Function Generator Voltage too high - Page 1
Forum section: electronics / projects

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 11, 2018, 10:02:11 pm
content: Guys,I have a Newtronics 200MSTPC that no one seems to know/have. It can deliver a sine wave of around up to 30V pp. Now, as Im trying to study Opamps, I would like to amplify that sine wave but when I go down to the region of around 6V pp or lower, the wave gets SUPER distorted to the point that none of my scopes can recognize the frequency or PP voltage of it.My question is, is there a way that I can get a sine wave from my generator of around 20V pp (this particular generator outputs a very nice a clear wave at this range) and compress it to around 1V pp and then amplify it again with the opamp? I know this seems counter intuitive but the idea here is to study the opamps because if I set the gain of it to around 3 or 4, with the "high" voltage coming of the signal generator I cant supply enough voltage to the opamp to give it enough headroom to amplify the signal without distorting it or simply burning it.Im not asking for a full schematic or anything, if you could just point me in the right direction to study a particular circuit or IC oe concept it would be great.thank you very much

author: JS
author_rank: Frequent Contributor
timestamp: June 12, 2018, 12:12:54 am
content: Some attenuators are the way to go here I guess, at least would be my first option if you want to do it outside the device and not modifying the internals.For the ranges you are talking about you might want the steps to be 10dB or less, as it can't swing that much which seems strange. Can you measure in some way the THD or SINAD of the output in different ranges?From 30Vpp to 6vpp is about 14dB, your attenuators steps should be smaller than the range you can swing effectively from it so you can get any output you want for a good quality signal at the output.JSLooking at picts for your device, it seems that has 20dB and 40dB pads, if you use them you should be getting a smaller waveform with the higher range quality.

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 01:01:13 am
content: Quote from: JS on June 12, 2018, 12:12:54 amOk, Im a begginer so please bear with me.When the -20db and/or -40db buttons are not pressed, I have the full range of amplitude available on the amplitude knob. If I go down to the region of 6Vpp or less I get the exact same result as using the attenuator of the device, it doesnt matter if Im using the attenuator or not, whenever I get to the 6vpp region the signal either gets week or full of noise...Do you think it might be my generator? I know this is not a top notch machine but I dont see why I would have this problem in other generators.

author: JS
author_rank: Frequent Contributor
timestamp: June 12, 2018, 01:36:04 am
content: When you press the 20dB switch you shouldn't be able to get to 6Vpp at all, max range should be more like 3Vpp if the max range without it is 30Vpp. 20dB is 10 times in voltage last time I checked.Let's check the attenuators. Set the generator for a low freq sine wave, under 100Hz if possible, at some level the oscilator has low distortion, let's say 20Vpp. Measure the output voltage with your DMM, switch the attenuator in. Do this for one, the other and both together and write the values, post them here for future reference.If all things are fine, you should get 7.07Vrms as your reference, with attenuators off. 707mVpp when the 20dB attenuator is on. 70.7mVrms when the 40dB attenuator is on. 7.07mVrms when both are on (if this is a viable option)Other result than that means your attenuators aren't working properly. If you can find a user manual and even better, service manual and schematics add a link or file so we can use them to help you.JS

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 02:24:53 am
content: Quote from: JS on June 12, 2018, 01:36:04 amI know I said the FG goes to 30V, that was a mistake, I had the 10x probe of the scope on, sorry about that. I've done some measurements for you guysOk, so let me describe everything here.Plugging FG directly into my Tektronix TBS 1102B (that was bought 4 months ago and came with a callibration certificate) and outputing a 100Hz sine wave:First the max amplitude with a sine wave the FG gives is 16.2VppThe RMS given by the oscilloscope is 5.41VThe frequency reading is stable at 100Hz varying around 0.5HzNow I will set the amplitude of the FG right in the middle and add -20dB of attenuation:I get a reading on the oscilloscope of 74mV RMS and around 270mVPPThe frequency reading of the scope jumps from 104Hz all the way to 197Hz (then comes back to 103Hz and then jumps to 188Hz then to 103.5Hz then to 193Hz... crazy)Turning off the -20dB and adding the -40dB attenuator, keeping the frequency knob and range at 100Hz:It all goes to s&%t, no stable readings on any parameter.Now measuring the output of the FG with a fluke 87-Vampliture of the FG set to MAX and no attenuation at 100Hz: around 5.32Vamplitude set to the middle, no attenuation: 0.78Vamplitude set to the middle adding -20dB of attenuation: around 71mVamplitude set to the middle adding -40dB of attenuation: around 6.6mV (I think this is not a very precise range for this DMM)

