[time-nuts] How are non-Trimble oscillators disciplined?

[Hal Murray]

hp_ciscovss at yahoo.com said:
> One thing I am wondering about is disciplining - how much of this is HW and
> how much is SW?

> How are non-Trimble oscillators disciplined? It it common practice to
> provide a GPS antenna input? 

I think you are missing the big picture.

There are two different types of boxes.

There are oscillators.  They come in all types of quality.  Many of the good 
ones include an oven to keep the crystal at a constant temperature.  OCXO is 
a common term: Oven Controlled Crystal Oscillator.

There are GPSDOs: GPS Disciplined Oscillators, for example the Trimble 
Thunderbolt.  They generally start with a good crystal, then add a GPS unit, 
some hardware to compare the crystal output with the GPS output, and a 
microprocessor and software to control everything.

Crystals can be tuned slightly by changing the capacitance in parallel with 
the crystal.  You can do that by using a diode for the capacitor and changing 
the back-bias across the diode.  Usually, that voltage comes from a DAC.

"Discipline" just means making it do what you want.  Usually, that's put out 
the right time and/or frequency.  GPS is a handy way to get both time and 
frequency.

GPS and a good crystal are a good fit.  GPS has lots of short term noise but 
very very good long term accuracy.  Crystals have good short term stability 
but lots of long term drift.


> How are non-Trimble oscillators disciplined?

Any way you want.  Whatever fits your application.

One approach is to adjust something with a screwdriver.  You have to do that 
frequently enough so that it meets your needs.  Usually, crystals come with 
specs, things like max drift over a month or year.  So you can figure out if 
you need to calibrate it monthly or annually.  It's easier to get better 
accuracy with GPS.


You might have fun browsing data sheets.  Feed OCXO to google and see what 
you get.

There is lots of info available on the HP Z3801A.
  http://www.realhamradio.com/GPS_Frequency_Standard.htm
(Time sink warning.)






-- 
These are my opinions.  I hate spam.