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 Post subject: Video to oscope adapter
PostPosted: Mar Thu 15, 2018 12:47 pm 
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Location: Warner Robins, GA
I have had a video to oscope adapter circuit I found on the internet several years ago with every intention to build it, but never actually doing it.

At work I have an older HP specan with a display that is much dimmer than what it should be so because the workload is quite hot I just connected a Tektronix XY display to it for the time being until the next cal cycle.

That got me interested again in the circuit and I'm interested enough to actually build it.

Attachment:
Video oscope adapter 1.png
Video oscope adapter 1.png [ 102.65 KiB | Viewed 864 times ]


Attachment:
Video oscope adapter 2.png
Video oscope adapter 2.png [ 70.05 KiB | Viewed 864 times ]


The basic circuit is for composite video, but after looking it over I saw where I could mod it for a VGA input by using a 4PDT switch.

Will the transistor circuitry generating the sawtooth waveforms actually work for frequencies other than the standard NTSC vertical and horizontal frequencies?

Also concerning the intensity mod output if I use it for instance with a solid state scope or Tektronix XY monitor with Z axis input it will be fine, but if I wanted to use it with an older oscope that has a Z axis input where it is just cap coupled to a part of the CRT circuit I don't think the output will be high enough, although with it being cap coupled more than likely the resultant picture wouldn't be nearly as good as it would be if it was dc coupled. Would it be possible to add some sort of amp that would increase the signal while keeping the dc and video signal proper?

Now if it will work with a VGA signal I might use it with my Tektronix XY monitor and connect it to my computer as a second monitor just to see what it will look like.


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 Post subject: Re: Video to oscope adapter
PostPosted: Apr Fri 13, 2018 9:41 pm 
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Location: Warner Robins, GA
The circuit as drawn doesn't work for VGA.

I'm not exactly sure I have it wired right, but I really don't have any composite source to test it with other than an Interface Technology 553 bus analyzer at work.

I thought that I could feed he vertical and horizontal sync signals to the transistors that generate the sawtooth waveforms, but apparently not.

I will test with that composite signal tomorrow just to see if the circuit is working right.

If it is then I need to figure how to feed the vertical and horizontal sync to the sync separator chip.

If not then I need to double check my work and see where I screwed up.

Looking at the data for the sync separator chip

http://www.ti.com/lit/ds/symlink/lm1881.pdf

I will need to increase the value of the 680K resistor for it to properly handle a VGA signal.

1 meg according to the data should work.

So what I need is a circuit that will combine the video signal along with the vertical and horizontal sync.

http://www.nexusuk.org/projects/vga2scart/circuit

Third picture

http://www.nanohawk.com/orange-pi-zero-and-vga/



EDIT:

Got to looking after I measured 12Vdc B+ from a 7809 regulator and saw where I did not ground the ground pin of the regulator.

That fixed the vertical, but the horizontal is still messed up.

EDIT 2:

Found the AD8032 wasn't connected to ground nor was pins 1, 2 & 5 tied together.

Finally gfot it working halfway decently off a composite signal, but there's some issues.

Think I'm going to need shielded wiring for all the internal signal wires and also I might need to address the grounds as I had to conenct all three grounds from the cables going to the X/Y monitor in order to get the best display.

Also I had to insert a capacitor in series with the vertical and horizontal outputs which didn't affect the display any that I could tell.

This presents a problem.

My Tek X/Y monitor only has two internal switches to set the gain between two levels far as I know so I will need to add level control pots to the output of the circuit.

Would two 10K pots work or should I use a larger value?


EDIT 3:

I tried a 10K pot on the vertical and the output looked screwed up so I disconnected the ground from the pot and the image cleaned up a good deal more so apparently the evrtical circuit with the .1uF cap was being affected by the cable on the vertical out.

So I will need to add a buffer.

For the TV frequencies I am thinking perhaps a TL-082 will work.

I will have to suss out the VGA Monday.

Concerning the TL-082 I need to measure the output voltage of the vertical and horizontal to see what the peak to peak is so that I know if 9Vdc is fine for the op-amp or if I need a higher voltage.

Now it is possible for me to get a 7909 which will give me a dual supply for the op-amp and eliminate the need of coupling caps on the output.

That said I am tempted to just build the circuit in the Tektronix X/Y display, although I don't think I have the room to do so. I could use panel mount trimpots or PC board trimpots for the controls. Then again I want this to be able to be connected to any scope with a Z axis input so it being in a separate box is just fine.

