Help with eliminating crosstalk between two precision Quartz oscillators - Page 1
Forum section: electronics / rf-microwave

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: March 18, 2021, 02:31:17 am
content: Hi,I'm working on a project and experiencing some crosstalk between two 5Mhz Quartz oscillators.Crosstalk won't be a problem in the finished unit as the oscillators will be phase locked.The oscillators will be in close physical proximity.However it would be nice if I could get rid of it so I can do testing on the oscillators before I enable the phase locking.At first I mounted each oscillator on its own subassembly in the chassis.The oscillators are in hermetic cans with PC mount pins sticking out.If I connect either oscillator to my DMTD unit and run phase plots I can see the beat note between the units.I then regrouped and installed the oscillators into shielded cans with feedthrough caps for the DC power signals and SMB connectors for the RF out, the Ref voltage out and the EFC in.Externally the Ref voltage and the EFC connectors use shielded cables from and to.Each oscillator has its own 7812 voltage regulator.Also attached a 50Ohm load the oscillator output I was not testingStill had the crosstalk!Then I added some bypassing internal to the module but this did not help either.The caps are all .1uf except one on the output of the 7812 that is a 1uF Tantalum.The inductors are 220uHAny ideas on how to reduce or eliminate the crosstalk?If I can't figure it out I'll have to power only one oscillator at a time for testing.Once the phase locking is working the issue will be moot!Cheers,Corby

author: geggi1
author_rank: Frequent Contributor
timestamp: March 18, 2021, 05:31:24 am
content: Looking at the schematics I would start with the powersupply.Maybe go for a bifilar choke on the input and get the Vreg inside the same shield or duble shield the whole circuit.Do also look at how the ground is made.If the oscillator is sitting on a project board or a PCB without a propper groundplane yu can get problems.Post some pictures of your setup.

author: LaserSteve
author_rank: Super Contributor
timestamp: March 18, 2021, 04:59:01 pm
content: Power up the one oscillator off a pair of 9V batteries and a discrete, loose, regulator and see what happens. . I fear you may have mechanical phase lock as well as electrical. I've had a 32.768 Khz tuning fork crystal start resonating and syncing to another passively up to a meter away from the active crystal. Had the bare part coupled to just a scope probe, get it close to the active one and it starts ringing with no direct drive. The Q is that high.Steve

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: March 19, 2021, 06:38:37 pm
content: Here are a couple PIX,The oscillator module case is connected to the electronic ground pin.The new case is also connected to electronic ground.Case is a conducting media.Cheers,Corby

author: TimFox
author_rank: Super Contributor
timestamp: March 19, 2021, 06:55:34 pm
content: Since crystals have an extremely highQ, it takes very little coupling to disturb the oscillations. Generally, with weak but sufficient coupling between two oscillators, as you tune one towards the other’s frequency, there will be a discontinuous jump where the two oscillators quickly lock to each other.

author: Grandchuck
author_rank: Super Contributor
timestamp: March 19, 2021, 09:03:01 pm
content: Entrainment:https://www.math.ucdavis.edu/~jasnyder/Coupled_Entrainment_Poster.pdf

author: TimFox
author_rank: Super Contributor
timestamp: March 19, 2021, 09:26:53 pm
content: I first encountered this many years ago in a discussion of piano tuning. A typical piano key hits three identical strings very close to each other, and the sudden disappearance of a beat as you tune one towards another is the same phenomenon.

author: David Hess
author_rank: Super Contributor
timestamp: March 21, 2021, 05:18:53 am
content: I have had that problem before with oscillators injection locking through the common power supply connection. My solution was to add LCR decoupling at each module to isolate them from the power supply and each other. Separate regulators are unlikely to be good enough because of limited line regulation at high frequencies.An interesting alternative is sometimes to use a notch filter for decoupling to remove the offending harmonic.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 22, 2021, 06:37:45 am
content: You could try xformer coupling the output of one or both oscillators to see if there's a ground loop involved. A few years ago I picked up a few Mini-Circuits FTB-1-1 transformers for just this application. They're intended for balanced to unbalanced conversion, but the effect is to break the ground loop.What level of crosstalk are you seeing? Is it possible that you've found a low-level problem in the DMTD unit?