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 02:38:09 am
content: My main issue seems to be that the wave form gets so polutated (distorted? week? I dont know the right word because I dont know what is happnening) that the scope doesnt see it as a sine wave anymore and cant even give a frequency reading.

author: JS
author_rank: Frequent Contributor
timestamp: June 12, 2018, 02:43:58 am
content: So, your 20dB attenuator is giving 34dB of attenuation, you have something there. The level should be closer to 1/10th of the level without the attenuator.Could you upload pictures of the scope screen and your setup?JS

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 03:15:30 am
content: Quote from: JS on June 12, 2018, 02:43:58 amI made a video, I think it is quicker and easier.https://www.youtube.com/edit?o=U&video_id=TXBg4EuaNuoAlso, attached I have the manual

author: KrudyZ
author_rank: Frequent Contributor
timestamp: June 12, 2018, 03:24:40 am
content: Like most generators, this one has the output voltage specified for driving a 50 Ohm load.If you load it lightly, as with a scope probe, you will get twice the output voltage from it.This might explain some of the issues you are seeing.

author: JS
author_rank: Frequent Contributor
timestamp: June 12, 2018, 03:41:48 am
content: Quote from: KrudyZ on June 12, 2018, 03:24:40 amEven without a load, a low freq sine should keep it's shape fine.I can't jump to the video link from tapatalk, it throws me to my channel, I'll try a different way laterJS

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 03:54:56 am
content: Quote from: KrudyZ on June 12, 2018, 03:24:40 amDo you think that could drive the attenuator to be all over the place like it is? Should I put it through a 50ohm resistor before the scope?

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 03:56:10 am
content: Quote from: JS on June 12, 2018, 03:41:48 amYou mean that a low frequency sine should not loose its shape just because there is no load?(sorry, my english is not great, just trying to understand if that was what you said)

author: Aspin
author_rank: Newbie
timestamp: June 12, 2018, 03:58:14 am
content: I would look at the scope spec.You may find that it is 8bit and down in the mV range it is work only a couple of bits.This will look like noise (because it is sampling noise).Also the auto measurement can be a bit hit and miss.

author: JS
author_rank: Frequent Contributor
timestamp: June 12, 2018, 03:58:28 am
content: Quote from: AngraMelo on June 12, 2018, 03:56:10 amYes, that's whst I'm saying. If you go for sharp MHz sqare waves you need the 50Ω load and a 50Ω coax to connect things.JS

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 04:19:44 am
content: Quote from: Aspin on June 12, 2018, 03:58:14 amYou are right, it is a 8bit scope.What would you suggest I do?I'm a bit lost now

author: Aspin
author_rank: Newbie
timestamp: June 12, 2018, 06:50:29 am
content: Quote from: AngraMelo on June 12, 2018, 04:19:44 amTry a 1x probe, a 10x probe will have divided the voltage by 10 before it hits the scope.You are trying to study op-amps at a gain of 3-4 so a 0.5Vpp will be fine and should be in the range of the scope.Also for signals in the < 50mV an op-amp may be needed to move them into the range a scope works well (this is one of their applications.I find that once you are looking at 5mV signals it is very hard for the scope to pick out the signal from the noise and then you would need to move to a Spectrum analyse (which a you don’t have and b don’t go under 100KHz generally)

author: AngraMelo
author_rank: Regular Contributor
timestamp: June 12, 2018, 06:59:35 am
content: Quote from: Aspin on June 12, 2018, 06:50:29 amI was plugging the FG directly to the scope, should I still try with the probe?