I am going to try shielded wire for the inputs, outputs and all wiring to the intensity and contrast pots just to see if it cleans up the signal any further, although it is looking real good as is and is possibly as good as it is going to get.

Concerning the vertical and horizontal sync lines from the VGA connector I will try the resistor method first to combine them then feed that to the sync separator chip.

I then reduce the switch to a DPDT or if I wanted to mix the sync with the VGA video a SPDT switch.

Also will replace the RCA jacks with BNC jacks except the composite video input which will remain RCA since most common composite video outputs are RCA jacks.


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 Post subject: Re: Video to oscope adapter
PostPosted: Apr Mon 16, 2018 5:11 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 19342
Location: Warner Robins, GA
If all else fails I can just get a VGA to composite converter box.



EDIT:

Needed a Motorola to Mini GM antenna adapter and found one on Amazon, but it was an add on item so I went ahead and ordered a VGA to composite box.

That will simplify things as I don't need the switch and I don't need any circuitry related to the VGA at all.

I tried the TL-082 OP-AMP and the horizontal seemed to work just fine, but the vertical didn't work and I found out I had pins 7 and 8 tied together.

Will fix that and see how well it works.

Provided it works ok I got a couple 10K multi turn panel mount pots I can use for the vertical and horizontal size adjustments.


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 Post subject: Re: Video to oscope adapter
PostPosted: Apr Wed 18, 2018 7:39 pm 
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Location: Warner Robins, GA
I found out that phase matters with the vertical and horizontal plus I have some ground issues.

When I disconnect either the vertical or horizontal ground going to the X/Y display I notice the display is not quite right even though the video ground is connected.

Also when I turn the vertical adjust pot full CW I get a display where the horizontal lines are slightly wavy, but if I back off the vertical adjust pot slightly that goes away.

I used .1uF caps to couple thevertical and horizontal to the buffer and the vertical seemed fine, although I noticed I could see the scan lines in a highlighted part on the display which is white edges with the letters black and the vertical sawtooth waveform was slightly curved up at the bottom.

I tried a 1uF cap for the vertical and that problem was gone.

The horizontal I tried a 1uF cap for it and saw no improvement in the horizontal sawtooth.

The horizontal at the start of the waveform has a slight curve to it but I can't see where the display is being affected.

Also if I adjust the bright and contrast controls I can notice a slight wavyness to the display and the waviness is in the vertical plane.

Here's the final schematic.

Attachment:
Video to oscope adapter final A.png
Video to oscope adapter final A.png [ 77.48 KiB | Viewed 575 times ]


Attachment:
Video to oscope adapter final B.png
Video to oscope adapter final B.png [ 79.27 KiB | Viewed 575 times ]


Decided it was easier to go with an external VGA to composite adapter which seems to work well, although I did have to install a cap between the composite in and the contrast control as the adapter had a little DC on its output which caused me to have to set the bright and contrast controls to vastly different positions than with a standard composite signal from a 1553 buss analyzer I have at work that has a composite out jack.

Tested the VGA using a LeCroy scope with VGA output and it worked quite well and everything was readable, albeit with some slight flicker.

Overall I believe this circuit is working good at least on a Tektronix X/Y monitor.


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 Post subject: Re: Video to oscope adapter
PostPosted: Apr Mon 23, 2018 10:00 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 19342
Location: Warner Robins, GA
Here's some pictures. The pictures do not really do the display justice.

This one was taken a few days ago with no light in the room.

Attachment:
20180420_021026-800x600.jpg
20180420_021026-800x600.jpg [ 73.05 KiB | Viewed 553 times ]


These were taken this afternoon with light from the window in the room.

Notice how the scan lines start to be farther apart near the top and also how the top seems slightly brighter.

Attachment:
20180423_161748-800x600.jpg
20180423_161748-800x600.jpg [ 74.98 KiB | Viewed 553 times ]


This picture is inverted landscape from the display settings in Windows.

It shows how the brightness of the top of the window is better balanced at the bottom of the screen and that the scan lines still expand some near the top. Now it could be that the scan line issue is causing the brightness issue.

Attachment:
20180423_162048-800x600.jpg
20180423_162048-800x600.jpg [ 71.24 KiB | Viewed 553 times ]


That said I can minimize the effect of the brightness issue if I set the brightness and contrast controls properly.


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