The USA Cal Club package has been sent back. I didn’t get a chance to do everything I probably could have but got enough done and gathered enough information to keep me busy for some time.
I had ordered all the parts for the GPIB interface a couple of weeks ago so I assembled the bare board I received and got to the point where the LEDs seemed to indicate a handshake and I got a 4 line message on my laptop indicating that the board was recognized but not programmed.
On measuring the 10 volt output of the FX supply, I probably did it differently than most people would. I wired the FX supply in series opposed with my LTZ1000ACH 10 volt Dr. Frank supply so that if the output of the two supplies was equal, the voltage difference between them would be exactly zero. To measure the difference I used my 50+ year old HP419A null meter which can measure down to 3uV full scale. To see a short video of the 2 being compared, go to:
https://youtu.be/wTGxBJNNhWc Although I had no history on it, I believed my HP3457A was pretty close but that was more of a gut feeling than hard fact and I don’t know when it was calibrated last. Judging by the CALNUM it had been calibrated about 20 times. The resistance readings of my standard resistors seemed very close to what I expected and the voltage readings also seemed to agree with my 5.5 digit Fluke 8840 (what do you expect for $16 at a hamfest). The HP3457A and my LTZ1000ACH 10 volt supply had been powered for several months and the readings seemed to be a stable 10.00000 VDC with maybe a 1 for the last digit.
When I connected the two supplies and the null meter up the difference was far greater than I expected. What I discovered is that if I connected the HP3457A across my supply while it was still connected to measure the null, that the difference on the null meter was much less. The adjustment on my supply is a 30 turn 5K pot that is wired so that 1 turn is between 5-6uV and exactly 10 volts is close to half way. By adjusting the pot I could regain a null on the 3uV full scale range and at null my HP3457A read about 9.99996 occasionally up to 9.99997 volts. If I read the graph on the FX supply correctly at 24°C the output would be 9.9999585 to 9.9999600 volts. This leads me to believe that my HP3457A is pretty close, or close enough so I don’t want to mess with the calibration. Apparently there is some small loading and/or leakage that affects the readings and where the 2 supplies agree that close with my HP3457A connected across my 10 volt supply, I assume that is pretty much the correct reading.
I decided to check the 2 supplies to see how they reacted to light loads and I knew mine could survive a direct short for some time because I designed the added transistor buffer within the opamp feedback loop to handle a direct short and also protect the opamp. With a 100K load both supplies dropped 1uV; at 10K, mine dropped 10uV, the club’s 20uv; and at 5K, mine dropped 20uV, the club’s 40uV so the output resistance of mine is less than the club’s but neither would be loaded like that in any testing situation. There may be some capacitive leakage from line powered devices causing part of the problem. Both my supply and the HP419A null meter can be run from internal batteries but the HP3457A is line operated and digging out 2 batteries and running the FX supply from batteries was more than I wanted to do. I’m satisfied that my HP3457A is reading close enough for my needs so unless I get a higher accuracy meter (which I don’t see in the budget) I’ll be satisfied with what I have now.
More to follow later tomorrow.
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