DIY Schematics: Sandin Function Generator & Differentiat

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lizlarsen
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DIY Schematics: Sandin Function Generator & Differentiat

Post by lizlarsen » Mon Apr 04, 2011 11:25 pm

Hi everyone, I mentioned this a while back, but as promised here's some quick schematics for the Sandin IP Function Generator circuit, updated for use with the LZX Visionary video synthesizer, and using attainable ICs. It's also been updated with "Span" and "Bias" controls. These controls could be made optional and various resistor values could be tweaked to taste (or divided down a bit for higher bandwidth -- these are mostly the original values from the Sandin schematics.) An output Bias control could be good too, depending on how much control you want over what the signal scaling is on the output.

LM6172 and 1N5711 could be substituted for TL072 and 1N4148, but with drastically poorer performance for video.

Anyway, it may get revised, but I wanted to get something up for anyone itching to build onto their system. :tu: It's a great DIY circuit with only two dual op-amps and lots of control.

EDIT: I've added a version of the "Differentiator" module as well, with expanded controls for adding the input back in ("edge enhance") and an inversion/mix control for the differentiator output as well.

EDIT: Attachments to this post have been deleted. Files are revised in following post below.
Last edited by lizlarsen on Wed Sep 07, 2011 10:17 am, edited 4 times in total.

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Post by andrewF » Tue Apr 05, 2011 3:10 am

Thanks Lars
I have the parts....just need some time and its built!

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Post by lizlarsen » Tue Apr 05, 2011 8:38 am

Awesome! Keep in mind this circuit is expecting incoming video that has already been DC restored, sync stripped, and scaled to 0-to-1V levels (typically a function of the Video Sync Generator module) and then at some point encoded back to Composite Video (a function of the Color Video Encoder module.)

Andrew you may be able to tap off of your "video palette 2" circuit for some sort of "send/receive" that'll give you a signal that you can run through processors like this.

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Post by andrewF » Tue Apr 05, 2011 9:22 am

good, I was hoping to do something like that. my palette already has the 'receive' input.

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Post by lizlarsen » Tue Apr 05, 2011 9:41 am

Since diode drop is 0.7V, you want a ~4.4V bipolar signal, centered at 0, going into the core of the circuit. The first 2 op-amps and the span/bias controls are designed for scaling the 0 to 1V signal to a +/- 4.4Vpp signal. And the positive offset at the output amp is attenuating and biasing the signal back to an approximate 0-to-1V scale.
Last edited by lizlarsen on Tue Apr 05, 2011 11:17 am, edited 1 time in total.

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Post by qu.one » Tue Apr 05, 2011 10:20 am

just want to say that i was completely impressed with the LZX modules this weekend. the look and build quality are nothing short of superb - and the results were instantly gratifying.
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lizlarsen
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Post by lizlarsen » Tue Apr 05, 2011 10:56 am

Thanks so so much. :) Lotsa passion went into them.

I added Differentiator schematics to this post too.

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Post by johnnywoods » Tue Apr 05, 2011 11:04 am

Ha! I was just going to ask for differentiator schematics!!! Awesome!

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Post by daverj » Tue Apr 05, 2011 11:07 am

You used the same RC values for each of the HP filters in the differentiator. They should be different.

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Post by lizlarsen » Tue Apr 05, 2011 11:15 am

Woops! Thanks Dave, fixed that now.

These cap values are right out of the Sandin schematic, so approximate values should be fine, and experimentation is probably the best way to decide what you like.

Keep in mind guys, these are just quickly put together -- they should work just fine, but I haven't built them up or refined them yet. The intention is to get some sort of information up for those of you who want to start messing with this stuff. Many variations on these circuits are possible.

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Post by lizlarsen » Thu Apr 07, 2011 8:34 am

Here are some further notes related to building video circuits with wired pots (rather than PCB-mounted) and probing signals with your scope from my partner Ed:
LM6172 and other video op-amps will start oscillating wildly if not bypassed (100n is OK). Also if the output is connected to a long lead or wire let's say to a front panel mount pot, the capacitive load can cause it to oscillate. To prevent this put a small series resistor on the output 100-470R is OK. Putting a CRO probe directly on the output pin of a video-amp (i.e a CRO probe looks like a capacitive load) will also cause oscillation, so put a 100-220R resistor in series with your CRO probe when measuring.

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Post by fletch » Wed Sep 07, 2011 10:03 am

I'm attempting to build these two circuits. The schematic for the function generator calls for 1n5711 schottky diodes, but would 1n4148s work? Also, I have ad8038 opamps that I am going to substitute, I imagine they should work fine?

thanks

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Post by lizlarsen » Wed Sep 07, 2011 10:07 am

Hey Fletch! No, 1N4148 will not have the right bandwidth performance to get good results from this circuit. I haven't tested with AD8038, but it will probably be fine. 1N5711 is available from digikey and mouser. LM6172s are available from digikey.

Before you build though, let me post some updated schematics! There were some fixes between the last version I posted and my final prototype models I believe, and I forgot to update them.

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Post by lizlarsen » Wed Sep 07, 2011 10:15 am

Here are the revised schematics.

FYI, if you are willing to wait 1-2 months, I will have bare PCBs for these projects available for sale. They have wiring points and standoff mounting holes for alternate applications other than board-mounted parts and a eurorack module if you are building a DIY system in a different format.

Also please keep in mind these circuits are expecting video in LZX standard impedance, already DC restored and 0 to 1V range representing black to white in the video signal.
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fletch
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Post by fletch » Sat Sep 10, 2011 10:44 pm

Thanks a ton Lars! Maybe I will just wait for the boards if it'll be that soon! :hyper:

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