It can take a GPS disciplined oscillator a while to stabilise and settle down. When I first tested my new free running rubidium reference the GPS disciplined rubidium reference had spent no more than an hour making it’s observations and corrections. So I left it running over night with the intention of measuring things again in the morning.
So after 16 hours the frequency error has reduced more than three decades, should be good to go. I also have a precision TXCO reference (SDI FEL-10A) so I thought I’d measure this first and see what sort of stability and accuracy this can achieve. Both the free running TXCO and Rubidium were switched on 30 minutes and left to stabilise before measurements were made;
So that is quite a respectable result, being within +0.318Hz of our 10MHz target. Then for the main event it was time to re-measure the free running rubidium;
So this is a little higher that what was measured last night, but +0.009Hz or +9mHz is still a great result. To put this into perspective if I were to use this reference to lock a series of transverters this is the final frequency accuracy I would expect;
F(carrier) | F(error)
[MHz] | [Hz]
50.195 | +0.45Hz
144.135 | +1.30Hz
432.070 | +3.88Hz
1296.070 | +11.66Hz
I think if I were to go higher than 70cm then a GPS disciplined oscillator is in order (there is a Trimble Thunderbolt in a box somewhere), at least I have some idea now what the upper workable limit is. One of these days I may yet get in and tweak the external C-Field input and see if I can tighten this frequency error up a little, that will have to wait for a 12-digit counter.
The free running TXCO also has a reference input so for a laugh I attached the free running Rubidium to the TXCO and waited to see what happened. The lock LED lit pretty much instantaneously and the result was;
Exactly the same as the Rubidium on it’s own. What I do like about the TXCO is the 6 channel output with >120dB of isolation between channels. At least now I have some idea how stable this oscillator is, so now I can start thinking about what I’ll do with it.