Electronics > Repair
Fluke 8840A Faulty CPU
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View[+]Finder:
Here’s a little experiment: connect two meters (Fluke 8842’s) to a GPIB network and feed them with a reliable 10VDC reference. Just for comparison, feed the same reference to a ’new’ Keithley 6500 meter set-up to record DCV at NPLC = 15 continuously. Then trigger the two 8842’s using PyVISA over GPIB with the ‘group execute’ command: 

 intf.group_execute_trigger(DMM8842a, DMM8842b)

Capture 10,000 observations at roughly 1-second intervals for the 8842’s and for the same time period for the 6500 (not exactly the same periodicity) and have a look at the resultant data.

The average (mean) value for measured DCV was calculated for each meter as well as for the average of each observation for the two Fluke meters. The standard deviation of the data (sigma) was also calculated as a measure of the consistency of the measurements.

As shown in the attached table, Fluke meter ‘A’ sigma was about 50 micro-volts, meter ‘B’ 40 micro-volts and the average of each observation, about 32 micro-volts. By comparison, the Keithley 6500 measurements had a sigma of 3 micro-volts.

While this is by no way an exhaustive test of the Fluke 8842, it is strong evidence of the value in a properly maintained ‘vintage’ voltmeter in comparison to a state of the art meter with a ‘one-digit advantage’ in terms of advertised precision.

From a user’s POV, the Fluke —due to its specification of voltage ranges—is capable of ‘digit equivalency’ with the 6500 in many lower voltage measurements, particularly in the under 2VDC area. And if the reference is in a double-wall, metal case with no vents or fan, how much does the lack of an on-chip heater really matter?
Kleinstein:
The limited resolution of the 884x may already contribute to the noise.  If random enough the quatization noise from 5.5 digits (and thus 100 µV steps) would be 29 µV RMS.  With a fixed input and little other noise, the quatization noise can be higher or lower. If the meters use digital cal factors there could be additional internal rounding errors.

One might get slightly lower noise when using the medium speed (20 SPS) mode and averaging on the PC side.
bdunham7:

--- Quote from: View[+]Finder on May 21, 2021, 05:17:28 pm ---Capture 10,000 observations at roughly 1-second intervals for the 8842’s and for the same time period for the 6500 (not exactly the same periodicity) and have a look at the resultant data.

The average (mean) value for measured DCV was calculated for each meter as well as for the average of each observation for the two Fluke meters. The standard deviation of the data (sigma) was also calculated as a measure of the consistency of the measurements.

As shown in the attached table, Fluke meter ‘A’ sigma was about 50 micro-volts, meter ‘B’ 40 micro-volts and the average of each observation, about 32 micro-volts. By comparison, the Keithley 6500 measurements had a sigma of 3 micro-volts.

--- End quote ---

How many digits do the readings have over GPIB?  If there are no more than displayed, then you will have what I commonly observe with these which is that at on a very stable reference with a fully warmed up meter, you either get a flickering reading that varies by one count or a steady reading with no variation.  This could be caused by a low noise level that can result in a one-digit variance when the voltage is right at the threshold between two possible values while resulting in no variation if the voltage source is far enough from a threshold. 

It might be interesting to do the same experiment with either a 1.5V or 15V reference so that both the 8842A and the DMM6500 are displaying the same number of digits.
View[+]Finder:
The underlying purpose of the experiment--other than to just check out the 8842A meters--was to see if averaging two meters' simultaneous readings would reduce the effect of noise. It did a bit, but nothing to brag about. The published spec for the meter is 0.0030% of reading + 2 counts which is a bit over 300 micro-volts (if I did the math right). As @bdunham7 pointed out, the ticking of the 6th digit is consistent with the sigmas that I reported. Getting +/-50 microV out of a thirty-year-old meter with a 90-day spec 6X that is a testament to the quality of the design and construction and probably a bit to survivor-bias as the duds were junked years ago.

The test was done with my best meter (HP3458) and the current best reference at 10VDC (sigma ~1.7microV). The Fluke range is 2V and 20V and the 6500 is 10V and 100V, so how about 1.999V? The 2V range doesn't much above 2V and the 100V range in most modern meters is not their best spec.
Kleinstein:
The 100 V range on modern high resolution meters is usually with a 1:100 divider and than measuring in the 1 V range. For a 5 digit or lower end 6 digit meter this is not a problem. However with the better 6 digit meters the noise of the divider alone (100 K at the lower leg) gives noise comparable to slightly higher than a good meter in the 1 V range. So the 100 V range is a bit limited and there is kind of a natural cap on how good it can be.

It can help quite a bit to have a 20 V or similar range and does not switch to the 100/200 V range so early.

The noise usually does not get much worse with age. It is only the available parts and expectations that have changed over the years. Aging may cause drift in the scale factor, especially with the high voltage and ohms ranges. The prime range (20 V for the 8840) is usually quite stable over long time.
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