I have a no-name OCXO with pins marked by seller (VCC, GND, RF OUT, Vadj). I have checked its output frequency against rubidium oscillator using the following method:
I connected them both to different channels of the oscilloscope, set triggering on the rubidium oscillator signal and adjusted the frequency of an OCXO by connecting negative terminals of the lab power supply together (to get a common ground) and changing one channel voltage between zero and +4V. At about 2.5V (exact value doesn't matter for my question) frequencies seem to match.
Now, I'm looking for a way to provide an adjustable voltage reference to control its frequency when this OCXO is placed in some device. My first guess was to add some resistance in series, but I failed – current consumption of Vadj pin is way too low. I was actually unable to measure it with my Fluke 289 in microamps range.
My second guess was to use an adjustable voltage regulator, for example LM317 rated at 100 mA. My concern is – is this LM317 precise enough for the purpose I described (assuming I use precise and temperature-stable resistors for adjustment)? It has minimum load current of 3.5 mA which is far more than I'm gonna have, relatively high temperature drift and 0.1%/V line regulation.
If using LM317 for that purpose is not a good idea, I would be grateful for any better solution.
Best Answer
Don't use an adjustable reference, use a static 5V reference and a resistive divider (using an adjustable ~4k7 trimmer). This should be good enough for what I think you describe. If phase noise matters, put a capacitor on the Vc input.