[time-nuts] Low noise power supplies for time nuts circuitry

Azelio Boriani azelio.boriani at screen.it
Tue Nov 22 15:45:47 EST 2011


Why is the shunt regulator push-pull? Because of the series regulator first
and the shunt regulator then?
On Tue, Nov 22, 2011 at 8:59 PM, <lists at lazygranch.com> wrote:

> If you want low noise voltage regulation, use a shunt regulator. If the
> load is low current, feed it with a floating current source such as a
> depletion mode jfet (or several in parallel). These are also sold as
> current regulating diodes.
>
> I've used this scheme in IC designs. You also find it in high end audio
> design.
>
> The shunt design is push pull. If the shunt is designed well, the ultimate
> high frequency feed through from the power supply, assuming ideal caps, is
> simply a capacitor divider based on the bypass cap and the capacitance
> across the current source.
>
> The bozos at Broadcom have actually patented using shunt regulation in
> their chips. Good luck enforcing that patent.
>
> -----Original Message-----
> From: SAIDJACK at aol.com
> Sender: time-nuts-bounces at febo.com
> Date: Tue, 22 Nov 2011 14:18:20
> To: <time-nuts at febo.com>
> Reply-To: Discussion of precise time and frequency measurement
>        <time-nuts at febo.com>
> Subject: Re: [time-nuts] Low noise power supplies for time nuts circuitry
>
> As Bob Pease used to say: Spice plots are good for padding bird cages, not
> much else :)
>
> Hittite has a fantastic new ultra low noise LDO for VCO's. Haven't had a
> chance to check that out, but it looks very promising. Has anyone tried
> that
> part here?
>
> Also, remember that those caps are microphonic most of the time, and could
> actually worsen supply noise in the presence of vibration.
>
> I get noise floors of below -170dBc with just simple RC or LC filtering on
> the power supplies, say 10 Ohms into 100uF Tantalum in parallel with some
> 10nF  to 100nF ceramics (or better Polyester caps), all following the
> typical
> Linear  Technologies low noise LDO's. That will cut off at 160Hz already,
> and go down at  40dB per decade if two filters are cascaded. Use two 100uF
> Tantalums or  22Ohms resistors for a <80Hz cut-off.
>
> At close-in frequencies, the crystal will likely be the worst noise source
> and overpower the supply noise by far. It's hard to get better than -108dBc
> at  1Hz, and -138dBc at 10Hz at 10MHz anyhow.
>
> For high frequency switcher noise, use shielded (TDK etc) 33uH inductors in
>  series to a 10 Ohm resistor, into 100uF Tantalums with 10nF and 220pF  in
> paralell. Cuts off <160Hz and has very good isolation at high  frequencies
> without radiating.
>
> For the best performance against supply noise, simply use differential
> techniques.
>
> bye,
> Said
>
>
> In a message dated 11/22/2011 10:52:29 Pacific Standard Time,
> phk at phk.freebsd.dk writes:
>
> In  message <6F9E458F-B701-49C1-8D83-EBDA35784764 at ulrich-bangert.de>,
> Ulrich  Ban
> gert writes:
>
> >It seems to turn out as if the well known  Wenzel suggestions for voltage
> >regulator finesse were not state of the  art [...]
>
>
> I've played a bit with the Wenzel circuits and they can  provide truly
> outstanding damping, my best was 80dB using a HP VHF  transister I can't
> remember the number of.
>
> And yes, they are very  sensitive to just about everything, in particular
> temperature.
>
> But  getting at good solid 30+dB damping is not _that_ hard, in particular
> for  very low-current constant loads, such as X-tal oscillators.
>
> --
> Poul-Henning Kamp       | UNIX since Zilog Zeus  3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC  956
> FreeBSD committer       | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by
>  incompetence.
>
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