Hello all - first of all, I would like to thank you for the many replies I got regarding my first posting a couple of weeks ago where I described my homebrew Rubidium oscillator based on an LPRO-101. During the last couple of days I found enough spare time to dig deeper into some issues. In addition to that I was lucky enough to find a Tracor 527E (literally from a scrap heap) for 50 EUR which looked horrible and was non- functional with a large red sticker "REJECT". After two evenings of digging through its circuitry it is now in perfect working condition again (actually there were only two faults: a defective transistor in the mixer of the first error multiplier which essentially rendered the machine useless, and a cold solder joint in the single shot which caused erratic operation after fixing the first bug). Since some of you mentioned that my (way too?) simple digital divider chain would produce output signals with non-neglectible phase jitter, I had a deeper look into this issue using the Tracor 527E and an old HCD 1519 precision oscillator which I assume to have better phase stability than my initial dividing circuitry. First of all, you were perfectly right - there was substantial jitter which I got rid of by inserting a 74LS175 between the respective divider stages and the output drivers of my divider circuit. The common clock line of the 74LS175 is driven by the TTL converted output of the LPRO-101. (I am aware of the problem that with enough temperatur shift it might happen that my divider chain might "slip" a whole clock period, but at the moment this simple solution seems to work really nice.) What came quite as a surprise to me was that my stupid idea of having a LED which blinks once per second would cause me so much headaches. Using the Tracor 527E and the HCD 1519 which was running for more than a day, I was able to adjust the quartz oscillator to the Rubidium clock with an error of < 1 in 10**10. After switching the Tracor to a resolution of 10**11 it became apparent that there still was a substantial phase shift every second then the *...* LED blinked. This problem was eventually solved by driving the LED with a discrete transistor instead of a free 74AC14 gate and decoupling this driver with an RC-combination. All in all, I think my Rubidium oscillator is now way better than its first incarnation, so thank you all very much for your help and hints. Best regards - Bernd. :-)
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