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

[Andy Talbot]

Or just plenty of L/C low pass filtering in the supply rails
Saves having to tune a notch

Andy
www.g4jnt.com



On Tue, 20 Feb 2024 at 10:25, john.haine--- via time-nuts <
time-nuts at lists.febo.com> wrote:

> 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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