[time-nuts] Low noise power supplies?

Max Robinson max at maxsmusicplace.com
Fri Feb 1 19:53:32 UTC 2013


This is a keeper.  Note the strong peak at 60 Hz for the unfiltered 
darlington.

Regards.

Max.  K 4 O DS.

Email: max at maxsmusicplace.com

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----- Original Message ----- 
From: "John Miles" <jmiles at pop.net>
To: "'Discussion of precise time and frequency measurement'" 
<time-nuts at febo.com>
Sent: Friday, February 01, 2013 12:03 AM
Subject: Re: [time-nuts] Low noise power supplies?


> Awhile back I ran some baseband plots of various supplies with an HP 3048A
> (image attached).  In my experience measuring actual OCXOs, an LM317T or
> LM338K is quiet enough to avoid influencing oscillator PN.  With these
> variable-voltage parts, you can bypass the reference pin for some 
> additional
> improvement, but I don't believe I did that for these plots.
>
> It's easy to spot the difference between a 7812/7815 and an LM317T (see 
> red
> versus green/white traces).   As a lazy approach, try measuring the
> oscillator with both a 78XX and an LM317T.  Because the 78XX is about 10 
> dB
> noisier across most of the spectrum, If you don't see a difference, you 
> can
> assume that further optimization is pointless.  Near 1 Hz this call may be
> questionable.
>
> If you don't need an LDO, don't use one.  If you do, use the quietest part
> you can find.  The best LDOs seem to be about as quiet as an ordinary
> LM317T.
>
> I've mentioned before that you need to be careful with large LC filters
> downstream of the regulator.   A good power source will exhibit a low
> impedance at ALL offsets of interest.
>
> You sometimes see NIST circuits where the power is conditioned by a
> Darlington emitter follower whose base is fed with an RC-filtered Zener
> diode.  The purple and orange traces are pretty informative with regard to
> that approach.  On the orange trace, where the only filtering is the RC
> network between the Zener and the base, notice how the noise becomes worse
> than all of the other sources below 10 Hz.  Here, the RC filter on the 
> Zener
> becomes less effective and the Darlington pair obligingly amplifies the
> diode noise.
>
> An additional LC filter after the regulator may have the effect of herding
> the entire noise spectrum into a high-Q peak, even though the LC corner
> frequency is much higher than the RC filter in the base circuit (violet
> trace).   Depending on your OCXO's supply rejection characteristics this
> could be a good thing or a bad thing.
>
> Finally, make sure the OCXO has good RF bypassing where its power supply 
> pin
> enters the case.  If in doubt, solder a 0.1 uF ceramic right at the point 
> of
> entry.  I've seen $2000 Wenzels that didn't bother doing this.  I'm sure
> they looked good in a screen room.
>
> -- john
> Miles Design LLC
>
>
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-
>> bounces at febo.com] On Behalf Of Richard (Rick) Karlquist
>> Sent: Wednesday, January 30, 2013 6:17 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: [time-nuts] Low noise power supplies?
>>
>>
>> I know this topic has been discussed in the past on the list, but
>> a colleague is asking if there are any off the shelf low
>> noise power supplies for testing oscillators.  Something
>> a cut above an HP "brick" lab power supply etc.  They are hoping
>> to avoid having to homebrew a power conditioning circuit.
>> Did we ever arrive at a concensus as to the state of the art
>> in homebrew power conditioning circuits?
>>
>> Any help would be appreciated.
>>
>> Rick Karlquist N6RK
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-
>> bin/mailman/listinfo/time-nuts
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>


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