From: "Didier Juges" <[EMAIL PROTECTED]> Subject: Re: [time-nuts] FMT on October 13 Date: Wed, 26 Sep 2007 05:57:14 -0500 Message-ID: <[EMAIL PROTECTED]>
Didier, > I just wanted to point out that with reciprocal counters, you can get > resolution much better than the 1Hz/s you would get with conventional > frequency counters, even though the actual accuracy of the measurement may > be way off. These days conventional counteras are reciprocal counters. It is only the old-school counters which is not reciprocal. Nothing wrong with old-school, but a conventional counter of the shelf today is probably a reciprocal jobbie. > The original question seemed to imply that with a short transmission time, > you could not guarantee a frequency accuracy of 1e-6 Hz, which you probably > can't anyhow, but the limit is not the resolution of the instrument or the > measurement method. > > I do not know how far off calibration my HP 5370s are, but the 20pS > resolution is at best only usable under some circumstances that I have not > isolated yet, due to jitter. > > When measuring a 3.5 MHz signal (@1dBm) from my HP 8657B through 1 meter of > good coax cable (with counter and generator phase locked to the Thunderbolt > GPSDO) in Frequency mode with a 1s gate time, the resolution is 1e-5Hz, with > about 1e-3Hz p-p variation. When measuring over 1 period with 10,000 periods > sample size, the resolution is only 1e-1Hz with a standard deviation of ~400 > Hz (or about 0.1%). Of course, over the air, it will be much worse due to > noise, let alone propagation, fading and multipath. When measuring over a longer period you see a different spot on the ADEV/MDEV curve. Chances are that you are more unstable there for an OCXO. Both linear and noise products will make things harder. It can be a challenge to separate the drift rate due to signal path shifts and that of the OCXO. Cheers, Magnus _______________________________________________ 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.
