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 Post subject: Ground loop question
PostPosted: Apr Sun 05, 2020 6:38 pm 
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I have a box I made that takes a composite video signal and converts it to X Y Z outputs for use with an oscilloscope or X/Y monitor.

It works great, but I noticed I had what looked like interference in the video.

Now initially I had that issue only worse, but I redid the grounds of the circuitry and that made it better.

I figured it might be a ground loop so as a test I took the X/Y display and plugged it into one of those ground adapter plugs.

Bingo the picture now looks as good as the monochrome portion of a S-video signal can look.

So it looks like I may need a video ground loop isolator installed before the box or is there another way to fix it?


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 Post subject: Re: Ground loop question
PostPosted: Apr Sun 05, 2020 7:01 pm 
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Try clamp-on ferrites on the cords.

Chas

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 Post subject: Re: Ground loop question
PostPosted: Apr Sun 05, 2020 7:15 pm 
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The laptop with the S-video jack doesn't use a power supply with a grounded plug, but it is connected to a USB sound card which is connected to my main stereo and my desktop which does use a grounded power cord is also connected to the stereo through a USB sound card.

So perhaps it has to do with the fact that the X/Y display has a direct path to ground, but the S-video signal doesn't have that direct ground path.


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 Post subject: Re: Ground loop question
PostPosted: Apr Mon 06, 2020 12:50 am 
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That is almost certainly the issue -- incompatible grounds. The 3 prong adapter is one time-honored way of dealing with that. Running both devices through an isolation transformer is another.

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 Post subject: Re: Ground loop question
PostPosted: Apr Mon 06, 2020 12:55 am 
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I hate to run the X/Y display without a ground.

I may see if I can find a video ground loop isolator and try that.


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 Post subject: Re: Ground loop question
PostPosted: Apr Mon 06, 2020 1:53 am 
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We used to make many thousands of these, integrated into the set-top boxes for the analog VOD system we used to make back in the day. We installed thousands of these systems into schools.
Later versions were optically isolated, I think I still have some in the garage.

Might not be practical to ship one to you, even if it's free....

Your thoughts?

Simple circuit, earlier ones used an NE592.

Another thought - can you run all the gear from the one power outlet and route the power cable to the display along the same path as the signal cable?
The trick is either to break or to squash the loop.

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 Post subject: Re: Ground loop question
PostPosted: Apr Mon 06, 2020 3:36 am 
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Everything is powered from one outlet. It wouldn't be easy to squash the loop so I would need to break it.

The circuit you are talking about sounds interesting.

I'd like to know more.

Would something like this work or would it cause video signal issues?

https://www.amazon.com/Hxchen-Female-Co ... cs&sr=1-10

It's more of a fun thing to see what I could do with the X/Y monitor.

I really wanted a circuit that would take the VGA V and H sync signals and convert them into the proper sawtooth waveforms for whatever resolution I selected, but that isn't easy to do for multiple resolution selection automatically as it requires oscillators and manually altering the oscillator frequency. That would get a better picture quality.


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 Post subject: Re: Ground loop question
PostPosted: Apr Mon 06, 2020 3:47 am 
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RF racket may be coming from the audio jacks at the computer. Ferrite those, try before building stuff that can cause video delay issues like ghosting...

GL

Chas

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 Post subject: Re: Ground loop question
PostPosted: Apr Mon 06, 2020 3:58 am 
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The only audio connected is a USB sound card.

Pretty sure it has to be a ground loop as disconnecting the ground of the X/Y display fixed it.

I'd leave it that way, but if there was ever a hot side of the line fault to the chassis, I would damage one of the connected devices the fault current would travel through on its way to ground.


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 Post subject: Re: Ground loop question
PostPosted: Apr Mon 06, 2020 6:06 am 
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That "balun" is probably just a common-mode transformer. Video needs to go close to DC so a conventional isolation transformer doesn't have the bandwidth.

This device picks up earth loop noise and injects an equal and opposite signal to the active line to cancel out the noise. It does not isolate the input and output the way you might expect. Only an active device (e.g. a differential amp or an optoisolator) can do this.

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 Post subject: Re: Ground loop question
PostPosted: Apr Mon 06, 2020 12:27 pm 
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Oh ok so just injecting an equal opposite signal might not be enough to stop the problem?


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 Post subject: Re: Ground loop question
PostPosted: Apr Tue 07, 2020 12:50 am 
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It may, depending on the design of the "receiver". Earth currents might get into the internal circuitry and cause trouble anyway. But if it works for you....

I have never seen one of those passive devices that does better than attenuate, rather than remove, the problem.

An active device breaks the loop and reads just the difference between the inner and outer conductors. You can do this with a high speed op-amp or an optoisolator like a 6N136. You won't get much more than 4 or 5 MHz from a 6N136 though.

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Last edited by irob2345 on Apr Tue 07, 2020 1:38 am, edited 1 time in total.

