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

[john.haine at haine-online.net]

How about putting a band-stop filter with a notch at the oscillator
frequency but passes DC in the supply to each oscillator, as close as
possible to the supply pin?

But I think you would need anyway to lay out the supply and ground
connections very carefully to make sure there isn't any signal bypassing the
filter.

John.

-----Original Message-----
From: Matt Huszagh via time-nuts <time-nuts at lists.febo.com>
Sent: Tuesday, February 20, 2024 7:13 AM
To: Discussion of precise time and frequency measurement
<time-nuts at lists.febo.com>
Cc: Matt Huszagh <huszaghmatt at gmail.com>
Subject: [time-nuts] Injection locking via common power supply

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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