Alarming quirks with new function gen.

[Nothing]

I got a new function gen on ali express, I know, but price point was an issue and I was just looking for entry-level stuff. Anyways, I found this JDS6600 FPGA based one which sounded about like what I was looking for. Just a $100 little entry level function gen. User manual can be found here: https://supereyes.ru/img/instructions/JDS6600%20Quick%20Guide.pdf

Anyways, on to the problem. I noticed the other day when the unit is powered off, but I leave the leads plugged in, I get a periodic wave with a ~170vpp voltage. But only when I just have the positive lead connected to my scope. If I connect the negative (black) lead to the ground lead on my scope, the voltage reads a steady zero volts. It's only when it's not referenced to ground that I get the high voltage. which makes me wonder if this is something I should concern myself with on not.

I would really appreciate any advice or opinion you guys might be able to offer me on this. Thanks very much. And take care!

[TimFox]

Assuming that voltage with only the red lead connected is at your line frequency (60 Hz), it looks like the normal capacitance through a line filter capacitor (before the power switch) to the chassis with that chassis ungrounded.
Does your unit use a two or three prong power cord, and is your outlet properly grounded?  The manual doesn't discuss the mains input to the external DC power supply module.
To verify that it is actually the mains, trigger your oscilloscope on "Line".

[Nothing]

It does indeed only have 2 prongs for the power supply. It's just your bog standard 120v to 5v adapter that for some reason isn't USB. Never understood why companies still do that when USB's become the de facto standard in terms of 5v power supplies from mains.

Anyways, that's good to hear it's likely just from capacitive coupling inside the unit. Part of me worried they cheaped out and used a capacitive dropper. But on the other hand, I couldn't see them going THAT cheap even for a cheap, entry level function gen.

[cdev]

Is your function generator a Feeltech one? If so what model is it? Thats a problem with mine. The floating voltage can get as high as 70v.

I got a new function gen on ali express, I know, but price point was an issue and I was just looking for entry-level stuff. Anyways, I found this JDS6600 FPGA based one which sounded about like what I was looking for. Just a $100 little entry level function gen. User manual can be found here: https://supereyes.ru/img/instructions/JDS6600%20Quick%20Guide.pdf

Anyways, on to the problem. I noticed the other day when the unit is powered off, but I leave the leads plugged in, I get a periodic wave with a ~170vpp voltage. But only when I just have the positive lead connected to my scope. If I connect the negative (black) lead to the ground lead on my scope, the voltage reads a steady zero volts. It's only when it's not referenced to ground that I get the high voltage. which makes me wonder if this is something I should concern myself with on not.

I would really appreciate any advice or opinion you guys might be able to offer me on this. Thanks very much. And take care!

[Gyro]

It does indeed only have 2 prongs for the power supply. It's just your bog standard 120v to 5v adapter that for some reason isn't USB. Never understood why companies still do that when USB's become the de facto standard in terms of 5v power supplies from mains.

Anyways, that's good to hear it's likely just from capacitive coupling inside the unit. Part of me worried they cheaped out and used a capacitive dropper. But on the other hand, I couldn't see them going THAT cheap even for a cheap, entry level function gen.

Don't assume that a USB charger / adaptor would be any better in terms of Y-cap filter leakage current. In fact, many no-name Chinese ones have positively dangerous SMPS transformer construction.

You have to hope that the Function Generator manufacturer has chosen a reasonable quality safe adaptor to partner their product.

[Doctorandus_P]

I've also got an JDS6600, and it's quite a nice function generator (for it's price). The power adapter is atrocious tough. Luckily the case is almost empty and there is plenty of room to build in some decent  5Vdc power supply, and as it's measurement equipment a simple low-capacitance transformer combined with a linear voltage regulator is a simple upgrade.

Another upgrade would be to replace the resistors in the resistor based DACs with hand sorted resistors, but that would be a finicky task as this has very small resistors.

[MikeK]

The JDS6600 looks like one of the many FeelTech clones.  Known power supply issues.  I have watched a couple of YT videos showing the problem and showing fixes...And I don't even own one. lol

[MarkMLl]

Belated comment. I've got a JDS6600-40 in the UK, and note that the wall wart has a plastic ground pin i.e. no attempt is being made to shield it or provide a ground reference to the instrument (which is, obviously, a mixed blessing).

I've been working on a Linux-based controller program, and have noticed that if one of the BNCs has a lead on it the internal USB interface chip resets at random.

I can only assume that the internal electronics is highly susceptible to RFI, and allowing for that I'm hardly surprised that it's letting through random crap it's getting from its PSU.

