[time-nuts] Low noise power supplies?

[gary]

Are 4 and 5 regulated? Or is the zener picked to compensate for the VBE 
drops?

Any of these circuits where the output transistor is in an emitter 
follower configuration will have its noise effected by the load current, 
since that current directly effects the output transistor transconductance.

On 1/31/2013 10:03 PM, John Miles wrote:
> 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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