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TZFM SAW VCO
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Luka



Joined: Jun 29, 2007
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PostPosted: Fri Oct 03, 2008 2:22 am    Post subject: Reply with quote  Mark this post and the followings unread

It would be very cool to have 2 or 4 of these things on a single pcb
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Dego



Joined: Apr 22, 2008
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PostPosted: Fri Oct 03, 2008 2:40 am    Post subject: Reply with quote  Mark this post and the followings unread

Luka wrote:
It would be very cool to have 2 or 4 of these things on a single pcb


Word! I thougt that was going to be a good idea when I first read about this project...
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frijitz



Joined: May 04, 2007
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PostPosted: Sun Oct 05, 2008 10:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Another update.

I've been mostly been working on the waveshape/waveshapers. The up ramp and down ramp sections of the core were not quite the same. It turned out that the problem was an unusually large amount of charge injection into the integrating cap during switching. The reasons for this were twofold: (1) To try to get the OTA current modulator stable and accurate, I set it up to use lower than usual current levels. This meant going to a smaller than usual integrating cap. (2) Resetting the positive and negative ramp sections requires two FETs in tandem and a large voltage swing on their gates. Since charge injection is proportional to the gate voltage swing, this lead to a larger than usual amount of injected charge.

The result of these two factors was that the zero points of the ramps were offset upward by ~0.4 V. This meant that the magnitudes of the up and down ramps differed by ~ 15%. I stewed for quite a while thinking about all the possible ways this problem could be fixed and finally decided to try the method of injecting a countercharge. This required just a transistor to invert the switching pulse and a small cap to inject the inverted pulse into the integrating cap. Surprisingly, it was real easy to get this to work. Of course, there are still small switching spikes, but the big problem is taken care of.

All this also made it easier to shift the positive and negative ramps into alignment (to produce a final waveshape exhibiting proper phase reversal).

Next I built a SAW-TRI shaper. This is just a full-wave-rectifier plus a level shifter / amplifier (although they don't usually tell you this). I found a FWR circuit that is a little simpler than what is usually employed. Its limitation is that its input impedance is different for positive and negative signals, but that doesn't matter for this application. (Jung, IC Op-Amp Cookbook, Fig. 5-13) It might be possible to save an opamp here with an even simpler circuit, which I may eventually try. This shaper produces nasty spikes at the switching point (as usual). I tried to figure out a simple way to use a countercharge injection pulse to eliminate the spikes but wasn't successful, because the input discontinuities are positive and negative, whereas the rectifier's spikes are all negative. So I ended up choosing to use fast opamps to keep the spikes narrow and some filtering caps to attenuate them. The resulting triangle is OK, with just small transient spikes.

A short noodley demo using the TRI output is attached. This was actually made before the charge-injection corrector was built, so the triangles have fair-sized steps on them. But it definitely sounds useful.

Since I have half an OTA let over, I'm going to try adding a SIN shaper following the TRI. It will be interesting to see if the waveforms have a small number of harmonics as you could get with a sinusoidal core.

So this project is still very much in the whiteboard stage. Probably a couple more weeks of testing before making a protoboard.

Very Happy

Ian


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phdinfunk



Joined: Jun 04, 2008
Posts: 115
Location: Taiwan

PostPosted: Mon Oct 06, 2008 9:24 am    Post subject: Thanks for the update! Reply with quote  Mark this post and the followings unread

Wow, I appreciate the details in your message!

Funny thing, I always imagined that method for saw to tri conversion in my mind (I'm not a designer, but I'm learning. I've got lots of spare time so I'm studying harder right now). For a guy that's reading Randy Stone's Tab Guide to Electronics while building projects like a madman, your posts are totally cool!

