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An idea for a new kind of DIY VCO
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Uncle Krunkus
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PostPosted: Fri Feb 17, 2006 8:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here's a shot of what's coming out.
It's just on Winscope so it's not accurate etc.
Each time division matches a step on the 4017.


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Uncle Krunkus
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PostPosted: Fri Feb 17, 2006 8:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

And here's the same wave and timing coming through a low pass filter (on my mixer) the vertical scale has been bumped up.
(edit) I just noticed that the position of that wave has changed, (trig adj)


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PostPosted: Fri Feb 17, 2006 9:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
The other one would be an enormous diode/DIL switch matrix which lets you force the wave shape.


or a ROM?
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Uncle Krunkus
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PostPosted: Sat Feb 18, 2006 1:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Yep,
maybe even a ROM. I do want to keep the basic circuit easily put together with minimum knowledge/money. But yes, the potential for expansion is definately there.

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Uncle Krunkus
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PostPosted: Sat Feb 18, 2006 3:51 am    Post subject: Reply with quote  Mark this post and the followings unread

This is what I'm trying to work out now. In the attached pic the blue lines represent the times that the signal is moving towards one of the voltage levels on the divider (ie, one or more switch in the 4066 is closed) Where the blue line stops, the signal starts to move towards 0V.
My question is why are there gaps between those times? If there weren't, the wave would look a lot more like the red line, which is what I'm really chasing.
First I thought that the 4066 takes longer to connect a switch than to open it. But according to the data sheet the propagation delay for 9V operation is about 65ns for both low-high and high-low control signals, which is way to fast to be seen at audio frequencies (~15Mhz?)
So the next suspect (should have been the first) is the speed of the diodes. (And as you said Jan, their associated capacitance) I've looked at the 1N4148s data sheet but it only quotes the reverse recovery time, which is down around 4ns anyway.
I'll keep experimenting, and let you know if I find an answer. I'm going to start a stripboard layout of the DIL switch version so I can play with other waveshapes too.


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Blue Hell
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PostPosted: Sat Feb 18, 2006 5:54 am    Post subject: Reply with quote  Mark this post and the followings unread

The scoop shots have been puzzling me as well . ... it's as if the DAC is only driven half the time. From what I've seen in the schematics sofar I've no explanation for it.

How do you control the switches in the diode matrix, could there be something wrong with that circuit ?

What speed are you running the main oscillator on ?

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Scott Stites
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PostPosted: Sat Feb 18, 2006 8:01 am    Post subject: Reply with quote  Mark this post and the followings unread

I may be way off base here, but....

Is you Winscope performed with your sound card? In that case, I imagine it may be AC coupled, and it would not 'hold' a DC level, but move back towards 0V once the level became stationary.

The best way to trouble shoot it would be to pulse the input slowly, step by step, observing what comes out of the DAC.

Is it possbile to post a schem of how you have things right now?

Cheerio,
Scott
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Uncle Krunkus
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PostPosted: Sat Feb 18, 2006 8:25 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm still pretty useless trying to use Ray's schem publisher so I'll try to describe it. It's basically the same as the first drawing in this thread, except I've added a 220nF de-coupling cap followed by a 10K volume pot on the output, and there are 5K pull down resistors on each 4066 control line. The control lines are fed by 10*1N4148 diodes on various 4017 outputs to combinations of 4066 control lines.
Yes the winscope goes into the soundcard, but there's definately a point where the signal stops heading towards the expected output, and starts heading towards 0V. I'm sure that means the 4066 switches aren't staying on long enough.
If I crank up the main frequency the trace shows much straighter lines from one voltage level to the next.
Thanks for the ideas anyway, I'll keep on the case!

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Uncle Krunkus
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PostPosted: Sat Feb 18, 2006 8:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Taking the de-coupling cap out flattens the tops a lot, but at low frequencies, it still just looks like a lot of different sized spikes. It's like the 4066 switches only stay on for a brief moment.
I'm starting to think you're right Scott. Some kind of capacitance effect is allowing the signal to bleed back to 0V. It doesn't get enough time to at higher frequencies. The question is, how do I "hold" it, at least down to say 200Hz?

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PostPosted: Sat Feb 18, 2006 8:53 am    Post subject: Reply with quote  Mark this post and the followings unread

In this case, you might try avoiding any capacitance and probing on the DAC output itself without buffering. Isolate the DAC with a buffer and do all of that stuff downstream from the buffer. A TL07X set up as a voltage follower will work fine for that. Connect the output of the dac to that, and just do all of your coupling and probing on the output of the buffer.

Cheers,
Scott
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Uncle Krunkus
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PostPosted: Sat Feb 18, 2006 3:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

The words "voltage follower" were actually rolling around and around in my head as I tried to get off to sleep last night! Laughing
Now how do I make a TL071 etc into a voltage follower with a single +9v supply again? Embarassed
I know some things well, but there's other stuff which I don't.
How about I go in to the non-inverting input, and feed the output back into the inverting input? (maybe via a resistor?) Is that right?

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PostPosted: Sat Feb 18, 2006 4:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just a heads up, might not have any relevance, but hey...

