[time-nuts] Low noise power supplies?

[John Miles]

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
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