[time-nuts] Re: General discussion of PID algorithms applied to GPSDO control loops (continued 1)

[Bob kb8tq]

Hi

Even if you *do* have a number for your OCXO EFC, what is that number?

The nominal sensitivity is fine. The next layer is the ratio of maximum slope
to minimum slope. This is a very different thing than the “linearity” that is normally
specified. Even the linearity is something you need to look carefully at the 
Mil-O-55310 definition. It’s not quite what you would expect it to be.

Bottom line: the sensitivity can easily vary 2:1 or even 4:1 on a pretty good part. 

Bob

> On Apr 15, 2022, at 5:13 PM, Skip Withrow <skip.withrow at gmail.com> wrote:
> 
> One of the things that cannot be ignored are the statements made in
> item #10 regarding the DAC and OCXO EFC.  If the DAC is 16 bits that
> is 2^16=65,536 steps.  If the DAC outputs 0-4 volts that is 61uV/step.
> 
> Now you have to take this number and multiply it by your OCXO
> sensitivity (not given), and you have the minimum frequency step that
> can be made.
> 
> This is the best that you can ever do.  So, your TIC/1pps resolution
> does not need to be any better than this.  Waiting a long time to get
> higher resolution may (probably) buys you nothing.
> 
> This is where all the magic comes in - higher resolution DACs, more
> stable oscillators, more stable GPS PPS, etc.
> 
> To a first order, overlaying the line from item #6 with the ADEV of
> the oscillator used and setting the loop time constant at the
> crossover tau works pretty good.
> 
> As has been said many times, if it was easy everybody would be doing it.
> 
> Regards,
> Skip Withrow
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