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

[john.haine at haine-online.net]

Indeed, but remember that what seems to be "plenty of low pass filtering"
may not turn out as you intended.  A series L will be capacitive above its
self-resonant frequency and shunt Cs inductive above theirs.  Careful choice
of components and good layout is needed.  Since what you are aiming for is
to reject a single frequency it can be better to focus the filter's
attenuation around that frequency.

-----Original Message-----
From: Andy Talbot via time-nuts <time-nuts at lists.febo.com>
Sent: Tuesday, February 20, 2024 10:32 AM
To: Discussion of precise time and frequency measurement
<time-nuts at lists.febo.com>
Cc: Matt Huszagh <huszaghmatt at gmail.com>; john.haine at haine-online.net; Andy
Talbot <andy.g4jnt at gmail.com>
Subject: [time-nuts] Re: Injection locking via common power supply

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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