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

[Bob Camp]

Hi

A very normal way to do this is with decent ferrite beads / cores. Yes, they will be a bit bulky if you don’t want them to saturate. You also need to get the right material for your target frequency range. Since they are going to be resistive over the “target frequency range” there is very little concern about resonance. 

If your capacitor is down in the < 10 mili ohm range, a bead that provides “only” 10 ohms would give you > 60 db of attenuation. 

Since you also want a low source resistance for your supply, the “zero impact” of an add on bead / core has other advantages as well.

Bob

> On Feb 22, 2024, at 8:54 AM, john.haine--- via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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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