Function Generator frequency vs measured frequency? - Page 1
Forum section: electronics / beginners

author: sequoia
author_rank: Unknown
timestamp: September 26, 2015, 12:25:16 am
content: I came across cheap (apparently Chinese) function generator, apparently some "FG-100 DDS" clone since no brand/manufacturer markings whatsoever....When the generator is set for 90Hz Sine Wave output and hooked-up to a multi-meter, I noticed the meter is showing ~30Hz, so something is not right.... It would seem like the Hz ratio between what is set on the generator vs. what is measured by multi-meter is roughly 3:1.I was expecting getting measurements roughly in 90Hz range. Am I missing something? (Or is this just bad/out of adjustment function generator...)

author: xrunner
author_rank: Super Contributor
timestamp: September 26, 2015, 12:38:54 am
content: Quote from: sequoia on September 26, 2015, 12:25:16 amHard to say which one is wrong, unless you are sure the DMM you have has been proven to be reliable in this mode. Do you have a "tiebreaker" available - or a scope ...

author: Maxlor
author_rank: Frequent Contributor
timestamp: September 26, 2015, 01:08:30 am
content: Note that many multimeters don't allow a DC offset when measuring frequency. If you set your function gen to switch between 0V and 5V or something like that, you have a DC offset of 2.5V and might get erroneous readings on your multimeter because of that.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: September 26, 2015, 02:49:58 am
content: R-divider, maybe a DC blocking C into your PC soundcard audio input, some free SW gets you a good view up to a few 10s of kHz

author: sequoia
author_rank: Unknown
timestamp: September 26, 2015, 05:15:47 am
content: I was using Fluke 117 to measure the frequency (Voltage measurements seem dead-on, but no idea how accurate the frequency measurements really are).DC offset on the generator seems to work, and adjusting it doesn't have any effect on the Hz measurement shown on multi-meter.Meter seems to measure "AC" from the signal ok, so that would point to DC offset being close to 0V when its set to 0 on the generator.I'll see if I can get this hooked up to a scope, that should show what is going on.

author: KJDS
author_rank: Super Contributor
timestamp: September 26, 2015, 05:36:37 am
content: I'd trust the frequency measurement of a multimeter about as much as a politician. It could be DC offset, mains interference or many other ills. They're usually very good at measuring odd harmonics of a distorted signal.

author: DimitriP
author_rank: Super Contributor
timestamp: September 26, 2015, 07:07:13 am
content: For fun, try measuring a different frequency like 500 Hz or 1000 HZ

author: sequoia
author_rank: Unknown
timestamp: September 26, 2015, 07:59:01 pm
content: It seems indeed that there is linear relation between the frequency set on the generator and what is actually produced which is roughly 3:1.Attached should be pictures showing the signal, when generator was set for 90Hz and 900Hz....

author: xrunner
author_rank: Super Contributor
timestamp: September 26, 2015, 09:26:39 pm
content: Well that proves it. It's a little hard to believe they madethatbad a design, but I guess the old saying "you get what you pay for" applies.

author: cncjerry
author_rank: Unknown
timestamp: September 27, 2015, 12:25:20 am
content: Does you fg have the option to set its internal clock? I suspect its clock or PLL is off or not set correctly.