20-Dec-2006

Introduction

The Oncore VP has a 9.54 MHz oscillator and thus exhibits a sawtooth of approximately ± 52 ns (1 / 9.54 MHz = 104 ns).

Below is a 10 minute trace of the 1PPS output of a VP (log30177.txt). The data was obtained with a 53132A TIC and there is a deliberate ~150 ns offset in the 1PPS reference (to avoid negative numbers).

Observations

Take a look at the plot and observe the following:

• The measured 1PPS value varies from just under 100 ns to just under 200 ns. The total jitter is on the order of 100 ns, as expected.
• Immediately visible in the plot is the sawtooth effect -- the cyclic ramping of phase.
• The magnitude of the teeth, or the height of each ramp, is quite consistent at 104 ns. This is like cycle wrapping when using a phase meter or a TIC on a CW RF signal.
• The period of the teeth, the slope, the pitch, on the other hand, varies quite a bit over time.
• There is moderate pitch, perhaps several teeth per minute, for the first few minutes of the trace.
• There is high pitch, or rapid wrapping, near the end of the trace; perhaps a new tooth every few seconds.
• In the middle of the trace the pitch is low with infrequent wrapping; perhaps only one or two teeth per minute.
• For GPS time averaging one can see that slow ramps are not ideal, as they very much skew any time interval average, unless the averaging period is many minutes, even tens of minutes long.
• Note also the pitch is either slanted forward or backward. There is a sign transition near sample 330.

Heater

Now, here's the cool part. Near sample 480 there is a slow ramp, the kind of sawtooth trend that is not ideal for GPS time interval averaging. What happened right at sample 490? Suddenly the slow, GPSDO-unfriendly ramp turned into a fast, GPSDO-friendly sawtooth. Why? 

The answer is that I was watching the sawtooth go into this slow ramp starting around second 420 and a minute later, I aimed a hair dryer at the bare Oncore VP from a meter away for a few seconds. Bingo! The slow ramp goes away and the sawtooth gets good again.

If one were to do this for real a small resistor mounted on top of the XO would work much better. The idea is to monitor the phase, or the phase differences, and when a slow ramp is detected (which can be done within, say, ten seconds), a pulse of power sent to the resistor to temporarily heat the XO, to break the close synchronization between GPS and the XO.

Additional Traces

10 minute VP trace #2

10 minute VP trace #3, showing some nice hanging bridges:

Links

• Motorola GPS M12 Sawtooth
• Motorola GPS VP Sawtooth
• Motorola Oncore VP Sawtooth with Heater