[time-nuts] HP 3586 entirely referenced to 10MHz: A solution II

Greg Broburg semiflex at comcast.net
Mon Apr 4 14:50:23 UTC 2011


Hello Didier;

I don't have a copy of the schematic however I was
wondering if there are trim caps installed for these
crystals?? If so, then a varactor can tune above and
below in place of the mechanical cap if you remove it.
Undoubtedly there is some loading cap in the circuit
that could be tweaked a tad. Crystals drift over time
though, this could have an effect on this concept. It
may be that they have already drifted too far to tweak
with the VCO concept.

Also, there are many ways that a PLL ideas interact
with phase noise. If the PLL reference source is quieter
than the intrinsic noise of the VCO then the loop band
width does govern the close in noise spectrum. However,
there are many systems wherein the reference bandwidth
is very small and the VCO noise governsl. Take for
  instance the PPS control of the GPSDO OXCO. The PPS
frequency reference is unable to reduce the noise of
the OXCO above about one half Hz, so the ultimate phase
noise above that frequency is that of the oscillator itself.
This can be a creative brain buster when working on
designs for these ideas.

My vote is to do a DDS synthesizer, this is a fine evolution
low in cost and easy to implement, to displace approximate
oscillators. What frequencies are you generating?

Regards;

Greg




On 4/4/2011 7:29 AM, Chuck Harris wrote:
> Hi Didier,
>
> If you want to convert the Xtal to a VCO, you will have to adjust the 
> parallel
> capacitance so that the crystal can get above the desired frequency, 
> and then
> design the varactor circuit so that it can pull from there to below 
> the desired
> frequency...which should be possible if the crystal was capable of 
> being tuned
> on frequency.
>
> The crystal might be high enough Q to run in a PLL with a very high 
> division ratio,
> but I would still expect it to hiccup around about the period of the 
> divider.... but
> as you say, it might not matter.
>
> The DDS works out to be such a simple solution, I think the minuscule 
> offset is
> hardly worth getting worked up over.
>
> -Chuck
>
> shalimr9 at gmail.com wrote:
>> "Basically, the higher the division ratio in a PLL synthesizer, which 
>> is what you
>> are describing, the greater the phase noise."
>>
>> In that case, that may not be a problem. Since the oscillator is a 
>> crystal, phase
>> noise should be low enough.
>>
>> One other issue is that most crystals only want to move in one 
>> direction (with the
>> varactor pulling trick), so if it is on the wrong side of where you 
>> want it, that
>> won't work.
>>
>> Otherwise, I have been thinking about that myself.
>>
>> Didier KO4BB
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>




More information about the time-nuts mailing list