Whither Holdover

A project log for GPS Clock

A simple desk clock that gets extremely accurate time from GPS

Nick SayerNick Sayer 11/05/2017 at 17:340 Comments

A few people have asked about holdover as a feature. Holdover is the ability of a radio clock (and the GPS clock is a radio clock, of course) to free-run when reception fails.

It's not been a priority because with proper antenna placement GPS should always be available, and holding over should not be necessary. This assumption has been a driving force in keeping the hardware and firmware simple. When there's no GPS lock, you just display "no GPS" and wait.

The problem with holdover is that the statement of how accurate the clock is gets a lot more complex. Right now, I can say that the clock is "within 200 µs." In actual fact, the clock is somewhere between 70 and 170 µs slow because it takes 70 µs for it to process the PPS pulse to completion and the display raster cycle is 100 µs long.

But holdover introduces the possibility of longer term drift as long as reception remains unavailable. The magnitude of any potential drift will be proportional to how long ago the last sync occurred. You can "tune" the crystal oscillator while reception is available - that is, determine the exact number of clock cycles between PPS interrupts - but you need to quantify the magnitude of any potential changes in that offset.

You can limit the scope of the problem by limiting how long you're willing to hold over before giving up. But even then, if your crystal has a 10 ppm stability, that's still 36 ms in an hour.

You can throw money at the problem. A TCXO can provide a frequency stability of 50 ppb. That would give you a maximum drift in an hour of 180 µs. But a DOT050V adds $30 to the BOM cost (so $60 to the retail price).

In any event, you'd definitely want to give some feedback of how long you've been holding over. You could just do this with a single LED - on constantly for GPS being available, and mostly off with a blink code indicating how long holdover has been in effect (n blinks in a row mean n time periods of holdover, and after 5 time periods the clock should give up).

But again, all of this is just an effort to mitigate poor antenna placement. I'm just not really convinced it's worth the effort.