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Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
SilverSolder:
--- Quote from: dietert1 on March 12, 2021, 03:00:27 pm ---There are 7 byte registers in the 8080A. The ADC code in the 8502A disassembly is register based and operates on two 24 bit quantities. They use A for shifting in ADC bits and H to address the backplane, so it's really very tight. Guess that was the reason why they left it like it was in the 8505/6A.
And when you look at the ADC code, they first collect 12 bits of data with control codes 0C and 00, then another 18 Bits with control codes 08, 00 and 00. So it isn't about making a loop six rounds instead of five.
Meanwhile i have a data logging run of 16 hours with a total of about 20 million samples, so that seems to work now. With 30 second logging interval i have 10 000 samples to average and the stdev of those averages gets down to 0.03 ppm. Of course systematic errors add in on top of statistics, like misaligned ADC.
Regards, Dieter
--- End quote ---
Sounds like we're good for another two digits, LOL
Maybe it is possible to read the ADC with 6 rounds, then "spit it out" on GPIB all from inside the same tight loop... doesn't seem like it would be much code?
We could load that code into RAM and run it from there...
Would be cool with a mod where the controller could simply load some external code and execute on boot...
1audio:
These efforts seem to have just stopped. I have an 8506A and just acquired a less than working 8506A/CT. I would really like a pointer to the GPIB software that is getting so much data from these.
My 8506a and my 732A seem to agree to .2 PPM which seems like pure chance. The AC performance seems quite good. The specs actually seem to match what the reference thermal converters were capable of at the time.
dietert1:
Yes i stopped working on that when i got an Advantest R6581T as a "present" of M. Reps.
The fast GPIB communication mentioned above was using a proprietary GPIB to USB interface that we made in 2009. I guess you will reach similar speed with other adapters, e.g. a PCI card. What made a difference was blanking the meter display and using binary ADC data - doing the floating point conversion to physical units on the host. The poor 8080A in the meter is slow with floating point.
Regards, Dieter
trobbins:
With respect to the first post in this thread, another factor influencing comparison accuracy would be the 50 ohm termination used.
Silversolder, what would be interesting is with the measurement made at the 8506A frequency extremes. One hassle with cross-comparing sine amplitude for the 3325A is that few 5-6 digit meters have frequency response below 1Hz and above a few hundred kHz (well not the meters I have, so I have to resort to a 3400A for anything approaching a MHz, and beyond).
1audio:
I would be happy to make some measurements at the frequency extremes. Which extremes are you interested it? 10 Hz and 1 MHz?
However the problem I have been contending with is confirming accuracy at those extremes. I have a 540B, 931B a collection of thermal converters and still have limited confidence in any of this. The accuracy and uncertainty of the converters is a real limit to any AC measurement.
Ballantine converter spec: https://static1.squarespace.com/static/560bfd14e4b01fdcd8c45b5f/t/5a0a092f24a6942e842dd165/1510607151655/1395B.pdf
Fluke 540B spec
A55 specs
They are pretty close to the 8506A spec.
This is from NIST on an effort to make a JJ DAC to improve accuracy on AC measurements: https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=32363
8506A low frequency measurements are limited by the time constants of the thermal converter. HF by all the frequency effects from amplifiers to skin effect which could influence the accuracy. Its a real and complicated problem.
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