Phillips PM 5136 function gen Cal question - Page 1
Forum section: electronics / general

author: Homer J Simpson
author_rank: Super Contributor
timestamp: August 21, 2012, 09:22:20 pm
content: Hello,Picked up a PM5136 off EBay. Frequency is spot on but amplitude is off, more so the higher you go.Looked for a service manual but all I can find is the operators.Does anyone have a service manual for this series or has done a cal?Thanks, KT

author: amspire
author_rank: Super Contributor
timestamp: August 21, 2012, 11:54:42 pm
content: I just have to ask the obvious questions.Have you got a proper 50 ohm termination on the output? (Otherwise the output voltage will be about double the set value)What are you using to measure the output voltage? If it is a digital multimeter, then the issue is probably the digital multimeter's AC response and not the signal generators accuracy.How much error are you talking about?Richard.

author: Homer J Simpson
author_rank: Super Contributor
timestamp: August 22, 2012, 12:16:26 am
content: Thanks for the responce.I am using a BNC to crocodile rated at 50 ohms to the probe on the scope. The scope is a TEK 2430A.I was in such a rush to try this out that I was not paying attention. Sine and Triangle measure on the scope as indicated on the generator. I thought the square wave off but looks like the scope was just measuring the over and undershoot.Also, after a warm up it settles down even more.Any other feedback let me know. I am a "newbie"Thanks KT

author: grumpydoc
author_rank: Super Contributor
timestamp: August 22, 2012, 08:26:16 am
content: QuoteYou will need to make sure that the 'scope input is terminated at 50 ohms - I think that the 2430 has a selectable 50 ohm terminator.To be honest the BNC-to-croc-clip things aren't all that much use as 'scope probes. The cable capacitance means they have very low bandwidth and the long wires to the croc clips have considerable inductance (maybe what's producing the "overshoot" on the square wave) so they are somewhat worse than an x1 'scope probe.If the signal generator has a BNC output then use a BNC-BNC lead to connect to the 'scope and either set the 'scope for 50ohm termination or add a tee piece to the scope input with a terminator on it, and then have another look at the waveforms.

author: Homer J Simpson
author_rank: Super Contributor
timestamp: August 22, 2012, 04:02:59 pm
content: Thanks again for the responces but now I have some more questions.The main reason for me getting into this stuff is to learn how it works.Looking closer at my BNC patch cables they are 75 ohm not 50 ohm. How does this effect the signal and reading on the scope?Here is what I am seeing so far;1) I connect the 50 ohm BNC to crocodile to the FG then connect the scope probe to that. Voltage measures correctly but the signal is noisy.2) I connect the 75 ohm BNC patch directly from the FG to the scope. Signal is clean but measured voltages are high.3) Same connection but activate the 50 ohm termination on the scope. Signal is even cleaner but measured voltage shows even higher.Another question is on the use of the BNC splitters. What effects will those have on signal and voltage? What is "50 ohm termination" doing? "grumpydoc" mentioned using a BNC splitter with one output having 50 ohm termination vs us using the 50 ohm termination on the scope.Thanks again for all of your help, KT

author: T4P
author_rank: Super Contributor
timestamp: August 23, 2012, 06:59:49 am
content: 50 ohm does the impedance matchingFG's usually run at 50 ohm impedance so with a 50 ohm passthrough terminator the FG sees a 50 ohm impedance (Well actually in parallel with 1M) but anyway it being parallel with the 1M equates 50 ohmsNasty

author: AndyC_772
author_rank: Super Contributor
timestamp: August 23, 2012, 07:48:09 am
content: Quote from: Homer J Simpson on August 22, 2012, 04:02:59 pmIt means that the transmission line (cable) isn't correctly matched to the function generator or the scope. This will result in some of the signal reflecting off the point where one joins the other, and as these reflections travel up and down the cable they will interfere with the main, wanted signal. This results in distortion, which you should see on the scope. How bad this distortion is will depend on how fast the edges of your square wave are, the length of the cable, and the bandwidth of your scope.QuoteNot surprising. Crocodile clips aren't controlled impedance, ie. they're not 50 Ohm, or 75 Ohm, or anything else which can be reliably stated. Expect distortion which varies depending on how the clips are attached.QuoteThat's odd, the voltage should be about the same, since all you're doing is changing one cable for another. Set the function generator to a low frequency, so transmission line effects aren't important in determining the amplitude of each pulse once it's had time to settle, and try again.Also check, are you using a 10:1 scope probe but then replacing it with a cable (which, of course, is 1:1)? That would give you an immediate factor of 10 measurement error.QuoteYou change the scope from 1M input impedance to 50 Ohms, and the signal actually gets bigger? That makes no sense at all, something isn't right there. The signal amplitude should drop to about 1/2 what it was before.Quote50 Ohm termination should prevent the signal from being reflected at the point where the cable connects to the scope. It's the same as setting the scope to its 50 Ohm input mode. If your scope has a 50 Ohm mode then you shouldn't need the T-piece and 50 Ohm terminator.