[time-nuts] Homebrew Rubidium oscillator, jitter and other tales :-)

[ulmann at vaxman.de]

 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. :-)