--Jonathan

PS: I like the sounds this thing's been putting out so far, mate!
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Tim Servo



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PostPosted: Mon Oct 06, 2008 3:13 pm    Post subject: TZFM SAW VCO Reply with quote  Mark this post and the followings unread

Hey Ian,

Neat stuff! I'm glad you're doing this. One thought on the Saw-Tri waveshper: how about using a "crappy" low bandwidth op-amp like a 741 or 1458 to reduce or perhaps even eliminate the switching spikes? It's a common technique, and might just do the trick. Other than that, rock on Ian.
Smile

Tim (just used up my ONE good thought per day) Servo
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frijitz



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PostPosted: Mon Oct 06, 2008 6:22 pm    Post subject: Re: TZFM SAW VCO Reply with quote  Mark this post and the followings unread

Tim Servo wrote:
One thought on the Saw-Tri waveshper: how about using a "crappy" low bandwidth op-amp like a 741 or 1458 to reduce or perhaps even eliminate the switching spikes? It's a common technique, and might just do the trick.

Thanks! Yeah, I know that one. (In fact I've suggested it here before.) The LM324 or LM3900 also work well for that method. I might end up doing that, but i didn't want to add another chip. Maybe it would work if I put it in the level shifter/amp. I'll see if I can't find an old 741 around.

Very Happy

Ian
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frijitz



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PostPosted: Tue Oct 07, 2008 11:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Yesterday I put in the sine shaper and got it adjusted. The circuit I used follows the OTA-opamp with feedback approach. I showed previously that this can have very low distortion. The tradeoff is that adjustments for drive amplitude and symmetry must be included. But it seems worth it.

I recorded some deep FM tones using sine wave modulation and put them through a spectral analysis program. All the main peaks were where they belonged, and I didn't see any problems with unwanted extra features.

Here's an mp3 with some percussive sounds.

Next I have to figure out what to do about sync. It needs to be quite gentle so to not distort the sine waves and at the same time reliable enough to keep the phases locked.

Very Happy

Ian


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bridechamber



Joined: Oct 06, 2007
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PostPosted: Tue Oct 07, 2008 3:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

That's fantastic!

I thought I'd need ten, but let's see... If I make a six-operator FM with four voices... I guess I'll need 24!
Very Happy

Scott
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LektroiD



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PostPosted: Tue Oct 07, 2008 11:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'd be interested in some of these too, not sure how many as yet Wink
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phdinfunk



Joined: Jun 04, 2008
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PostPosted: Wed Oct 08, 2008 4:18 am    Post subject: Please, Reply with quote  Mark this post and the followings unread

Ian, yes, Please. As everyone else has said, please put more than one OSC per board. It's only natural!

--Jonathan
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frijitz



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PostPosted: Wed Oct 08, 2008 7:03 am    Post subject: Re: Please, Reply with quote  Mark this post and the followings unread

phdinfunk wrote:
As everyone else has said, please put more than one OSC per board.

Well, we will keep an open mind on this. But more than one on a board is asking for trouble with them locking together in phase. Especially with a SAW core. Plus, if you need two together you can always connect them with screws or even glue, na?

Very Happy

Ian
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bridechamber



Joined: Oct 06, 2007
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PostPosted: Wed Oct 08, 2008 7:23 am    Post subject: Reply with quote  Mark this post and the followings unread

I forgot about phase.
Besides sync, there's really no way to address that, is there?

For stacking multiple boards, you could add "power-through" pads for a power bus for them all. Then it's just a matter of extra standoffs, and the individual user could make one VCO or twenty.

I don't know a ton about FM and such, so let me know if I'm off on this:
For a typical FM patch, you'd still want the same controls as the "final" VCO, right? I.e., fine and coarse tune, fm attenuator(s).
If so (and I know it's early to be thinking about panel stuff), maybe a 1U tall-format panel could hold one VCO, with board-mounted pots. That would be simple, fast and inexpensive.
That would also give you maximum patching flexibility for different multi-op set-ups.
I'm sure a similar solution is there for Frac/ Euro.

What would it be like to insert a phaser between two FM operators? I can't wait to find out! Smile

Cheers,
Scott
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fluxmonkey



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PostPosted: Wed Oct 08, 2008 8:04 am    Post subject: Re: Please, Reply with quote  Mark this post and the followings unread

frijitz wrote:
phdinfunk wrote:
As everyone else has said, please put more than one OSC per board.

