[time-nuts] First success with very simple, very low cost GPSDO

Bob Stewart bob at evoria.net
Thu Apr 10 19:52:12 EDT 2014


FWIW, I set the DAC to midpoint and use a binary search to get to frequency.  On my system, I start off with a change of 0x80 and work my way down to 0x04 in powers of 2 (0x80, 0x40, 0x20, etc).  Once I get to 256 seconds between DAC changes, I light the PLL fuse.  I could probably get away with less than 256 seconds.  Before I built the TIC, I worked down to 0x01 which resulted in pretty long times between changes.

Bob



>________________________________
> From: Chris Albertson <albertson.chris at gmail.com>
>To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>Sent: Thursday, April 10, 2014 6:05 PM
>Subject: Re: [time-nuts] First success with very simple, very low cost GPSDO
> 
>
>On Thu, Apr 10, 2014 at 10:55 AM, Charles Steinmetz
><csteinmetz at yandex.com>wrote:
>
>>
>> Why even try to discipline an OCXO before it's warm?  Just leave the
>> control loop off for a predetermined time at startup.  You can light up a
>> bright red "unlocked" LED, and even inhibit the 10 MHz output until lock is
>> achieved if you want.
>>
>
>Even if you wait long enough you still need to set the DAC to "something"
>as a first try.    What if your first guess is off by a large amount.   The
>answer is not to guess better because you still need some way to determine
>if the guess is correct.
>
>>
>> Alternatively, you could figure out the EFC voltage needed to zero the
>> cold oscillator
>
>
>How to do that?  Can you write code to do this without counting overflows?
>You can be off by 256 cycles and never know it.   Let's assume someone is
>building a GPSDO because they need a frequency reference and don't have
>one.  A good assumption, I think.   How could such a person "figure out the
>EFC voltage needed"  with out either coining cycles?
>
>
>I'm thinking about turning the overflow counter off after lock.  But
>without one I think we have a chicken and egg problem where you would need
>an accurate frequency reference to build a GPSDO.
>
>-- 
>
>Chris Albertson
>Redondo Beach, California
>
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