I have been following the EEVBlog on YouTube and saw a video about a Rubidium Frequency Standard. So I thought it could be fun to get one of those. On eBay I found a guy in China selling a version from Efratom...I think. It was described as a 10 MHz Rubidium Frequency Standard model number FRS-C. I was able to find a manual (with schematics) and started reading about it and I decided to buy one.
When I received it I could not get it working. I contacted the seller and he told me that I could not use that [Lock Status] pin. If you connect anything to this output the frequency unlock's. This output (pin 1) is a status output. It should not be affected by a multimeter being connected to it. But I decided to do a 48 hour test to see if this was actually true. It would not lock in the 48 hours that I tested it.
I figured the schematics might help me solve the problem. But I could see that the [Lock Status] pin was connected directly to the output of an Op Amp. I tried again and again to measure the [Lock Status] pin when the unit was locked-in. And every time my probe touched the [Lock Status] pin the frequency would unlock.
I couldn't figure out what I was doing wrong. I thought maybe I should try to isolate the Op Amp's output from my other circuits. It didn't makes much sense but I tried it anyway. EUREKA!
Rb-STD Lock Indicator schematic
As shown in the schematics I tried connecting an optocoupler to the output. I found it strange that the Rubidium Frequency Standard would unlock when I measured it. So connecting an optocoupler to the output seem like an equally strange idea. But it actually worked. Now I'm able to measure the [Lock Status] signal all the time.
I have uploaded a couple of videos of my Rubidium Frequency Standard to YouTube.