Beginners Function Generator / Signal Generator?

[BillyO]

No funny ground business with my precious TDS 340.

Your TDS340 will be grounded if it's attached to a proper outlet.

The power supplies that come with cheap and cheerful AWGs leave a bit to be desired.  They are noisy and can produce leakage voltages.  Measure between the outputs and your house ground.  The last one I saw had about 30V AC leakage voltage and a few uA current.  We ended up putting in a switch to ground the negative lead on the DC side of the supply to eliminate the leakage for using it with ground referenced DUTs.  We will eventually replace the power supply altogether with one that creates less noise.  Unfortunately it looks like there is not enough room inside for a suitable linear supply.  That would be best, but these things can draw nearly 2 amps @ 5V when driving both channels as hard as they can into 50 ohms.

[bdunham7]

It measures infinite resistance between the output and input terminals and there is no connection to mains earth too, which means the generator's outputs are floating.

The problem with that method is that any link between primary and secondary is likely to be capactive, so it would measure open on an ohmmeter.  You would need to measure the voltage between the secondary and earth ground with an AC voltmeter.

[shapirus]

The problem with that method is that any link between primary and secondary is likely to be capactive, so it would measure open on an ohmmeter.  You would need to measure the voltage between the secondary and earth ground with an AC voltmeter.

Good point.
Tested this: it measures 96 V AC between mains earth and any of the DC output terminals (230V being the nominal mains voltage). The generator's DC negative is common with the output ground. Capacitive coupling, yeah.
Unfortunately, my DMM doesn't have a Low-Z mode, it would be interesting to measure that.

However, I have also tested 4 (whatever I could grab around) other PSUs, some with a 2-wire mains connection, some with 3-wire (with earth). One was a cheap(ish) noname, one was a genuine Japanese laptop PSU, other came with usual devices like any other.
*All of them* measured 93..115 V AC between any of the output terminals and the mains earth, just like the one in question. I measured them all without load.

I don't know what to make of it. It looks like any PSU will be the same in this regard.

Yes it can be a problem if you touch a small MOSFET's (such as 2N7000) gate with the generator's ground (maybe the signal wire as well) -- it may (or may not, or may partially) kill it, I did experience this in the past.

My oscilloscope measures about +/- 36 Vpp if I touch the generator's ground with a 10:1 probe (ground clip being disconnected), which may be just enough to damage small transistors.

But is there a PSU that won't be capacitor-coupled to mains? Sounds like this can be only solved by making the generator's output ground connected to mains earth, which will create its own set of well known problems.

[bdunham7]

Unfortunately, my DMM doesn't have a Low-Z mode, it would be interesting to measure that.

Just put a 10K resistor in parallel with the DMM, measure the voltage and do the math to get the leakage current.

But is there a PSU that won't be capacitor-coupled to mains?

Well, there's batteries for one!  Also it is fairly easy to make a low voltage linear supply with a double-bobbin transformer that will have very, very little leakage.

[shapirus]

Just put a 10K resistor in parallel with the DMM, measure the voltage and do the math to get the leakage current.

That'll be for the others feeling adventurous, I have hidden those PSUs already for now .

Also it is fairly easy to make a low voltage linear supply with a double-bobbin transformer that will have very, very little leakage.

This is actually another interesting topic. I am seeing a significant leakage, or rather capacitive coupling (between the windings themselves), or maybe it should better be called "mains interference" on the output terminals of my DIY lab PSU that is powered from a toroidal transformer -> bridge + caps -> Chinese DC/DC adjustable module. Not usually a problem, but can kill small MOSFETs if I'm not careful enough. I wonder if there's a way to get rid of it somehow except for connecting one of the terminals to mains earth. Hmm.

[.RC.]

I am in the same boat as the OP in this thread.  I would like a simple function generator that can put out basic waveforms. Square, Triangle and Sine.  So I thought I would start with one of the cheap ICL8038 kits.  Well the sine wave it puts out barely resembles a sine wave.

So I looked at what is cheap and popular, and that is the FY6900, 20MHz, but it is $135 delivered. Then you read about fixing the issues it has.  To fix them adds to the price, so then you think about the UTG932 which is about $100 more.   Reading on them some say it is fine, other say it has it's own issues.

Then you think.  Do you want to spend more on something that may be not suitable, then down the track you have to replace it anyway.

So I went back cheaper and found

this cheap one https://www.aliexpress.com/item/1005004618727924.html

Then a bit more expensive and not even a case https://www.aliexpress.com/item/1005004826478755.html

Then before long you are up to this sort of price point https://www.aliexpress.com/item/1005005295374464.html

And I am not sure I want to spend more then that at this point in time and how do I know any of them can put out a decent sine wave.

[coromonadalix]

i use 3x  Juntek JDS2900    at my job,  the 40Mhz variants   

work on 5vdc adapter and or usb ports with thoses woh as enough power ...    control software for them too

paid 125$ CAD shipped at the time, gone up a bit, and they are easy to control, interface not hard to learn


works well for what i need ... and have Siglents and a TTI or serious stuff

[.RC.]

Thanks.   In a moment of weak mindedness I ordered a FY6900.   Knowing full well it's issues.  With such a long thread on here about it with modifications and so on, it will be way overkill, but should do the job.