On 04/26/2014 06:34 PM, Said Jackson wrote:
Shane, The trade off for most applications is as follows: Rb has much faster stabilization time after power on. Ocxos suffer from retrace, that can take hours to days to get rid off. Retrace could cause a frequency shift of several ppb or more from say 15 minutes after power on compared to 10 hours after power on Retrace on a good Rb is very low, turn on a PRS-10 and after 10 minutes or so it will be stable and drift much less than 1ppb over the next days typically. 10x to 50x less retrace than on a good ocxo Is possible. This is important when you do not have a GPS to remove retrace error from the ocxo. If you run without gps (holdover) the best docxo will start to drift more and more after a day. Rb will stay stable for months or years. Important for base station applications where the amount of drift determines how much time can pass before a repair crew has to be sent. They do not want to send crews over the weekend for example because it could cost double overtime pay. Because the loop BW of a Rb is larger than a GPSDO (say 10Hz vs 0.001Hz) a typical Rb will have higher ADEV noise close in than a really good GPSDO due to the loop steering noise being additive to the ocxo noise. This is why a GPSDO can have significantly lower phase noise below 10Hz. But it depends on the Rb. For example the CSAC which works on the Rb vapor cell principle actually improves noise close in as Rick explained because it has a fairly low cost tcxo and the vapor cell thus is more stable than the tcxo by itself. On a PRS-10 one can see a steering hump below 1 Hz (around 20s or so depending on the selected loop time constant) that probably would not be there without the loop steering.. Most of the time Rbs are used because they require much less calibration, have much less g (tilt) sensitivity and much less initial retrace/warmup error. In the case of the CSAC they also have more than an order of magnitude less power consumption than a good Ocxo (0.12W vs 1.7W on a typical docxo)
The PRS-10 have a nice little trick in it, it stores the previous OCXO steering value, so on power-up it sets the OCXO to this and that gives it about the right frequency and only once the rubidium have heated up it locks to it. That gives a relatively quick stable signal for starters, which is quite quick.
Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
