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 [email protected] 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, > [email protected] writes: > > In message <[email protected]>, > 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 > [email protected] | TCP/IP since RFC 956 > FreeBSD committer | BSD since 4.3-tahoe > Never attribute to malice what can adequately be explained by > incompetence. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
