[time-nuts] First success with very simple, very low cost GPSDO
bob at evoria.net
Thu Apr 10 23:52:12 UTC 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.
> 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.
>Redondo Beach, California
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