Oscillators synchronizing themselves - Page 1
Forum section: electronics / beginners

author: WatchfulEye
author_rank: Regular Contributor
timestamp: January 06, 2022, 09:17:20 pm
content: I've been playing around doing some timing experiments with an interval counter made from an MCU. After cleaning up some software and attaching a cheap recycled OCXO from Aliexpress, I was ready to go with some experiments.Measuring a GPS PPS signal and coding up a PLL to make a GPSDO worked as expected.The next experiment was more interesting. I connected a spare identical OCXO to a frequency divider, and looked to investigate the stability of the two oscillators. This did not work at all as expected.When the oscillators were set to different frequencies (> 2 ppb (20 mHz) difference) things worked mostly as expected.When the oscillators were brought closer together using EFC, they would lock together, and would stay locked until a few ppb of EFC was applied to pry them apart.I'm guessing that this is something called "injection locking", and is due to some sort of coupling between the oscillators. This was a little surprising to me, as the oscillators were powered from different power bricks, but obviously the grounds were connected at the timer board. So, I'm guessing that there is some sort of ground bounce or capacitive coupling which is somehow getting coupled across the ground wires.Perhaps I need some more filtering on the oscillators. At present, I've just been using the power bricks and 7805s, but is this the sort of situation where proper LC filters would help? Or is proper practice with these sorts of clock sources to use RF isolation transformers and just isolate everything?

author: David Hess
author_rank: Super Contributor
timestamp: January 07, 2022, 12:30:48 am
content: That is injection locking all right. I have encountered it several times before.My usual solution is to add RLC decoupling for power to the oscillator and associated circuits, but RLC decoupling to the circuit generating the interference also helps, and both are not a bad idea. Since the injection locking could also be coupled in through the outputs by variation in load, the outputs may need to be isolated with a buffer that provides reverse isolation.In a precision timing application, I would also consider precision regulation of the supply voltage to the oscillator using a reference.

author: WatchfulEye
author_rank: Regular Contributor
timestamp: January 08, 2022, 06:55:08 pm
content: Many thanks. Well, I feel like a fool.My oscillators had been connected up "dead bug" style (the pin-out didn't fit perfboard/breadboads), and I had an undetected broken ground connection, which disconnected the decoupling and the shield return for the output coax. The stricken oscillator was generating nearly 2 V of 10 MHz ripple on the output of its power brick.Repairing the ground connection, and strengthening the decoupling with some small inductors and electrolytic caps seems to have done the trick.I had been working on a better power supply for the oscillators. The plan is to build a PCB with proper RLC filter, precision regulator (probably an LM317 using external reference and error amp), DAC and output buffer onto a single board , which could then go into a thermally controlled enclosure if desired.