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: March 22, 2021, 11:53:35 pm
content: Ed,Hard to Quantify the level as I'm observing the beat on the DMTD phase plot. I'm testing another fix right now. If it does not work I'll try the TX idea.Probably will then just proceed on the build and just put up with the crosstalk during initial testing.Once everything is locked up the crosstalk will no longer be a problem.I don't think it's a DMTD problem as I can test all my other oscillators without seeing this effect.Also if I power down the one I'm not testing the beat of course disappears.Cheers,Corby

author: Wolfgang
author_rank: Super Contributor
timestamp: March 23, 2021, 12:01:56 am
content: Quote from: 5065AGuru on March 18, 2021, 02:31:17 amJust working on a thesis about 5MHz oscillators. Some tips:- an 78XX has much to little PSRR. What you need is a LM723 supply with PNP pass transistor, followed by a cap multiplier filter.- you need buffer amps with extremely good reverse isolation. The buffer amps also need a superb low noise PSU.- paranoic shielding is a must (tinplate, soldered). If you can, use a MuMetal box for magnetic shielding as well.- for testing, independent battery PSUs would be great.- Put the oscillators physically apart.have funWolfgang-

author: bob91343
author_rank: Super Contributor
timestamp: March 23, 2021, 04:01:58 am
content: All the replies look reasonable. The decoupling is very important and ground is not ground. You need to establish a ground for each oscillator and keep it separate from the other one. The ground to the power supply could be a small inductor from each oscillator ground with bypasses everywhere.As for the piano string analogy, I have never had that experience. I've been tuning my piano for years and it's always tough finding zero beat among three strings. If they would lock it would make it a lot simpler but it's never done that for me.

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: March 23, 2021, 05:19:40 am
content: Wolfgang,Thanks for the thoughts.At 5 MHz PSRR looks to be hard to improve no matter how you do it.In the test setup the isolation amps have around -115db isolationAs you can see in the PIX the original hermetically sealed can oscillator which is ferrous is surrounded by the outer enclosure also ferrous, conductive, and tightly sealed.I may see if I can field a couple batteries to try that just to see what happens.As far a physically apart, that will not be possible.Bob, Not sure if I can "float" the oscillators to do independent grounds. Currently each oscillators ground pin runs all the way back to a "star" ground. I thought maybe trying a couple clip on ferrites around theground and power wires.Will see how my latest try works this week, another project just took over and bombed my workbench!Cheers,Corby

author: Wolfgang
author_rank: Super Contributor
timestamp: March 23, 2021, 11:16:27 am
content: Hi,why is the PSRR so hard to improve ? If you look at Wenzels, or HH there are some ideas to do that.I have improved some commercial atomic clocks (RS XSRM), and I was impressed about the amountof PSU decoupling and shielding. Everything is not just a metal can, but MuMetal win inner cans.You probably know the video about the metronomes on a plate ?Even in test equipment (RS FSWP) unwanted oscillator coupling is the limiting factor forimprovement by correlation. One factor is the synchronizing effect of common vibrations (fans).Thats why I suggested physical separation.RegardsWolfgang

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2021, 01:44:34 pm
content: One possibility to reduce coupling, after good power-supply decoupling, is to couple the outputs optically, possibly through a fiber through a long copper pipe. One paper I heard many years ago (about reducing coupling between a master oscillator and a high-power amplifier at the same frequency) pointed out that there was a microscopic coupling back to the LED from reflected light, but that it was not important.

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2021, 05:15:37 pm
content: One link about "oscillator injection locking" between crystal oscillators with almost equal free-resonant frequencies:http://www.ko4bb.com/~bruce/InjectionLocking.html

author: edpalmer42
author_rank: Super Contributor
timestamp: March 23, 2021, 08:14:55 pm
content: I found some interesting info on how clean the signal environment needs to be to get a clean ADEV graph. This is an HP App Note.Application Note 358-12: Simplify Frequency Stability Measurements with Built-in Allan Variance Analysishttps://www.hpmemoryproject.org/an/pdf/an_358-12.pdfAlthough this is aimed at the HP 5371/5372 Frequency and Time Interval Analyzer, it includes a discussion on how interfering signals can show up in the results.

author: fcb
author_rank: Super Contributor
timestamp: March 23, 2021, 08:47:19 pm
content: Agree with another poster. 7812 etc.. filtering at 5MHz is virtually zero.But are you sure that the leakage isn't through whatever you are using to measure the two signals?