[time-nuts] Count up/down DAC circuit

[SAIDJACK at aol.com]

Hi Steve,
 
not sure if the LPF would help, some issues with analog sample and holds  
with long time-constants were recently discussed on this group. I am not a fan  
of these to implement a GPSDO loop.
 
I guess it depends on your dac steps too. If you have a 16 bit dac over a  5V 
EFC, then you have +/-32K steps, or 32K seconds to adjust.That's a very slow  
slope.
 
Also, take into consideration that the GPS and the OCXO will "jump" over  the 
course of a typical day, and you need to track these jumps. Your tracking  
speed may not be fast enough here.
 
Our best double-oven OCXO's still have ~+/-50 microvolt variations, and a  
typical single oven may have +/-15mV variations over a 24 hour period. If you  
don't have enough resolution, you will unnecessarily introduce quantization  
noise, and if you do you would be lagging behind these natural changes.
 
What would probably work is if you over-sample the counter circuit to  say 
100x to 1000x (or in other words being able to take 100 to 1000 LSB  steps per 
second). That may prove to work out ok, and give you enough DAC  resolution and 
DAC speed.
 
With an M12+ set to 100PPS you may just make this work well enough! My gut  
feeling is it won't work with 1PPS, but may work with 100PPS.
 
And if your DAC LSB step is very small, say 5E-013, then your tracking  noise 
would be quite small. Let's see, 100 steps per second, at 5E-13 per step  
would give you a tracking speed of 5E-011 per second. That's probably good  
enough.
 
A typical crystal jump of ~1E-09 would thus take only 20 seconds to  correct. 
Sounds reasonable to me. Note that this is a frequency locked loop  rather 
than a phase locked loop.
 
Let me know if you try it, and if you get it to work.
 
bye,
Said 
 
 
 
In a message dated 12/30/2008 20:32:50 Pacific Standard Time,  
sar10538 at gmail.com writes:

Hi  Said,

Yes, I could see that my idea would suffer from this hunting in  the
locked state problem an was wondering if this could be perhaps  'cured'
with a simple low-pass filter stage between the count up/down DAC  and
the EFC, IE, the DAC would hunt up and down with a duty cycle equal  to
the difference between the two DAC levels filtered by the LPF,  thereby
giving a constant frequency.

This, of course, would further  exasperate any problems trying to lock
a fast moving oxco or achieving lock  in the first place. Some tuning
of the LPF time constant would be required  to stop ringing as the PLL
moved into lock.

Do you think that would  work as it would probably be easier to
implement than something to  implement a dead-zone?

As for achieving lock in the first place with  this idea, I'm thinking
now that it could take a very lon time for the DAC  to count up/down
with output from a phase detector at 1PPS.  I was  really only thinking
about this whole idea as it seemed to be a natural to  hold the EFC
voltage during lack of PPS if the GPS goes down. Without the  PPS, the
phase detector will output no pulses so the DAC would remain  frozen in
it's last state. Implementing a GPSDO via a phase detector  followed by
a LPF would obviously be easier but in the absence of the PPS,  I
imagine that leakage in the circuit would make the EFC voltage  drift.
I guess I could buffer it with a source-follower or something  like
that, or perhaps some form of sample and hokld circuit.

I was  just bouncing around ideas as I know there are a lot of great
brains on  this list.

73, Steve

2008/12/31   <SAIDJACK at aol.com>:
> Hi Steve,
>
> I played with such  a circuit a long time ago.
>
> The slope is limited to the  clocking of your circuit (one LSB digit per
> clock typ), which can  present an issue if you cannot follow the OCXO's EFC
> changes fast  enough. You could be chasing the OCXO voltage and this may 
lead to
>  instability.
>
> Even if it is "locked", the circuit will  constantly be "chasing" the OCXO,
> unless you implement a dead-zone  where the circuit stops counting up/down 
when
> you are close enough to  your target frequency.
>
> This chasing may cause the frequency to  modulate up and down, and could  
lead
> to large-scale  oscillations.
>
> Tough to get this to work properly, but with  circuitry to add a deadzone,
> and to dampen the lock, and maybe to  introduce gain (jump more than one 
LSB when
>  far off etc) it may  work. Then again implementing a standard PI controller
> (in a   micro etc), and calculating it's stability etc is much easier than
>  getting this  to work properly.
>
> bye,
>  Said
>
>
> In a message dated 12/29/2008 21:30:54 Pacific  Standard Time,
> sar10538 at gmail.com writes:
>
> As  part  of the current idea I have with the hockey-pucks, I'm  thinking
> about  feeding the D1 and U1 phase difference pulses out  of an MC4044
> out to some  circuit that could clock up and down an  analog output
> which would  ultimately go to the EFC of a ocxo, IE  D1 pulses when the
> phase of one  input signal is advanced and  visa versa for the U1 pin.
> Anyone seen a  circuit like that  please?
>
> Thanks & 73, Steve
> --
> Steve  Rooke  - ZL3TUV & G8KVD & JAKDTTNW
> Omnium finis   imminet
>
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-- 
Steve Rooke - ZL3TUV &  G8KVD & JAKDTTNW
Omnium finis  imminet

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