In message <[EMAIL PROTECTED]>, "Tom Van Baak" writes: >Warm-up time -- > Many Rb will lock in 5 minutes, typically. Some Qz > take much longer to get on-frequency from cold start. > This can simplify the initial loop locking algorithm.
Initial capture is best done with a looser timeconstant in any circumstances. Note that the integrator terms initial condition is undefined in a PLL, this can be used to achieve lock without the initial overshot: Clamp the integrator term to zero and let the proportional term drag the offset into range (use a high rate for this, there is no stability issues). Once the second derivative of the offset approaches zero, unclamp the integrator and switch to a normal but loose set of constants for the PLL. With properly chosen values, you can drag any frequency source into submission of a PPS that way in a fraction of a minute. After this the PLL can adapt its constants based on the statistics (remember what I said earlier about looking at the ADEV shape). >Power consumption -- > Probably Qz-based GPSDO have much lower power > consumption than Rb. Single oven: Maybe, double oven: certainly. >Hold-over performance -- > For mid- to long-term, Rb is vastly superior to Qz; > most Rb have daily drift rates 100x better than Qz. I is likely to be the difference between replacing the GPS antenna now or after the snowstorm is over. Given the price difference, this may be a no-brainer advantage to the Rb. >Stand-along performance -- > Without GPS lock, a free-running Rb can be trusted > to be orders of magnitude more accurate than Qz. Also, if the qz in the rb jumps, the Rb is very likely to tell you it lost lock. A Qz unit will jump and you will not know it, unless the resulting phase jitter kills your microprocessor or similar. >Environmental -- > Is it the case that Rb is less sensitive than Qz to > extreme environments? No significant difference with proper design. The Rb's cooling requirements are tricker to design for than an Qz units "bolt down and forget". >Cost -- > As a rule, Qz-based GPSDO are cheaper than Rb. That's actually not a given. For a decent Qz performance new price approaches $1k and a PRS10 is only $1.5k. >Phase noise -- > I'd guess that Qz-based GPSDO could have better > short-term stability and phase noise than Rb. Depends on your PLL more than anything else. >Lifetime -- > Is the MTBF of Qz much longer than Rb due to fewer > parts and simpler design? Yes, no chemical stress and with proper drive levels, no mechanical stress either. -- 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 time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts