HP 53131A FREQUENCY COUNTER HELP

[samson]

Hi, I have the HP 53131a Frequency counter with the genuine HP 010HS med oven oscillator fitted. My question is if I take the HP 010HS out of the unit and bench calibrate it against a 10mz gpsdo and  connected to a oscilloscope ,I can adjust the trim pot so it is exactly 10mhz. When put it in the HP 010HS back into the counter and do a time base calibration is comes up as failed. To fix this I have to turn the trim pot on the HP 010HS oscillator out of the 10mhz zone and do the test again and it will pass the test. Is this because the algorithm its to far off for the it to make the adjustment.? and why do they put the trim pot there.

The other question is when the HP 53131a comes from the factory fitted with 010HS oscillator does it still have the on board oscillator at the back of the unit or do they omit this.

Any help or advice appreciated

[artag]

Does the 53131 adjust the frequency control voltage both + and - from zero or does it need to adjust it into a +ve range only ? If so, setting the frequency such that it requires 0v input to get 10MHz would be on the limit of the adjustment range.


If you haven't seen this article already you may find it interesting (but he talks about making small random adjustments to the trimmer rather than setting it outside of the counter).
https://damien.douxchamps.net/elec/equipment/hp53131a/timebase_MDA/

[alan.bain]

I seem to recall the HS 010 is based around the HP10811.  When you calibrated externally what EFC voltage did you supply to the unit?  I seem to recall that when EFC was not used pins 5&6 are shorted which is a zero EFC.

The timebase board has a AD7243 configured in -5 to 5 mode to drive the EFC line.

The only 53131 I used had the HP-especially-drifty-timebase so no practical experience!

[snickers]

You can use two different methods of calibration:
1. Manual analog - with the potethiometer. In this case your board do not take care of DAC signal from MCU of the unit and all calibrations will fail.
2. Digital DAC. You need to turn OFF jumper of the potentiometer and then start origial calibration procedure. It will pass. EFC signal will be drived by DAC acording with the signal from MCU of the instrument. But the position of potentihometer will not care.