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x-4046 VCO mods?
MUFF WIGGLER Forum Index -> Video Synthesis  
Author x-4046 VCO mods?
smrl
Just got a couple of PCBs for Thomas Henry's X-4046 VCO. I'm looking at the schematics now and wondering if there's a straightforward way to modify this thing for video rates. Looking at the 4046 datasheet it claims 1.3mhz @ 10VDD (the HC series, OTOH, is capable of 17mhz). The standard 4046 CV in is ignored in this design, instead a current mirror is used to feed pin 11. I'm not sure whether that would affect the max freq. of the IC.

SCHEMS


I could kludge a 5v regulator to power an 4046HC & hard sync circuit or leave the standard one in... Otherwise my eyes tell me to lower resistor values around IC3c&d, and IC4c & d, swapping op-amps of course. Lowering values for C2 and the timing cap, and probably adding a switch to swap ranges. Also changing the gain of the output sections... Am I missing something? Does this look worthwhile or will I spend more time trying to tweak this thing into service than it's worth? Might this be a good design with high-freq sync in mind?
smrl
Hmm, just had a thought -- are there even decent quad DIP op-amps that would work here?
daverj
You just need to change the cap between pins 6 and 7 to change the frequency range. The current source to pin 11 should work fine as it is.

If you divide the cap on pins 6 & 7 by 10, the chip oscillates 10 times faster. Divide it by 100, it oscillates 100 times faster, etc...

The minimum allowed cap for the HC4046 is 40pf. That will get you up way into the Mhz range. So something a bit larger is probably better. Right now that schematic has a 10,000 pf cap between pins 6 & 7. So a 1000 pf will give you 10 times the current speed. 470pf will give you 20 times the speed, etc...
daverj
You'll want to change more than just the op amp on the output though. There are dual and quad video amps that are compatible pinouts, but those resistor values used in that diagram are way too large to use for video amps. You'll need to make them 10 to 100 times smaller values, and adjust their proportions to lower the signal level about 10 times smaller to feed into video circuits.
lizlarsen
I don't mind redrawing these schematics with some recommended values, I will mess with that tonight. Dave, which quad DIP amp would you recommend in this case?
daverj
creatorlars wrote:
I don't mind redrawing these schematics with some recommended values, I will mess with that tonight. Dave, which quad DIP amp would you recommend in this case?


The LT1254 is a quad video amp with the same pinouts as the TL074. But it's a current feedback amp, so layout is critical to prevent parasitic oscillations.

The OP467 might work too. It's a voltage feedback amp. Not as fast as some video amps, but 10 times faster than a TL074.

Very few good video amps are in DIP anymore.
lizlarsen
Cool -- OP467 looks like a good bet -- LT1254 probably wouldn't be a good drop-in replacement for this project, if the goal is to avoid modifying the circuit or PCB beyond just parts value changes.

I will go through the schematic and see what resistor value changes make sense to me, and then post and see if you guys have further suggestions. Beyond just frequency range, making sure the sync circuit is stable is a must, too. And then maybe smrl can try a build and we'll see how it works? smile
smrl
Yes! If you provide some alteration hints I will gladly report back post haste. May need to wait on delivery of some of these op amps. FYI the OP467 are in stock at AVNet and Newark.

Dave, I'm following you. But looking at the datasheets between the 4046 and the HC4046, a 40pF cap on the HC4046@5v yields a freq. well into the Mhz range, but a 50pF cap on the CD4046@15v gets me 1.6Mhz only. I'd be willing to give both ICs a try.
lizlarsen
Cool! I'm working on this now. How will you be using it in context, with LZX modules or something different?
lizlarsen
Also, I think a max frequency of 1.6MHz if you can get there would probably be just fine. This is the same frequency as the LZX Video Waveform Generator's core. It goes through a frequency doubler (full wave rectifier) to get to 3.2MHz.
smrl
Yes, also keep in mind this design generates a saw that is 2f, as well as the rampoid, so there's more complex/higher freq. patterns available that way.

