[time-nuts] RFDO - Experience and questions

paul swed paulswedb at gmail.com
Sun Mar 5 15:23:36 EST 2017


Gilles what signal is that at 162KHz. A European station? Nice thats its C
controlled.
Regards
Paul
WB8TSL

On Sun, Mar 5, 2017 at 5:54 AM, Gilles Clement <clemgill at club-internet.fr>
wrote:

> 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
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