[time-nuts] HP 5370B jitter
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sat Jul 14 20:13:25 EDT 2007
Magnus Danielson wrote:
> Further investigations have now shown that the 10811 output is clean, just a
> little 3rd harmonic but nothing to worry about. The INT test point is also
> clean. I suspect the output drive part, as other outputs to various parts are
> also free of the 5 MHz. The 10 MHz present detector circuit is currently my
> main suspect. It consists of a "one-shot multi-vibrator triggered by the 10 MHz
> signal". Having a RC time-constant of 100 ns makes it a suspect indeed. Probing
> it (pin 11 on A8U1) clearly shows a waveform wich looks like a 25% 5 MHz with
> a short spike on it. This little culprit of a detector is infact a wideband
> comb-generator which contributes its 5 MHz as sidebands to the output 10 MHz.
> It's only purpose in life is to light a LED only visible to the servicing
> engineer (me in this case). Thus, making a small modification to disable it
> during normal operation would improve the quality of the 10 MHz output
> considerably if I am right. Since I don't do ECL design on a daily basis, I
> will have to ponder a bit in order to come up with a good method of acheiving
Try shorting AU8U1 Pins 6 and 7 to ground.
This turns of the npn output emitter followers driving the longtailed
pair which drives the LED.
This is permitted and nothing will be destroyed or degraded even if the
shorts are permanent.
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