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

Rick Karlquist richard at karlquist.com
Tue Nov 22 14:47:10 EST 2011


The Hittite parts (there are two versions, one with 1 output and one with
4 outputs, HMC860 and HMC976) unfortunately have a foot note
on the data sheet that indicates
that the noise numbers are good in the application circuit, which
happens to include large capacitors.  The capacitors are really
doing the work, as indicated by that fact that the noise skyrockets
at low frequencies (or else the IC has a servere 1/f noise problem.
Therefore, the IC doesn't bring much to the party compared to
what you can do yourself with your capacitor and a decent low
noise op amp.  The Hittite data sheet says that the IC uses
a bandgap circuit.  A bandgap circuit is inherently noisy because
it requires that one of the transistors has to be operated at
very low current, and then the output of this transistor is
amplified by something like 30 dB and subtracted from the output.
Better to have a buried zener/avalanche diode.

Rick Karlquist N6RK

SAIDJACK at aol.com wrote:
>
> 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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