From the amateur's point of view, GPS-disciplined oscillators are a great way to acquire a 10-MHz or 1-pps "house clock" on a budget. Typical performance of a GPS clock's 10-MHz output is in the 1E-10 to 1E-11 neighborhood at a 1-second time scale -- often an order of magnitude better than an undisciplined crystal oscillator -- and of course there's no long-term drift at all.
If you know what a GPS-disciplined frequency standard does, chances are you either want one or you already have one. There are many options when it comes to purchasing or building your own GPS standard, but there's a lot to be said for the Trimble Thunderbolt model. Thunderbolts are exceptionally reliable, and they're largely "plug and play" in nature, requiring only an inexpensive power supply and GPS antenna to provide a time/frequency standard competitive with cesium or rubidium atomic standards without the associated maintenance concerns.
Thunderbolts were originally designed for E911 "enhanced 911" applications, in which precise time-of-arrival readings at multiple cell sites can be used to locate callers in distress. Thanks to telecom consolidation and ever-changing regulatory standards, a large number of Thunderbolts have appeared on the surplus market over the last few years. Typical prices range from US $200 to $400.
Because the GPS-disciplining process is meaningful only at timescales from several seconds to several hours, the OCXO is the heart of any good GPS clock. GPS handily overcomes the effects of long-term oscillator aging, but it does nothing for short-term drift and phase noise. If you are using your GPS standard as an external 10-MHz reference for a spectrum analyzer, synthesizer, or other application where it will be multiplied into the GHz neighborhood, you can imagine how important the reference's spectral purity is.
Recently (2008) some
2002-vintage units with particularly-high-quality OCXOs have appeared for sale in the
sub-$200 range, but most of the Thunderbolt units in the field are probably a few years older,
like the one shown here. The lower-grade OCXO in this unit had very good in-channel phase noise
performance, but poor stability at 100 Hz and below. So it made sense to try to upgrade
the OCXO, a process made relatively simple by the presence of unused SMA jack pads on the Thunderbolt's PC board.
After removing the factory OCXO, jacks installed at these pads were connected directly to the voltage-controlled tuning and RF output lines of an HP 10811-60111 OCXO. The SMA jack closest to the corner of the PCB (J10) provides the OCXO's tuning voltage, while the remaining jack (J9) is connected to the oscillator's 10 MHz output.
The Thunderbolt's 12V oscillator power connection was also suitable for direct connection to the HP OCXO, and because the Trimble control software allows the user to adjust the loop coefficients used to phase-lock the OCXO, it was easy to change the tuning coefficient from -5 Hz/volt to the 0.2 Hz/volt value needed for stable operation of the 10811.
The only complication was the need for a separate
24V oven supply in my temporary "breadboard"-style test bed. This test platform
will ultimately be housed with a single low-noise linear power supply.
As seen in the phase noise plot below, the replacement HP 10811-60111 OCXO is more than 25 dB cleaner than the original part at 1 Hz. This is a massive improvement, easily verified by noting significantly less wander in TBoltMon's PPS and ppb timing output fields.
The high-frequency spurs in the blue trace were caused by the Lucent switching regulator assembly in the original Thunderbolt housing. Replacement of this supply with an external +12/-12/+5 volt switching supply eliminated most of the original Thunderbolt's spurs, lowering its phase noise floor as well. Some of the improved noise/spur performance was lost when the 10811-60111 upgrade was performed, but the close-in improvement more than justifies the upgrade.
The orange trace is from one of the newer Thunderbolts from the 2008 TAPR group purchase, which, as noted,
already has a 10811-class OCXO. Improving the performance of this particular Thunderbolt would not be trivial.