If in doubt, find a mains isolating transformer (i.e. not an autotransformer). It that fixes spurious voltages etc., then look for a decent PSU.

MarkMLl

[Kleinstein]

The capacitive coupled voltage of something like half the mains voltage relative to ground is common to essentially all SMPS with a 2 pin plug.  They kind of have the chance to leave out the capacitors and instead get EMI problems and going beyound the limits there. A few cheap one may go for just 1 cap, and it depends on the polarity if you get near 100% mains or little. This may be OK in the UK with a polarized plug, but few cheap things are made specificly for the UK.

[Mechatrommer]

yes the issue is not specific to any device, brand or even this FG unit, it can happen to anything even gave me tingling shock on so many occasions when casing of the device is metallic. i remember the one that got me really pissed off is from an Epson Projector while doing a job in a hotel ballroom, mind you its not a cheap arse brand, its Epson! my favorite name for printer... if i really got pissed off and i can have access to the device into my lab, i will just solder a meg ohm resistor from secondary ground to mains earth line if its 2 prong, i'll make it 3 prong to get access to earth line i'm not sure whats good is the Y1 (or X2 whatever the type rating is) capacitor even it is there, still can give me a shock.

check out our Uni-T UTG AWG thread we modded powerbank to power our FG, sure this problem will be gone. unless your lab bench also capacitively coupled to nearby mains cabling, like my wooden bench here, but not enough to give me a shock. laying my probe on the bench can register some Vac. thats the good thing when the FG/AWG is readily accept a single 5V power input, it can easily modified to battery powered or wall wart psu replaced, not the built in type of smps offline psu with many power rails branching out to the device's pcb.

[MarkMLl]

I was looking at the price of isolating transformers a few days ago, to help out with this sort of problem and the occasional (smallish) PSU repair. Best I found was £75 for a 500VA toroid... add another £25 for minimal circuit protection and enclosure :-(

MarkMLl

[Mechatrommer]

Thats way overkill for a 10w device.. a $5 samsung phone charger could do it..

[MarkMLl]

It's overkill for a 10W device... but even a smallish PSU to be worked on is likely to need a couple of hundred. Ditto for isolating an arbitrary mains-powered 'scope or analyser.

MarkMLl

[Kleinstein]

Thats way overkill for a 10w device.. a $5 samsung phone charger could do it..

The phone charger would not solve the porblem: it has the same problem. The Supply in (or that comes with) the function generators is very  much comparable to a phone charger. The point that EMI filtering against common mode emissions of a SMPS essentially needs a capacitance (a few nF) from the secondary to the primary side.  With a reversible 2 pin plug (US, EU) there is nothing better they can do than using half the mains for this.

With the usually small capacitance one normally can not really feel that current. Some 150 V and 5 nF from half the mains voltage should also normally not be a problem to instruments if they can survive a more common ESD event (e.g. 100 pF and 4 kV).  One may get a more sensible small shock if there is additional electrostatic charge in this capacitance, so a special kind of ESD even with moderate voltage (e.g. 500 V), but a bit more capacitance. So ideally there would also be a electrostatic discharge path. It is sometimes found,  especially if they have PE available.

[Mechatrommer]

this is where the hunt for the old smaller 50/60Hz transformer is invalueable. its quite hard to get them as single electronic part from suppliers today, but they can still be found inside some of cheap devices and old wall warts. worth grabbing from dumpster anytime i see them around.

[Zero999]

I was looking at the price of isolating transformers a few days ago, to help out with this sort of problem and the occasional (smallish) PSU repair. Best I found was £75 for a 500VA toroid... add another £25 for minimal circuit protection and enclosure :-(

MarkMLl

That sounds reasonable and could also be useful for test purposes.


Why not simply connect one side of the output of the signal generator to something earthed?

[MarkMLl]

Why not simply connect one side of the output of the signal generator to something earthed?

Thinking in particular about the interface-resetting problem I've seen, I'd rather not prejudge what is responsible for putting what kind of voltage or RFI onto which piece of interconnecting cable: my 'scope is mains powered and even though it's an HP it might, after many years or sterling service service, have a dodgy filter on its IEC input.

So even leaving aside the issues of working on suspect PSUs (I've got a "fancy Chinese" one with a measurement circuitry fault) I think it would be worth getting to a situation where I can selectively isolate one or more devices from ground.

Of course, the one thing I've got a belt off in the last 20 years or so was an SMPS cap with high ESR, which had the fun property of recharging itself over... and over... and over...

MarkMLl