[time-nuts] 2.5V reference IC in HP E1938 oscillator.

Rob040 . rob040 at live.nl
Mon Nov 10 08:23:40 EST 2014

Dear Rick,
> When asking questions such as this, it is helpful to know why you need to know the part number.

Basically to complete my documentation, but there is more to it...
Please let me start with a nut shell introduction. I'm going to build a GPSDO and will use the following key building blocks:GPS receiver (Oncore UT+) with an outdoor antenna for a precise 1PPS.HQ 10MHz oscillator with EFC (initially the 10811, later switched to the E1938).Dividers (HC390) to come to 1Hz.PLL (HC7046) to compare 1PPS & 1Hz and tune the oscillator.Power supply, to feed the animals.
Nothing exiting so far. To check on the outside if the instrument is working properly, I want to have some checks:Oven error LED, is the oven on temperature (not too cold, not too hot). LED on = error.1PPS from GPS LED, only flashing when at least five satellites are contributing.PLL Lock LED, on when 1PPS & 1Hz are in sync.EFC voltage readout (3½ digit), to monitor excessive drift.
Now about the oven warning LED. The idea is to use the "Thermistor Bridge out" pin (8) on the 15P 'Puck' D-sub and the "Them. Bridge Reference" (pin 15) for this.So, I connected with two wires (40mm/1.6" length) both above mentioned pins with the not used pins 19 & 20 in the special D-sub. Now I can connect the oscillator board with one cable (20cm/8") to my own PCB and use two comparators to make a small window for an alarm. The reference I only will use to determine the 'on temperature voltage'.However, when reading your article "A LOW-PROFILE HIGH-PERFORMANCE CRYSTAL OSCILLATOR FOR TIMEKEEPING APPLICATIONS" I noticed that you talk about 'picking up noise due to a long path from reference to the controller board'. That brought me to the schematics of the reference voltage and the IC. My doubt now is that my leads will increase the chance on errors in the delicate control system.Can you please elaborate on that, is there indeed a risk?
> I do remember that I originally used some convenient reference which seemed OK from the data sheet, but turned out to be too noisy. I changed it to a different one.
That emphasises my earlier mentioned feeling, but also brings me to a next questions:I have two different versions of the E1938. One contains two DAC IC's, the other only one DAC.What is the reason for changing this?Was the noisy reference IC ever used in production (so, might it be present in the two DAC version)?Is it possible to publish an older revision of the schematics that reflects the two DAC version that was produced (and if applicable the oscillator unit)?
Best regards, Rob.


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