz/162kHz = 4E-5 so attenuation of 1E10 should knock it down to 4E-15 (well below 1E-11) ?

My questions to the respected audience:
- Can I get any better? 
- Have I reached the limit of what the RF signal can provide? 
(with all the perturbations encountered on the path from the transmitter to my antenna...)  
- Am I limited by the local oscillator? 
- Or the measurement gear?
- Or anything else…?  

Thanks very much for your feedback,

Gilles. 

Note: 1E-3 = 0.001