Rubidium frequency standard not locking - Page 1
Forum section: electronics / general

author: jadew
author_rank: Frequent Contributor
timestamp: February 27, 2014, 07:32:11 pm
content: Hey guys,So I got a FE-5680A, it turns on, it heats up, it starts sweeping and... that's it. It just keeps sweeping from 9.99986 MHz to 10.0001 MHz (according to my scope).I opened it up, tried playing with C217 as described here:http://ko4bb.com/dokuwiki/doku.php?id=precision_timing:fe5680a_faq#what_if_my_5680a_output_does_not_lock_up_after_several_minutesStill couldn't get a lock and I couldn't get it past 10.0001 MHz, so either my scope was reading fine and the standard does hit 10 MHz or my scope is wrong and the VCXO can't get to 10 MHz anymore.I also checked the lamp and it seems to lit up (violet kind of light).Any suggestions?

author: babysitter
author_rank: Frequent Contributor
timestamp: February 27, 2014, 08:32:08 pm
content: Does your scope read 50 Hz when connected to a mains transformer ?

author: jadew
author_rank: Frequent Contributor
timestamp: February 27, 2014, 08:55:18 pm
content: Quote from: babysitter on February 27, 2014, 08:32:08 pmI don't have a transformer around (as sad as that may sound), but I doubt the main frequency is that exact... it would have to be better than 10ppm in order to tell me that my scope's counter was off by 100Hz when measuring the 10Mhz signal.Am I missing something?Thanks

author: babysitter
author_rank: Frequent Contributor
timestamp: February 27, 2014, 09:01:28 pm
content: Although mains is quite accurate over longer periods, it is not on a short timescale due to load-dependency.But the momentary mains frequency can be monitored, swissgrid for example gives it for the european network, you can compare your reading to theirs.Another question which might be interesting for crowdsourced debugging: Does your current consumption at the 16V level drop during heatup, does it stay high, starts it off low or does it oscillate ?If you were closer, I would say just come along and lets compare, are there any hams or a university with EE around ?

author: jadew
author_rank: Frequent Contributor
timestamp: February 27, 2014, 09:06:43 pm
content: I see, thanks for the answer.The current starts high (about 1.7A) and drops down to about 600mA as it heats up. It oscillates a bit (several mA) when the sweep frequency changes.

author: babysitter
author_rank: Frequent Contributor
timestamp: February 27, 2014, 09:20:40 pm
content: High start and drop is typical, i guess mine is similar. recorded the power consumption during start-up of mine some day, dont know if i still have the file (was only a test for readout from Power supply) and my unit also shows this oscillation when hot and sweeping. Measuring for this oscillation would take some days until i get to my lab.

author: jadew
author_rank: Frequent Contributor
timestamp: February 27, 2014, 09:45:05 pm
content: Good idea, gonna record it as well.

author: casinada
author_rank: Frequent Contributor
timestamp: February 27, 2014, 10:13:19 pm
content: Those devices have a useful life, at some point they get tired for a lack of a better term.I'm sure the unit was used and had thousands of hours of operation.

author: jadew
author_rank: Frequent Contributor
timestamp: February 27, 2014, 11:04:42 pm
content: Quote from: casinada on February 27, 2014, 10:13:19 pmMost likely, but I hope it can be revived, if not I'd like to at least know what went bad.Attached is a CSV with the current consumption from cold to hot, as well as two pictures where something happens (the rest of the time the consumption just gets constantly lower).

author: edpalmer42
author_rank: Super Contributor
timestamp: February 27, 2014, 11:32:54 pm
content: Check the temperatures of the lamp housing and the Rubidium cell (the hot lump next to the lamp). My lamp housing is about 114C and the Rb cell is about 60C. If the temperatures look good, in case your scope is a bit off frequency, adjust C217 through it's range to see if you can get a lock anywhere. Tweak C217 a bit and let it go through a few sweeps. Repeat as necessary. If it locks anywhere, it's right and your scope is wrong!Ed

author: jadew
author_rank: Frequent Contributor
timestamp: February 27, 2014, 11:52:05 pm
content: @edpalmer42Thanks for the temperature info, but I can't find my temperature probes, I have two and they're in the same zip lock bag.I'll give it a try as soon as I find them.

author: jadew
author_rank: Frequent Contributor
timestamp: February 28, 2014, 12:23:29 am
content: I just checked and the lamp is at 102C while the cell is at 59-60C. I guess the temperature looks good.Edit: Tried adjusting C217, no luck.Edit 2: Seems that adding more capacitance to C217 makes it swing between lower frequencies. I'm going to remove it and measure it, maybe I can add a smaller cap to make it swing above 10 MHz (assuming my scope is wrong and it doesn't do that already).Edit 3: The cap is 2pf at it's lowest setting.Anyway, I give up. Thank you all for your input.