I recently took delivery of a Fluke 287 DMM and attempted to use the pulse width feature. The quickest available square wave on the bench was a 1kHz 0-2V probe compensation signal on a nearby scope. Well, despite being 50% duty it registered 45/55 when following the Fluke manual instructions:
"For 5 V logic signals, use the 5 V dc range. For 12 V switching signals in automobiles, use the 50 V dc range."
On the AC volts position, it measured an accurate duty cycle. Fluke's response to the inquiry was "It's wrong. Send it back." I'm more inclined to think this is a design issue with small pulse amplitudes or bad advice in the manual. To verify this, I manually set it to the 500mV DC range, and despite being out of range, the result was accurate.
Even though this is a bench measurement I'd almost certainly never make with scopes nearby, I'd like to know the limits of the device. Has anyone run into similar measurement issues with these devices? It may happen with other Fluke models as well.
Just tried this on my 289, with a 2.0Vp-p signal @ 1kHz, and it can't actually measure the duty cycle at all in DCV mode. It just says 'OL'.
With the meter set to ACV, it says 50.00% and 1.0000kHz, just as it should, and it works with the meter set to the 500mV range too.
I think the problem is with the manual. Frequency is an AC phenomenon, it doesn't make sense to measure a "dc frequency". Larger signals and lower frequencies do give a reading in DCV mode, but even so, it's not very accurate and I'm sure it's just trying to perform a measurement on a low-pass filtered signal.
I've also tried the exact same signal on my 89-IV, with the same result. It works fine on a 1kHz 2.0Vpp square wave if I put the meter in ACV mode, but not DCV mode.
I dont have my 289 in front of me nor has my signal generator come yet anyway, but I think its just a mis-wording in the manual as well. page 42-45 in the latest version of the manual do show the meter set in VAC mode for Frequency, Duty Cycle and PWM, though it does show any of the current and voltage modes would work too and I see the part about the 5VDC measurements which Im guessing is just is just incorrect.
I'd guess that it is applying a low pass filter to the signal and that the manual is wrong.
Thank you for the replies.
As a quick test, I increased the amplitude of the 1kHz pulse train, and the meter reads pulse width correctly at around ~3V amplitude when using the DCV setting per the manual. This is above a valid TTL logic high, so you might get burnt testing a 5V logic system operating on the margins.
Takeaway: Ignore the manual and use the ACV position for duty/pw unless you're sure the pulse exceeds 3V.
Okay, just tried this with a Fluke 289 and an Agilent 33220A waveform gen set to output a square wave at 1KHz, 2Vp-p, 50% duty cycle and a 1V offset to make it pulse from 2V to 0V and the 289 set to VDC mode, % and it seems to be reading the 50% duty cycle correct. Unless I set something up incorrectly...