Fluke 45 frequency measurement - Page 1
Forum section: electronics / general

author: willb
author_rank: Regular Contributor
timestamp: July 21, 2012, 05:01:04 pm
content: I have posted this on the Fluke Yahoo message board, however I figured I'd post here as well as this place is a lot busier.I have just picked up a Fluke 45 for rather cheap with the battery option. The unit powers up fine, displays no error codes, and the measurements seem accurate (compared to my 87IV and HP 3478A).However, with nothing connected to the input jacks, when doing a frequency measurement, the unit shows a frequency of 27.7Khz. When I ground the input jack, the unit displays 0.0 Hz, and when I apply an AC signal from my signal generator, the unit displays the correct frequency being input into the meter. I hooked up my scope to the front input jack of the meter, and there is indeed a 27.7Khz signal at roughly 150mVac p-p. Doing some digging through the service manual, it seems that the frequency of the inverter is right around 27khz. I was wondering if this could be a case of bad filtering (old caps) in the power supply/inverter or simply that the meter has very sensitive inputs and is picking up internal noise/bad design and is acting normally? This initially seemed odd to me as my other meters and frequency counters (Philips PM6665 and an old beat up Ballantine 5500B) all display 0.0Hz with nothing connected.2 users on the fluke board said their Fluke 45's are doing the same thing, and as one of them said, it's either normal or a common problem. I'm just trying to figure out which one.

author: willb
author_rank: Regular Contributor
timestamp: July 21, 2012, 05:11:39 pm
content: This is the waveform on the input jacks when doing a freq measurement.

author: saturation
author_rank: Super Contributor
timestamp: July 21, 2012, 07:07:30 pm
content: If you output a signal amplitude smaller than 150mVpp, does the contaminating signal produce an error?As a test, reduce the output of the generator to ~ 75mVpp and say 90 Hz. Does it count 90 Hz correctly? I forget the precise value but the counters do have a minimum input voltage required to trigger and this is maybe near it.

author: willb
author_rank: Regular Contributor
timestamp: July 21, 2012, 07:44:19 pm
content: Nope, no error is induced. I've repeated the test at 100hz, 1khz, 10khz and 100khz. It will count the proper frequency with signals as low as 15mVpp (in a 50ohm termination) with the proper RMS value in the 2nd display. Below 13-14mVpp it becomes unstable. The trigger seems to be around 4mVrms.

author: saturation
author_rank: Super Contributor
timestamp: July 21, 2012, 08:06:33 pm
content: That's great then. Even if you pick up artifacts, so long as it meets the published specification for minimum trigger voltage, min and max frequency limits, and accuracy it won't matter.Lastly, try doing the same tests directly into the Fluke 45 inputs, without a 50 ohm load. If there is leakage, the current must be fairly small so it can't generate voltage across 50 ohms, while the function generator can source, if not sink, current far better even at mV levels. Whatever the current is on the Fluke 45, it can generate a voltage across at least 1 megaohm, which was the input impedance of the Rigol.Quote from: willb on July 21, 2012, 07:44:19 pm

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: July 21, 2012, 11:19:37 pm
content: I guess it's only a problem when making measurements with no leads connected to the meter. Just avoid those and it should be fine