); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Tom Van Baak said the following on 09/24/2007 08:57 PM:
> I would think this is especially true for non-local frequencies, > such as one received over the air. I'll leave it to you FMT > guys to comment on the magnitude of degradation due to > transmission and reception noise. Absolutely. Probably the best real-world performance you can get with a skywave signal is on the order of 0.01 Hz. Propagation effects play havoc, but the longer the averaging period, the more short-term effects will average away. One of the reasons for the fairly long transmission periods is to both allow longer averaging, but also provide the opportunity to observe the atmospheric conditions at work. > While were at it, in the case mentioned above I'm a curious > about their FMT frequency standard -- if it's really accurate > to parts in 10^12, as they imply, over 10 minutes. I could > believe this if it were an Rb or Cs-based GPSDO. We're using an Austron 1250A OXCO that's been measured as better than 9x10e-13 for averaging times of 1 second out to 1000 seconds; over a broader range, it's better than 3x10e-12 from 0.1 seconds to 40,000 seconds. Now, an important point -- we're not trying to trim the Austron to be precisely on frequency. We're going to let it run at whatever offset it happens to be. That will help make sure that the signal doesn't have lots of zero's at the end, even though the resolution of the synthesizers driving the transmitters is limited to 0.1 Hz. We'll be comparing the Austron against a Z3801a (via my TSC-5120A analyzer) and logging the frequency difference for at least several hours prior to the test until several hours following. The TSC gives 16 digits over 1000 seconds; depending on how much jitter we see, we'll probably throw away the last two or three. Even though the Z3801A may be wandering around a bit, with successive 1000 second measurements we should have confidence in the actual frequency over 1000 second periods to at least parts in the 12s, ultimately limited by the Austron's stability. But since that's known to be in the 13s over the averaging period of interest, we think we're safe in claiming accuracy and stability of parts in the 12s. Tom, if I'm missing something in this analysis, I'm seriously open to education... By the way -- the synthesizers used to drive the transmitter amplifiers will be PTS 250 SX-51 low noise units, so hopefully the transmitted signals will have a better-than-the-average-ham-rig phase noise. The synthesizers will directly feed the driver and final amplifier stages of some vintage Kenwood TS-520 ham transceivers with no other mixing -- it'll purely be the synthesizer and a transistor buffer amp driving two vacuum tube stages to get up to about 75 watts (the rigs can run 100 watts, but we're derating -- and adding fans -- to support the long key-down times). John _______________________________________________ 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.
