[time-nuts] Count up/down DAC circuit

[WarrenS]

Steve

Here is another opinion to help add to your choices.

The hunting problem is EASY to fix many different ways.
It is basically done by anticipating what is going to happen next.

If it is done wrong It would indeed:
"exasperate any problems trying to lock a fast moving oxco or achieving lock in the first place"
If done correct, that does not need to be an issue.

To do it 'right' may not be easer to implement than something with a dead-zone BUT  more accurate.

As far "As for achieving lock in the first place with this idea"
This need not be a big issue, It needs to only depend on what the uncertainty of the 1PPS sync signal is,
so as not to resync on the wrong clock. That is max Jitter plus the down time phase error time. 

As far as down time hold-over, It need not be a big problem as long as you have 
something in there that is digital such as the DAC.
Just detect the fact that the GPS is down someway and shut down the Dac updates, 
but don't try to use an analog holdover circuit if you want to hold more than a few minutes.

And I must agree mostly with what "Said" said:
"implementing a standard PI controller (in a  micro etc), and calculating it's stability etc 
is much easier than getting this to work properly"
That is it is easier IF you are good at using micro's, mostly because that is the way it is usually done. 
If you don't or can't use a micro, then its easier to do it without one.

The real problem is just how do you get an accurate signal to lock on to in the first place?

WarrenS
***************
----- Original Message ----- 
From: "Steve Rooke" <sar10538 at gmail.com>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Tuesday, December 30, 2008 8:31 PM
Subject: Re: [time-nuts] Count up/down DAC circuit


> 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
> 
> 
>