[time-nuts] HP E1938A questions/remarks.

Rob040 . rob040 at live.nl
Mon Oct 13 16:44:02 EDT 2014

Dear all,
This is my first post, so a small introduction to start with... My name is Rob, 54 years old and living in the Netherlands. I am going to build a GPS controlled 10MHz oscillator which will be a reference for time/frequency measurements.It will be based on a some standard existing modules (e.g. Oncore UT+), 74HC390 dividers etc. For the oscillator I planned to use a HP 10811, but now I changed that into the HP E1938A. My questions/remarks are about that last one.
I purchased an E1938A at fluke.l in China. After receiving it, I studied the connections etc.Also I could find a lot of information on this site and also TvB's and Rick's webpage, thanks for that. But still some things are unclear. And information is scattered, so I might have missed some details.What I would like to know is:1. I have the version with two DACs. I assume that is the first edition. What was the reason of removing one DAC and add some diodes?
2. The ACOM/DCOM pins. Both Rick and Theo are talking about ACOM/DCOM in their pinning overview. In my edition ACOM (shield & pin 1) is separated from DCOM (pin 11&12). That means both have to be connected to ground in my instrument. According to me it would be better to separate it also in the pinning overview.
3. The five LED's, what do they indicate? DS3 is obvious, but the other four (DS4-DS8)?
4. Was there any software update for this unit? I did not connect it yet, so I'm not sure which version is in at the moment. If there is a younger version, where to find it and how to flash it?
5. To be honest, I was surprised to see that the oven is fed by 5V. That means a high current. Any special reason not to use a higher voltage? (just curious, that's all).
6. Vref, that's 2.5V. I would say, it's strange to use that, the EFC voltage range is 0 - 5V. To bring it on exactly 10MHz the voltage will be around 3.0V in my case. Normally the reference voltage is equal to the highest possible voltage (5V). Also here just curious what the thought behind it is. Now it seems a bit pointless to bring it outside the unit.
7. I want to have a warning LED that lit up when the oven is not on temperature any more (too high & too low). I want to use the pins 8 (Therm. bridge out) & 15 (Therm. bridge ref.) on the 15 Pin sub-D for this. If I understand correctly, if both voltages are equal the oven had reached its final temperature. Am I correct?
8. Are there also schematics available of the two-DAC version? I could not find it.
9. On my version some tracks were cut, both related to pin 2 of the 20P sub-D. Was it done at HP, or can it be a customers action?
10. I saw one adjustment point (R155), I always wonder when I see something like that, what can I do with it.
I know, a lot of questions in one post, but maybe it also helps others who are struggling with gathering all info about this unit.
Thanks in advance, Rob.


More information about the time-nuts mailing list