What to do about a glitchy OCXO frequency adjustment - Page 1
Forum section: electronics / general

author: rubidium
author_rank: Regular Contributor
timestamp: October 12, 2021, 07:03:28 pm
content: I have this Vectron 100MHz OCXO that hasn't been active for quite some time and, after keeping it powered up for about a week, I started to take a look at how stable it is. Unfortunately, the frequency adjustment screw (an internal pot) appears to have a glitch in "just the right place" where 100MHz should be. Making an adjustment is a lot more frustrating than the usual "sticking your tongue out." Sadly, this is a sealed unit, with the sole exception of a small capped hole to allow passage of the adjustment tool, so I don't see that replacing the pot is an option. Am I simply SOL?Also, I don't quite understand the apparent variation of the frequency with the termination that I place on end of my coax. I'm seeing a difference of about 0.1Hz between these 2 setups: 1. The line ends with a tee and a good-quality 50R terminator on my 53123A counter with 1Meg input impedance selected, and 2. The line ends directly at the 53132A with 50R input impedance selected. Should this OCXO be that sensitive, or am I perhaps mistakenly trusting that my 50 Ohm terminator isn't really 50 Ohms at 100MHz? Even so, why does the frequency change with termination?Jim

author: Kleinstein
author_rank: Super Contributor
timestamp: October 12, 2021, 07:17:13 pm
content: At 100 MHz the 1 M input impedance would not be really 1 M anymore - chances are there are some 20 pF or so that add to this. If you are lucky the 50 Ohms setting is actually close to 50 Ohm and not just 50 Ohm + the input.At 100 MHz the amplifier inside the OCXO will not have perfect reverse isolation. So a wave coming back has a good chance to also act back on the oscillator. This could be directly though the 100 MHz or harmincs part or indirectly loading the supply slightly different. So the refected wave changes the current the sources has to provide and this changes the supply current to effect the oscillator.Depending on the counter used for the adjustement, it could be the counter that is a bit sticky / nonlinear.Sometimes a pot get better when more a few times a little more than just the tiny bit aroung the point where you try to find the best spot. Chances are one would have to wait a little before one can read the frequency. The adjustment tool may also effect the frequency. So this may end up in a try and error mode.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 12, 2021, 07:46:35 pm
content: At 100 MHz, the Tee could be distorting the signal enough to give an error. Remember that the counter is counting the number of trigger points, i.e., the number of times the signal crosses the trigger threshold. If the signal is distorted for any reason, you could see more or less trigger pointsAlso, every oscillator has a spec for frequency change vs. load. A quick check showed a Vectron 100 MHz low phase noise oscillator with a spec of 10 ppb (1e-8) for a 5% change in load (OX-305). So, the change of 0.1 Hz (1e-9) that you're seeing doesn't seem unbelievable.Ed

author: bob91343
author_rank: Super Contributor
timestamp: October 12, 2021, 11:23:00 pm
content: If the pot seems to have a bad spot, you might exercise it to clear that. It's worked for me many times. At worst you could drip in some solvent and then exercise it again. Perhaps that will move the desired spot to a good place, or just polish it.

author: rubidium
author_rank: Regular Contributor
timestamp: October 13, 2021, 02:03:15 pm
content: Thanks everyone for the insights. I agree with the arguments on the load impedance - given the specs - which I failed to consider myself.I did try the usual practice of "rotationally exercising" the pot, but to no avail. There's just a bad spot there. The variation in frequency isn't even monotonic in the immediate neighborhood of that spot, where as frequency adjustment is well behaved on either side of that spot beyond the problem zone. I'll just need to tweak it the best I can and live with it. BTW, this OCXO is the time reference used in a Wavecrest DTS2070 that I acquired about a year ago and noticed the adjustment issue then. The frequency back then was about 3Hz off, but I just let it go for the time being. But I recently had to go into the unit (which weighs a ton by the way) to replace an apparent failed electrolytic in a 24V power supply that, among other things, actually powers this OCXO. While in there, I thought "let me adjust the frequency". Isn't there an old adage "let sleeping dogs lie"?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 13, 2021, 03:34:22 pm
content: If the problem is really just the pot / specific point, one could try to shift the frequency in a different way (e.g. with the load or supply voltage).

author: edpalmer42
author_rank: Super Contributor
timestamp: October 13, 2021, 06:19:34 pm
content: I have a DTS-2077 so I agree that it does weigh a ton - and it sounds like a jet engine!If the existing oscillator won't play nice, you could always replace it with one from ebay. It might also be worthwhile to upgrade it with a module that allows an external 10 MHz input to phase-lock the 100 MHz output. You can buy these from ebay in multiple designs from multiple sellers. I've always been annoyed that the DTS-2077 is the only piece of time/frequency equipment in my lab that can't be syncronized to a master source.Ed

author: rubidium
author_rank: Regular Contributor
timestamp: October 14, 2021, 01:46:59 am
content: That's exactly my dismay, Ed. Everything else in my lab can take a 10MHz external reference. I did look on eBay as you suggested and see a bunch of 100MHz oscillators phase locked to 10MHz reference inputs. Hmmm ... considering ...

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: October 14, 2021, 05:29:37 pm
content: Just recently completed a small circuit with help from (seller rf_gadgets on Ebay) for converting a Sync 10MHz GPSDO signal to enable a synchronized output at 60KHz... enabling obsolete WWVB Reference Standard radios to once again "see" the stable 60KHz signal, giving some lost functionality after NIST converted to BPSK. Works well...