Yeah, I will be using it with LZX modules but I'm dealing with mutliple formats & strange gear here, so if the output ends up being bipolar, for example, that's fine. I intend to build some voltage scaling and biasing modules soon. So yeah, intercompatibility is a plus but I'm aware it's an issue at least in the context of the current PCB...
smrl
Also, Lars, you describe the Voltage Interface I as "low bandwidth" - what frequency range does that module support?
lizlarsen
Cool! Well the good thing is that there are existing trimmers for gain/offset on the core, so getting to a 0 to 1V scale on the output shouldn't be an issue. But there's not really a problem with bipolar signals in the LZX modules, just so long as you realize what's going on and how that effects summing two signals, etc.

One issue is with the CV inputs, technically they should be buffered before attenuation, in order to lower all the resistor values and make things respond at video rates. So you might need to have a protoboard with some extra video op-amp buffers (LM6172 is a good dual). Or you can just leave it as-is with 100K pots which should be okay for LZX termination, but bandwidth of the CV inputs will be limited (which wouldn't be a huge problem.)
lizlarsen
Quote:
Also, Lars, you describe the Voltage Interface I as "low bandwidth" - what frequency range does that module support?


It uses standard TL07x series op-amps and 1N4148 diodes -- I haven't tested the maximum bandwidth, but it does at least pass video (but softens it up quite a bit.) So definitely above audio range, but you will see some low pass filtering in video range.
lizlarsen
Upcoming Triple Video Processor is designed specifically for in-system gain/bias manipulation, for this kind of thing... since the bias is applied before the polarizer, you can change the 0V point of the incoming signal before it's summed with the B input channel, allowing bipolar mixing. It will be $200 or under. The basic circuitry is the same as Video Blending Matrix.



I can do a higher bandwidth version of the VI1 on request. The reason why it's lower bandwidth is it's specifically for external input from audio/CV devices, and I didn't want to bloat the module's cost.

If you're wanting to do a high bandwidth oscillator, you'll get better performance keeping the voltage levels at 1V or 2V, and then you can always upscale it for audio use when desired. At least that's how I'd approach it.
lizlarsen
Well here's something, not much. Just everything up to the triangle output to start. Once we've got that looking right, I can help with all the waveshapers too. It's difficult to decide on some of these resistor values without playing with the circuit, so that may be in your ballpark -- give us a full report of how it looks after building up this far, maybe with some scope shots and notes on output. For the capacitor, I'd just experiment and take notes.

The Video Waveform Generator is scaled on the FM inputs so that 0 to 1V cv scales all the way from minimum and maximum frequencies to get biggest possible sweeps. So you may need to play with the expo circuit values to determine how you want that, too.

The biggest thing is gonna be how that sync circuit responds. One way to improve performance there may be to replace the current circuit with a high speed comparator set to a 0.5V threshold.
smrl
Yeah, 1v is good, this would work well with the weirdo RGB video encoder I've got. Speaking of which, I should probably post a picture of the PCB on that thing. You might be interested in taking a look.

Question: Is the input bias current of the opamps connected to C3 going to make a considerable impact on frequency? From TL074 to OP467 is a jump from 65pA to 600nA. Pins 6 and 7 are connected to the VCO, which the datasheet pretty much describes as a black box.
smrl
Wow. Thanks! That was fast. I'll get back with info when that much is built!
lizlarsen
Quote:
Question: Is the input bias current of the opamps connected to C3 going to make a considerable impact on frequency? From TL074 to OP467 is a jump from 65pA to 600nA. Pins 6 and 7 are connected to the VCO, which the datasheet pretty much describes as a black box.


Hmm, I'm not sure. I am not much of an engineer when it comes to things like this (that's my partner, Ed.) I do know that we generally put a 499R resistor between a signal and the noninverting input of an op-amp -- so maybe try that if you're having issues.

And here's an image of how I'd recommend the CV inputs should look before the attenuators (for pots I'd use 1K or 10K.)

daverj
smrl wrote:
Question: Is the input bias current of the opamps connected to C3 going to make a considerable impact on frequency? From TL074 to OP467 is a jump from 65pA to 600nA. Pins 6 and 7 are connected to the VCO, which the datasheet pretty much describes as a black box.


I doubt that it'll make any noticeable difference.
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