[time-nuts] Re: Injection locking via common power supply

[Bob Camp]

Hi

RF running back thru the supply lead is pretty unlikely. You have a lot of regulation and bypassing between the oscillator and other internals to that supply. If it worries you, put more bypass on the supply pins to the OCXO. You always should put some sort of simple bypass  there when you wire things up. 

RF looping through a common ground can always be a problem. That’s going to be a problem pretty much whatever you do. Handling that needs to be looked at as a separate issue. 

Injection locking when running two OCXO’s into one device (like maybe a phase detector in a DMTD) is indeed quite possible. This is a signal coming right back through the RF output pin. The power supply really isn’t in the act. Isolation amplifiers are the answer here (and possibly other items like transformers and common mode chokes ….). 

While we’re talking about wiring up OCXO’s and weird problems, here’s another one.It is a completely different issue that really isn’t injection locking at all (but is a form of locking ….):

OCXO heaters are a negative resistance. They pull more current as the voltage drops. This is the same thing as saying they are a constant power load. As we all know negative resistance is one of those things that you need to make an oscillator work …. hmmm …...

If you have a rack full of OCXO’s they pull more than a little bit of power, even after warmup.

Plug them all in through some (highly) questionable connectors and (badly frayed) wires. You get a bit of resistance between the supply and your rack of OCXO’s. Just how much resistance it’s going to take and how obvious those issues with the cables are ….. very much a “that depends” sort of thing 

That combination can indeed get the whole rack full of OCXO’s swinging back and forth current wise. They will “oscillate” up and down at some sort of 10’s of seconds sort of period. The period is going to depend on the mass of the oven, some fiddly details about the thermal gain of the controller, and the amount of resistance involved. 

If you have a current meter on your supply, it will be easy to spot. With just a voltage at the supply …. not so much. A good DVM on the rack of units will indeed spot what’s going on. 

Simple answer is to attack the offending cables and connector with a hammer. (yes, remove them from the supply and rack first …). Then toss them in the trash. Once you put in proper cables …. problem solved. 

If you don’t both destroy the cables *and* throw them away, they will somehow pop back up a few weeks later creating the same problem. I have data on this. Do they creep back and re-install themselves? Maybe they do …. :) :) :) 

Is this a rare problem to run into? Sure it is. It’s not exactly easy to spot without that current meter ….

Fun !!!

Bob

> On Feb 20, 2024, at 2:13 AM, Matt Huszagh via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> To what extent is oscillator injection locking a risk when powering
> multiple oscillators from the same power supply?
> 
> I measured a couple OCXOs to see how much of the output signal made its
> way back into the power supply input. A Datum 1000B showed a 10 MHz
> signal of -53 dBm for a +13 dBm output. A Wenzel 10 MHz ULN gave a
> similar result. Curiously, the Wenzel showed much better isolation
> (-74 dBm) with the output unterminated.
> 
> If we compare that to the reverse isolation of buffer/isolation
> amplifiers, that seems to put the power supply isolation in the "ok"
> territory, but possibly still a risk, depending on other characteristics
> of the OCXO. Since the PSRR of many linear regulators is quite poor at
> 10 MHz (and even worse at higher typical OCXO frequencies), I would
> expect using separate final regulators for each OCXO is probably only
> slightly helpful. The same is probably true for the regulators internal
> to the OCXOs themselves. There are exceptions to this, though. For
> example, the LT3045 provides 50 dB PSRR at 10 MHz, assuming the layout
> recommendations are followed.
> 
> I found Bruce's analysis page on injection locking
> (http://www.ko4bb.com/~bruce/InjectionLocking.html) quite helpful in
> this process. Unfortunately, I don't often know the Q factor of a given
> OCXO or the amplitude of the tank circuit, so I just tried to guess.
> 
> Any thoughts on ways to minimize the risk of injection locking via power
> supply connections? Am I right in thinking this is a genuine concern? I
> was hoping to place a couple Datum 1000Bs in an enclosure and power them
> all from the same AC/DC supply. Each would use separate LT3045 final
> regulators. Given the good PSRR of that regulator, I should be ok, I
> think. But I'm also curious about cases where the PSRR of the linear
> regulator cannot be relied upon. I suppose ferrite beads in the form of
> pi filters could be used, but ferrite beads generally aren't as
> effective at 10 MHz as they are at 100 MHz. There are also DC power line
> filters, sold by Schaffner and others. These can be quite effective at
> 10 MHz. I suppose these could be used if really needed.
> 
> Anyway, curious to hear thoughts and ideas. Thanks!
> 
> Matt
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