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 Post subject: Variable frequency sawtooth generator
PostPosted: Jan Tue 08, 2019 2:52 am 
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I'm looking for a variable frequency sawtooth generator circuit that can use a sync signal to sync the oscillator.

Frequency range needs to be whatever will cover up to 1280 X 1024 VGA.

Not exactly sure how to figure out what frequencies a typical VGA mode uses for a CRT monitor.

Doesn't need to be anything elaborate, but it does need to be stable far as the frequency is concerned.

Is there anything out there that can automatically detect the desired display mode and put out the proper sawtooth waveforms and Z axis signal?


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Tue 08, 2019 3:24 am 
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something like this ?. http://www.alltronics.com/cgi-bin/item/ ... erator-Kit


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Tue 08, 2019 4:11 am 
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The 7KHz maximum is one limit of that circuit plus it doesn't put out sawtooth nor does it have a sync input.

I do like its simplicity though.

From what I gather the maximum frequency I would be needing is 100KHz.

The closest thing to what I'm looking for would be a scope sweep generator circuit as it has sync, sawtooth output and variable frequency.

Also the output will need to be adjustable.

I have seen this circuit which has oscillators like what I would need, but I don't know how frequency stable it would be or is it like a CRT TV where the sync keeps the oscillators at the right frequency and is tweaked by the hold controls.

Image

http://www.electronixandmore.com/projec ... index.html

Here's the same basic circuit only it has a range switch.

Image

http://www.electronixandmore.com/projec ... index.html

If either of those will work I may use them.

That said I don't really know much about the vertical and horizontal frequencies of various VGA modes.

I would technically need 1280 X 1024 to be the proper aspect ratio for a 8 CM X 10 CM screen size. So I just really need to figure out the frequencies for that and what cap I will need for each oscillator to get the desired frequency.


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Tue 08, 2019 4:42 am 
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Quote:
That said I don't really know much about the vertical and horizontal frequencies of various VGA modes

http://martin.hinner.info/vga/timing.html

EL4511 is a better sync separator that supports a much wider range of resolutions. You can interrogate it to get scanning rates. Not sure that is particularly helpful, though, because it sounds like you want a ready-to-build circuit. Simple circuits seem to need various trimmers to make their outputs fit a particular resolution/refresh rate. Something automatic will be more challenging!

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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Tue 08, 2019 6:50 am 
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Thanks for that link.

Looks like I need one oscillator to cover from 59-100Hz and the other oscillator to cover from 30-100KHz.

I like that chip, but it wouldn't do me much good I don't think.


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Tue 08, 2019 9:14 pm 
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From your description of what you are looking for, the above circuit is exactly what you need. You don't have to worry about the stability of the oscillators as they will be externally triggered each frame of your video signal. However, you still would want to use some good quality timing capacitors and current source resistors.

The only time the 555s will be free running is if you don't trigger them externally. If you don't have a composite video signal then you wouldn't need the LM1881 and you would apply your trigger signal (square wave) into pin 2 of the 555s through the 0.1uF and the 555s will trigger on the negative edge of the signal.


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Wed 09, 2019 1:51 am 
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I'll try that circuit then.

I will be using it with a VGA signal so I will already have the vertical and horizontal sync pulses.

For the timing resistors I figure 1% metal film should be ok.

The timing caps I suppose should be 1% as well.

Only thing I wonder is what the maximum output voltage will be?

I do plan on adding a TL-082 OP-AMP as a buffer and can add some gain if necessary.

The OP-AMP will be operating on + and - 9Vdc supplies.

Now is it possible to operate the 555 on 9 Vdc as well or should it be operated on the 5Vdc the circuit calls for?


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Wed 09, 2019 2:06 am 
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Capacitor tolerance is not critical, just use a good quality film type, metal film resistors are good. No problem powering the the 555s from 9V, just make sure it is regulated. As far as the ramp output, it will swing from 1/3 to 2/3 of 555 power supply.


Last edited by upsss on Jan Wed 09, 2019 2:14 am, edited 1 time in total.

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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Wed 09, 2019 2:11 am 
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So the 9 Vdc won't affect the 3906 circuit any?


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Wed 09, 2019 2:18 am 
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No, the 2N3906 is a simple constant current source and you will have to adjust the current source any way to get the correct frequency timing.


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Wed 09, 2019 2:32 am 
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Oh ok.

Here's the circuit.

I took a circuit that same website had for a VGA to oscope adapter and added the feedback resistor so the oscillators would free run in the absence of sync pulses. I also a dded a buffer on the outputs and a multi turn pot so I could adjust the size of the raster.

Attachment:
Sawtooth.png
Sawtooth.png [ 11.77 KiB | Viewed 1833 times ]


Now I could simplify things and use a single +9Vdc supply, but I would then need to bias the OP-AMP and also use output coupling caps.


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Wed 09, 2019 4:37 am 
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If you don't mind adjusting trimmers when display resolution is changed, 555 timers should work well.
Quote:
Is there anything out there that can automatically detect the desired display mode and put out the proper sawtooth waveforms and Z axis signal?

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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Wed 09, 2019 1:36 pm 
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Going with 555 timers.

I could always hack a CRT monitor, but I don't know if I would get sawtooth waveforms and most monitors are a single board and quite large.

Got a nice metal box to put it all in.


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Jan Fri 18, 2019 1:30 am 
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I had thought of using a two pole rotary switch with caps for 640 X 480, 800 X 600, and a few other lower resolutions, but then I remembered that the display size is a 5:4 aspect ratio and that only 1280 X 1024 plus a couple much higher resolutions would be able to be used due to the screen not being perfectly square.

I may yet include caps for the most common resolutions though so that I am not limited to just this Tektronix X/Y display.

What I may do if the oscillators are stable enough is get some multi turn trimmer caps and use those to set the oscillators for the various resolutions I choose.

That way I can simply switch resolutions in Windows and then set the rotary switch to the proper resolution without having to adjust anything.

I figure on using no resolutions under 800 X 600.

So the panel will only have two 10K multi turn pots, the brightness and contrast controls and the rotary switch on it with the trimmers for the resolutions on the rear.


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Nov Tue 12, 2019 1:41 am 
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Just found your post on this subject. I am playing with an old data scope ,and have got the CRT sort of working, but I need some sawtooth generators. Looks like the one you planned to use may do the job for me. Did you have any success with it?

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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Nov Tue 12, 2019 3:19 am 
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Never did build that circuit as I decided to go with a VGA to composite adapter and just use the composite to oscope adapter I built.

Those should work though I think.


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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Nov Thu 14, 2019 1:21 am 
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I just finished putting together a couple of sawtooth generators using the circuits you showed above and it went very well. I now have a raster on the CRT, although the horizontal generator output was not high enough, so I added an Op Amp.
Next thing is to try some video.

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 Post subject: Re: Variable frequency sawtooth generator
PostPosted: Nov Thu 14, 2019 1:57 am 
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Nice.


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