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 Post subject: Re: Ground loop question
PostPosted: Apr Tue 07, 2020 1:12 am 
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Oh ok. I'd rather not spend money on something that may not completely solve the issue.


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 Post subject: Re: Ground loop question
PostPosted: Apr Tue 07, 2020 1:56 am 
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You need to ground loop isolate a complete VGA signal, is that correct?

If so, what bandwidth do you need?

Maxim make some very high speed op-amps and there may be a reference design on their site for just what you are trying to do.

Or if cost is no object, see if Extron have such a device off the shelf.

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 Post subject: Re: Ground loop question
PostPosted: Apr Tue 07, 2020 2:12 am 
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No I only need to ground loop isolate the luma signal of an S-video output.

I will eventually figure out a way to get the VGA done right to where it can display on that X/Y monitor.

That said are there any affordable devices out there that can turn a VGA signal into a proper X/Y/Z signal?

Sure I can build something as there's projects out there to do just that, but something already built would be better.


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 Post subject: Re: Ground loop question
PostPosted: Apr Tue 07, 2020 2:36 am 
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If it's VGA, try a cable with proper heavy shields. Ground loop noise isn't normally an issue for 2 or 3 metre VGA runs, unless you have a very poor quality cable (and believe me, they do exist!)

So, are we talking about standard bandwidth (<4MHz) video signal?
Easy-peasy!
I'll find a circuit for you or draw one if so.

Attachment:
DiffAmp.png
DiffAmp.png [ 12.76 KiB | Viewed 1235 times ]


The LMx18 will not drive a video load, you'll need a newer part for that, such as an LM7171, and a +/- power supply. The Video Out ground is the CT of the power supply.
Use SMD bypass caps close to the chip.

To drive a video load, you need 2 volts PP and 75 ohms in series with the output. You also need to add a 75 ohm load for the source. And increase the gain to x2 by changing the 10k resistors that go to the inputs to 4.7k.

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 Post subject: Re: Ground loop question
PostPosted: Apr Tue 07, 2020 3:54 am 
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So does the ground of the composite video cable connect to one input of the OP-AMP?

First I need to connect it back up then unplug my USB sound card from the laptop to see if the problem goes away. May do that tomorrow.

There are USB ground loop isolators which if the problem goes away when I disconnect the USB cable to my sound card might be a better solution than trying to ground loop isolate the video.

Of course the ground loop issue could be due to the fact I built the circuit on a piece of perfboard versus something professionally made. Initially it was worse until I redid some of the circuit grounds on the perfboard with lower gauge wire.

So it could be that if I had gotten someone to design a PC board, I may not have this problem.


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 Post subject: Re: Ground loop question
PostPosted: Apr Tue 07, 2020 4:29 am 
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Yes, the cable ground goes to the - input of the diff amp.

This circuit is quite effective, we'd use it for composite video running up to a kilometre or more, with several volts of common mode hum.

The later optoisolated version was also used in TVs where the chassis was at 120 volts AC above ground. The circuit used 3 optos, a 6N136 and two 4N35s. One 4N35 handled audio, the other was used in reverse for feedback to set the bias for the cold-side video driver. This was needed to compensate for shifts in 6N136 characteristics.
In this case the isolated cold-side power was obtained from 4 turns of wire around the flyback core.

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 Post subject: Re: Ground loop question
PostPosted: Apr Tue 07, 2020 1:31 pm 
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Ok thanks.

Traco makes a nice isolated +/- power module which would work great for this circuit and the circuit can be mounted in the same box as the composite to X/Y/Z converter.

Now with the converter I have a 75 ohm resistor on the video input.

If I can fit the circuit inside the converter box do I need the 75 ohm load resistor or will it be fine without it?

Here's the circuit as a reference.

Attachment:
Video to oscope adapter .png
Video to oscope adapter .png [ 281.88 KiB | Viewed 1206 times ]


Now given I will be adding the circuit and a +/- supply I can then alter the TL-082 buffers to eliminate the capacitors and the bias method I used.

Also the 160uF cap was used because the VGA to composite converter I originally used had a slight bit of DC on the output which messed with the circuit so if the luma output of the S-video cable has no DC on its output I could eliminate the cap.

Or can I simply alter the AD8032 circuit to have a differential input provided I can switch it to use a +/- supply without impacting the operation of the brightness control?

That might not work due to the sync separator chip being fed the video signal as well and it would need the signal referenced to ground somehow.


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 Post subject: Re: Ground loop question
PostPosted: Apr Tue 07, 2020 2:04 pm 
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Looks like your box has about +18 volts unregulated and +9 volts regulated. You could run the diff amp off this just by using the +9 volts as the reference ground and the ground as the -ve supply. Just add 47uF caps to the inputs. The output will need the 75 ohms in series as well as about 470uF to remove the DC offset.

Would you like me to draw this up?

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