Well, we will keep an open mind on this. But more than one on a board is asking for trouble with them locking together in phase. Especially with a SAW core. Plus, if you need two together you can always connect them with screws or even glue, na?

Very Happy

Ian


one per board would be fine by me... keep yr modular modular, eh? anyway, for FM i thought you only needed one thruzero osc., the 2nd one can be "normal"... at least, thats what i do with one zeroscillator and one blacet.

b

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frijitz



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PostPosted: Wed Oct 08, 2008 9:07 am    Post subject: Reply with quote  Mark this post and the followings unread

bridechamber wrote:
I forgot about phase.
Besides sync, there's really no way to address that, is there?

Right. Phase control is very important. I think it's important the the osc's not sync spontaneously, but that there is a way to implement a gentle (probably adjustable) sync that doesn't affect the waveform too much. I'm going to be looking at some ideas I have for this. Wink

Quote:
For stacking multiple boards, you could add "power-through" pads for a power bus for them all. Then it's just a matter of extra standoffs, and the individual user could make one VCO or twenty.

Maybe. They'll try to talk to each other through the common power leads if you do it that way, though. Also, there is a possibility of RF communication between the boards. It might be better to force the user to handle these problems, since power distribution and conditioning is largely a system issue.

Quote:
I don't know a ton about FM and such, so let me know if I'm off on this: For a typical FM patch, you'd still want the same controls as the "final" VCO, right? I.e., fine and coarse tune, fm attenuator(s).

The controls I'm counting so far are:
coarse freq tune
fine freq tune
initial freq
lin FM level
expo FM level
sync level
I'm assuming that we do *not* include a built-in VCA for dynamic depth. (This is still open for discussion.)

Quote:
If so (and I know it's early to be thinking about panel stuff), maybe a 1U tall-format panel could hold one VCO

It's never too soon to think about panels, from what I've seen so far. Wink

Very Happy

Ian
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e-grad



Joined: Sep 12, 2008
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PostPosted: Wed Oct 08, 2008 9:38 am    Post subject: Reply with quote  Mark this post and the followings unread

bridechamber wrote:
I'm sure a similar solution is there for Frac/ Euro.

That would be great if the pcb would fit behind a Frac/Euro-panel!
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frijitz



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PostPosted: Thu Oct 16, 2008 11:46 am    Post subject: Reply with quote  Mark this post and the followings unread

This project marches on. Most of the past week was spent on developing a sync circuit. It turns out that two circuits were needed, one for the up ramp and another for the down. I also looked carefully at whether a fixed type of sync could be used.

I expected that a hard sync would always work reliably, but I found that some fm waveforms work better with weak sync pulses. I couldn't find a soft-sync scheme that always worked well, either. So I ended up using an adjustable soft-sync setup that can go from no sync to a strong sync that is quite similar to a standard hard sync.

I've been able to get locking at all kind of oddball frequency ratios, such as 5:7 and so on. Most of these need a fairly weak sync and will not track over a wide frequency range. But of course, that depends on how well the two oscillators track each other. This is one area where beating hard on the tracking problem pays off.

Here are a couple of demos. The first illustrates master/slave tracking over three octaves with modulation index in the 8-10 range. (This means that the change in frequency is up to ten times the modulation frequency.) Followed by a little doodle.

The second demo shows some results obtained by abusing the sync feature to produce dynamically changing waveforms. Lots of horrid sounds here for you noise freaks.

I'm wondering how to do the Initial Frequency (bias) control. Right now it is just a pot. Would a multiposition switch be preferable? The continuous variability doesn't really seem necessary, although it works fine. Fixed levels would make it easier to to explore different modulation depths and to get back to previously dialed-in sounds, but It would be an additional expense.

Opinions?

Very Happy

Ian


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machine.cuisine



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PostPosted: Thu Oct 16, 2008 3:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:
Fixed levels would make it easier to to explore different modulation depths and to get back to previously dialed-in sounds, but It would be an additional expense.

Opinions?

Very Happy

Ian


What do you mean by "expense"? I like the idea of fixed levels, but it depends if there's a large enough range of levels to choose from...It would provide a more 'repeatable' experience - which I think would be nice for FM-ing.