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Why do you have the diodes in there, out of interest?
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PostPosted: Sat Feb 18, 2006 4:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

So that each step of the 4017 only closes certain switches in the 4066. Without the diodes most patterns would wire back to close every switch on every step.
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PostPosted: Sat Feb 18, 2006 4:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ahhh, you diagram didn't show the same counter driving all 3 switches. Embarassed

I may just have to breadboard this then. Very Happy
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PostPosted: Sat Feb 18, 2006 9:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think, in the case of a non-inverting voltage follower, even if it's a single supply circuit you can still wire it up the same. Signal to non-inverting input, output connected to inverting input. Note: I am spoiled by dual supply designs - the Dim C is the only single supply thing I've done.

The 1N4148's should not be a concern here - this is the kind of circuit they love Very Happy

Cheers,
Scott
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PostPosted: Wed Feb 22, 2006 6:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

I've been away for a couple of days, but I've been thinking lots!! Confused
I reckon the bits where the signal moves towards 0V are points between steps where the last steps switches haven't dropped out yet. Almost every time this happens, there will be a small moment when too many switches are closed. (closer to 0V)
So I'm thinking that the capacitance of the diode matrix is holding the 4066 switches up just a bit too long. Now, could that be helped by putting pull down resistors on the 4017 side of the matrix (ie: 8 of them) instead of, or as well as on the 4066 side (ie: 4 of them)?
I'll get back home tonight and try it out.

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PostPosted: Thu Feb 23, 2006 11:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

I found the problem!!! Very Happy
A very screwy method of getting a square wave out of a 555 which was contaminating the supply rails something shocking. (I forced all the 4066 control lines low so that no switching could happen, the output still showed a whopping big spiky square wave at the main frequency)
Now I've put a 220uF smoothing cap on the supply and incorporated a 10K/4n7 LP before hitting a TL071 voltage follower.
Here's a pic of what's coming out now. A bit of overshoot, and a touch of switching noise, but it's 500% better than before.
I'll re-do the pulldown experiments tonight and see if it improves. Cool


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PostPosted: Fri Feb 24, 2006 6:14 am    Post subject: Reply with quote  Mark this post and the followings unread

That looks better indeed !

Glad you found it.

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PostPosted: Fri Feb 24, 2006 11:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, velly goood. Cool
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PostPosted: Sat Feb 25, 2006 2:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

A couple of things,
First, it now seems to make no difference whether I listen to the signal pre or post TL071 voltage follower. Is there a good reason to leave it there?
Secondly, if I connected the outputs of 4 AND gates would they work as if they had been OR'd together or would the other three "sink" the high on the fourth?
And thirdly,.... umm, I forget what the third question was!!

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PostPosted: Sat Feb 25, 2006 3:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
First, it now seems to make no difference whether I listen to the signal pre or post TL071 voltage follower. Is there a good reason to leave it there?


If the output of the circuit is arranged so that plugging it into inputs of varying impedance won't affect the performance, I don't see why you would need the buffer if it sounds the same with or without it. If the buffer is providing that bit of isolation from outside world influence, might not hurt to leave it in.

Quote:
Secondly, if I connected the outputs of 4 AND gates would they work as if they had been OR'd together or would the other three "sink" the high on the fourth?


If you're talking about connecting the outputs of logic gates directly together, that's generally verboten (and the reason tri-state logic was invented). Methinks you can OR them together with diodes.

Quote:
And thirdly,.... umm, I forget what the third question was!!


42

Cheerios,
Scott
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PostPosted: Sat Feb 25, 2006 4:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:

First, it now seems to make no difference whether I listen to the signal pre or post TL071 voltage follower. Is there a good reason to leave it there?


The main reason that springs to mind is that if you connect to something that has capacitance, without the buffer, it could affect the circuit.

My discovery of this was when I built a wien bridge oscillator. When connect to oscilloscope (extremely high resistance) I got my nice sine wave. When i then connected to a frequency counter directly, it distorted waveform bad, hence the need then for a buffer.

EDIT: Just occured to me- just check the slew rates of the buffer op amp, it needs to be really good. I used an LM358 once, and actually was disgusted at the poor performance due to the sloppy slew rate.
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PostPosted: Sat Feb 25, 2006 5:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey Scott,
Yeah, I had a feeling that might be the case. (with the gates) Sad
I suppose I was hoping (beyond hope) that you might say "Oh yeah! that's fine, no worries!!"
Anyway, the reasoning behind it was that the final circuit I'm looking at, will now be a complete condominium of CMOS cockroaches!!
8 * sqr wave clocks - NE556 * 4
driving
8 * 4 bit binary counters - 4520 * 4
switching
32 * dual input ANDs - 4081 * 8
matrixed against
an 8 step sequence - 4017 & 555 * 1
and OR'd - 32 * 1N4148s
into a 4066

I'm thinking of calling it Frankenstein's Oscillator! Very Happy Cool

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PostPosted: Sat Feb 25, 2006 6:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

Or simply : the Monster, We'll know what you mean Very Happy

You could consider ... well see : http://electro-music.com/forum/post-68326.html

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PostPosted: Sat Feb 25, 2006 6:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey Uncle Krunkenstein,

Fortunately diodes aren't too much of a hassle to put in there - just as easy as putting in a jumper, usually.

If you have any concern about it, you could try 7556's instead of the 556's - I love the 755X IC's - they operate on a mere fraction of the 55X IC's power requirements, and are sooo much friendlier to the power rails. Rene Schmitz turned me on to them with his VCO3 design.

Cheers,
Scott
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