BK Precision function gen. Frequency is fine. Voltage is off - Page 1
Forum section: electronics / repair

author: JBaughb
author_rank: Regular Contributor
timestamp: May 06, 2015, 08:28:28 am
content: So i picked up a non-working function gen (BK model 4079) recently. Wanted to get opinions of wether its just a cal issue or internal damage before i opened her up. Turns out the frequency is fine up and down the range but the voltage is off. It fluctuates sometimes being .8 or the input voltage setting(1Vpp shows .8V), sometimes multiple times the setting (5V+).I haven't opened her up yet to probe around for voltage test points or checked for anything obviously burnt out. I'm used to working on stuff that just doesn't turn on.

author: electr_peter
author_rank: Unknown
timestamp: May 06, 2015, 09:07:34 am
content: Set frequency to something low, say 200 Hz, and check voltage with DMM. It is likely scope is not setup right or there is some attenuation setting turned on in signal gen.

author: wraper
author_rank: Unknown
timestamp: May 06, 2015, 09:32:49 am
content: Stupid question, but isn't your scope set to 10:1 ratio? This signal gen doesn't seem to be capable of 40 Vpp at all.

author: dom0
author_rank: Super Contributor
timestamp: May 06, 2015, 09:47:25 am
content: Quote from: wraper on May 06, 2015, 09:32:49 am+1

author: JBaughb
author_rank: Regular Contributor
timestamp: May 06, 2015, 10:01:48 am
content: Okay, let me try to explain this better...and I hope its not just my lack of experience with function gens thats screwing me up.So first, gen is 4, scope is showing 40 (not the other way around)And, yes the scope is 10:1, but when i hook up my Siglent function gen, I've always kept it on 10:1 and the voltage on the scope has matched (close enough) the voltage setting on the generator (1volt on silent = 1 volt on scope).I changed the ratio to 1:1 and the voltage went down in equal measure but because its not always a 10:1 ratio (sometimes, I'm getting 8:1, now I'm getting 2:1) that didn't "fix" the problem. on 1:1 ratio I have 1V on the BK generator and showing 2V on the scope. this is all peak to peak.So, this could all by my ignorance on the topic but I was also sold this scope at asignificantdiscount because it wasn't functioning correctly so Im anticipating issues.

author: wraper
author_rank: Unknown
timestamp: May 06, 2015, 10:14:06 am
content: I'm not sure what you are doing with siglent signal gen but you cannot set 10:1 ratio on the scope and get correct reading unless using 10:1 probe. If you connect with BNC cable directly, for sure you will get 10x multiplied readings, this setting exists solely for this purpose FYI (multiplying the reading in software).Also at which frequencies you get wrong readings? If doing anything more than a few MHz, you will get completely wrong reading without using a 50 Ohm terminator. Also you must be sure that signal gen have the right setting of load impedance - HighZ (or the same with different wording) if basically no load or 50 Ohm if using terminator. Otherwise set value will not match with measured value.

author: JBaughb
author_rank: Regular Contributor
timestamp: May 06, 2015, 11:31:11 am
content: I should be more specific. Its direct BNC, terminated with a 50ohm terminator. all function gens are in their 50ohm mode as well. I'll take pictures later of both function gens with the scope. That might clear things up.I'd love to hear this is my own fundamental misunderstanding of how these function generators work rather than something wrong with it since that means I scored this at an absurdly low price.I modified my top post to avoid the confusion. The 10x that it was showing at that point was most likely because of the 10x input (meaning at that moment it was correct) but its rarely correct, showing different amounts everytime I restart it. Right now its showing 0.8V, earlier it was showing 3V and I've never changed the 1Vpp setting on the BK function gen.I stand by what I said about the Siglent function gen though. It was purchased brand new fairly recently and its always showed the correct Vpp even using a BNC, terminated in 50Ohm mode. I kept Siglent on 1Vpp and just changed the scope 10:1 setting and the scope went from showing 1Vpp dow to a mV setting. Maybe its the cheap Siglent generator thats always been defective, haha.

author: JBaughb
author_rank: Regular Contributor
timestamp: May 06, 2015, 11:59:24 am
content: Quote from: electr_peter on May 06, 2015, 09:07:34 amvoltage reading on the multimeter was jumping all over the place in between -1 and 1V and all numbers in between (Fluke 87-V). At that low frequency it was doing the same thing on the scope, unable to capture a stable waveform.

author: jlmoon
author_rank: Unknown
timestamp: May 06, 2015, 04:06:27 pm
content: Quote from: JBaughb on May 06, 2015, 11:59:24 amHope this is not a "stupid question" either.. you do have that DMM set to measure AC Volts right?I suspect it measures RMS voltage too.

author: wraper
author_rank: Unknown
timestamp: May 06, 2015, 04:21:13 pm
content: Quote from: jlmoon on May 06, 2015, 04:06:27 pm+1, I had a dodgy feeling about that. But now rereading"between -1 and 1V"that's almost sure DC range. You cannot get negative reading on AC range.

author: jlmoon
author_rank: Unknown
timestamp: May 06, 2015, 04:30:56 pm
content: Quote from: wraper on May 06, 2015, 04:21:13 pmlol.. depends on how much voltage you stuff into that meter.. enough and I'm sure you can get all kinds of strange readings.

author: electr_peter
author_rank: Unknown
timestamp: May 06, 2015, 05:01:46 pm
content: Seems like a PEBKAC...

author: JBaughb
author_rank: Regular Contributor
timestamp: May 06, 2015, 09:10:37 pm
content: @electr_peter - I am not discounting that.I of course tried the AC voltage setting on the dmm (it was outputing a sine wave after all) but I got absolutely no reading.I am just going to post some photos. That should clear things up.

author: tautech
author_rank: Super Contributor
timestamp: May 06, 2015, 09:33:45 pm
content: Be aware the Siglent AWG output as displayed on a scope with vary in amplitude IF the selected output impedence AND the terminated impedence do not match.Select desired output, select impedence, terminate to match and displayed waveform will be accurate.Results will be correct for Hi Z (probe) or 50 ohm (BNC).You probably know this already.Now unless the BK has the same features......Rip the covers off and check the PSU for ripple, might be just a re-cap required.