[time-nuts] Thunderbolt? (re simple gpsdo.)
Bob Camp
lists at rtty.us
Sun Jan 1 20:11:04 UTC 2012
Hi
Any real world capacitor will have a dielecric with an associated insulation resistance. It's a "more money gets better performance" sort of thing, but there are indeed limits. A 1000 uF cap that has a "good" insulation resistance number might cost you more than some new cars….
No free lunch.
Bob
On Dec 31, 2011, at 11:54 PM, Chris Albertson wrote:
> On Sat, Dec 31, 2011 at 5:56 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>
>>
>>> As soon as you say "Software" the device is no longer simple. Even a
>>> microprocessor is a very complex device and so is its development system.
>>> The software inside the uP is not simple either if you count the number
>> of
>>> possible paths through the code (2 raided to the power of the number of
>>> branches.)
>>
>> Yes and no...
>>
>> Software doesn't have to be big, bloated, ugly, and complicated. (But I
>> agree that it often is.)
>>
>
> If you have eight "if" statements you have 2^8 = 256 possible paths through
> the code. For a hobby application I goes you'd not bother to write up and
> run 256 test cases.
>
>>
>> This looks like fun to me, but I like writing that sort of code. Note that
>> it doesn't need an OS or even any libraries.
>>
>>
>> The context for "simple" wasn't well specified.
>>
>> Does simple refer to design or construction?
>
>
> I think "simple" means you can explain how it works in a few sentences.
> And if software is used you have to explain every calculation and decision
> point.
>
> With software design and construction is the same thing if you only build
> one unit.
>
>>
>> How good does the GPSDO have to be? (After all, this is time nuts.) What
>> sort of adev at what sort of time scale?
>>
>>
>> I think the main problem in this area is building a low pass filter with a
>> long time constant.
>>
>> The time constant of the filter has to be:
>> long relative to the noise from the phase detector
>> short relative to aging of the oscillator
>> short relative to environmental changes
>> (so the osc can track temperature and voltage
>> those changes may be in the PLL system rather than the osc)
>>
>> If we are starting with PPS (rather than 10KHz), the filter time constant
>> needs to be 10s or 100s of seconds. How do I build an analog filter with a
>> time constant that long?
>>
>
> Time constant is just R*C. If you have a 1000uF cap and a 1K resistor you
> have 1 second. In theory you could build 100s just by using a 100K
> resistor but I think real world components are not perfect enough.
>
> What's the input impedance of a VCXO or Rb unit? I assume we will need an
>> op-amp to buffer the filter.
>>
>
> I suspect you are right.
>
>
>>
>> The ugly problem in this area is that time constant to filter out phase
>> detector noise overlaps the time constant needed to let environmental
>> changes through. That doesn't matter if the filter is analog or digital.
>>
>> If the osc is stable (Rb) filter time constants of 1000s of seconds might
>> make sense. That might help take care of some of the hanging bridges.
>>
>
> The new $38 Rb units can only be adjust by RS-232 commands. So you need a
> digital controller. No choice there.
> The best oscillator for an analog controller would have to be a high
> quality ovenized crystal.
>
>
> About the time constants. If you are doing this in software then you can
> track performance inside the controller and adjust. Seems you shouod be
> able to tell the controller the "tau" you need and it should be able to
> optimize.
>
> Once you have a uP then more features are easy to do, like maybe using
> multiple GPS receivers or maybe fault detection and switching to holdover
> mode
>
>>
>> For those who aren't familiar with this trick, it's easy to make a low
>> pass filter in software:
>> X = X*(1-k) + k*new
>
>
> Designing filters seems like an art. What is the frequency response of the
> above for different values of k? I tend to like FIR filters because I
> think I understand them better. I think yours is an IIR.
>
>>
>>
>> Chris Albertson
> Redondo Beach, California
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