I've played with the Hittite chips before and obtained PN results in the same ballpark (see http://www.ke5fx.com/hpll.htm ), but at 8 GHz rather than 6 GHz. To save further head-scratching, the figure of merit on these chips works like this:
In-band phase noise in dBc/Hz = FOM + 10*log(Fcomp) + 20*log(N) This is the best-case noise level that you will get assuming a perfectly-clean reference and no VCO noise contribution. As usual, Hittite's less-than-ideal data sheet doesn't make that relationship clear. Specifically, the 5.8-GHz integer-N plot in figure 1 appear to have been made with a 50 MHz comparison frequency and N=116. -107 dBc/Hz - 77 - 41 = -225 dBc/Hz. >From that relationship, you can see that minimizing N is the most important thing you can do. Good-quality signal generators work on the basis of minimizing N at all costs. Fractional techniques are common but the ultimate performance still comes from cascaded or nested synthesis stages with an integer-N output loop. The HP 8672/8673 family, for instance, uses a comparison frequency (really a harmonic sampler drive signal) in the 200 MHz neighborhood. They still don't achieve inband PN better than about -100 dBc/Hz, because the earlier stages that generate the Fcomp signal are relatively noisy. It's safe to say that your R&S synthesizer uses a similarly-low N factor, cleaner reference synthesizers, and a cleaner output loop. The easy way to get a few more dB out of the Hittite chip would be to run with an Fcomp in the 100 MHz vicinity instead of 50 MHz. It is rated for Fcomp <= 140 MHz in the (quieter) integer-N mode. -- john, KE5FX > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > Behalf Of Matt Ettus > Sent: Tuesday, July 22, 2008 1:16 PM > To: Discussion of precise time and frequency measurement > Subject: [time-nuts] Test equipment-level phase noise PLLs > > > In looking into extremely low phase noise synthesizers, I have come > across the new HMC700LP4 chip from hittite, which seems to have the > best figure of merit I have found, -227 dBm/Hz. That gives you > -107dBc/Hz at 20 kHz offset at 6 GHz according to the datasheets. > > That sounds amazingly good, but my R+S signal generator does better. > Do they use a different sort of architecture? Do they not use > conventional dividers? Some other sort of phase detector? > > Thanks, > Matt > _______________________________________________ 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.