Then again, the wouldn't the 'purist' in me want a variable pot? (Nah, I've got enough of those by now).

Ian, you're the one doing all the experimenting, you should know what to do. Smile
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frijitz



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PostPosted: Thu Oct 16, 2008 6:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

machine.cuisine wrote:
What do you mean by "expense"? I like the idea of fixed levels, but it depends if there's a large enough range of levels to choose from...It would provide a more 'repeatable' experience - which I think would be nice for FM-ing.
MC -- Thanks for your thoughts. I need more play time with the system to decide on this issue. Right now I'm thinking of maybe six levels -- three positive and three negative. It may not be necessary to have both polarities, but it seems to me you might want to feed in an EG and have it either increase or decrease the magnitude of the frequency.

So maybe bias levels of 0.2V 1V and 5V. With a 5V modulation level this will give modulation indices up to about 25 (+/-, depending on the frequency ratio.) How much depth do people think they need? I've been working with m = 8-10 for a lot of my experiments. This seems pretty extreme to me, but maybe I'm being conservative. Tracking and frequency range get worse at lower bias levels, so I don't see any sense in trying to push for extremely high indices.

Opinions are welcome -- if I don't get any I'll just use what I like.

Very Happy

Ian
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machine.cuisine



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PostPosted: Thu Oct 16, 2008 8:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:
How much depth do people think they need? I've been working with m = 8-10 for a lot of my experiments. This seems pretty extreme to me, but maybe I'm being conservative. Tracking and frequency range get worse at lower bias levels, so I don't see any sense in trying to push for extremely high indices.


I think that there should be at least one "extreme" option. Because in my (non-thru-zero) analog FM experience, some really usable percussive pulses, thwaps, thacks and fun-unexpected weirdness are there in the "extremes". I'm talking noise-rock here Laughing , where tracking and range mean nothing.
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phdinfunk



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PostPosted: Thu Oct 16, 2008 9:12 pm    Post subject: Don't quite comprehend... Reply with quote  Mark this post and the followings unread

Ian,

I've used the Yamaha FS1r extensively for FM synthesis, so I'm pretty hip with how FM works, BUT...

...I'm not exactly sure what you mean by "bias"... is that the initial frequency of the osc, as in "frequency" or "tune" or is it the modulation amount? The way you describe it it sounds like a modulation depth control...

Sorry for the confusion -- email's not necessarily the best form of human communication Smile

--Jonathan
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widdly



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PostPosted: Thu Oct 16, 2008 11:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'd like to see some PWM control on there. Then it would really cover a lot of bases...FM, sync and PWM from one VCO.
Last edited by widdly on Thu Aug 05, 2010 10:05 pm; edited 1 time in total
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loss1234



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PostPosted: Fri Oct 17, 2008 4:04 am    Post subject: Reply with quote  Mark this post and the followings unread

ian

i am assuming you would do this with a rotary switch..which isnt really too expensive in my experience

but maybe you meant something else. i like the idea of a stepped switch. there are some moments i ge a bit tired of always having to work really hard to get back to a certain sound due to all of my knobs covering such giant ranges Wink

thnx for this awesome project
cant wait to add one to my synth (or three)

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frijitz



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PostPosted: Fri Oct 17, 2008 8:09 am    Post subject: Reply with quote  Mark this post and the followings unread

machine.cuisine wrote:
I think that there should be at least one "extreme" option. Because in my (non-thru-zero) analog FM experience, some really usable percussive pulses, thwaps, thacks and fun-unexpected weirdness are there in the "extremes". I'm talking noise-rock here Laughing , where tracking and range mean nothing.
Thanks. I appreciate the input. With ordinary FM you get up to 100% modulation. With a .2V bias setting you get around 2500%, but I still am not sure if that is enough for you. Perhaps I should try some more thwaps, etc. You will always be able to inject extra +/- bias through the modulation input to get any value you want.

Very Happy

Ian
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aerogramma



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PostPosted: Fri Oct 17, 2008 8:24 am    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:

I'm wondering how to do the Initial Frequency (bias) control. Right now it is just a pot. Would a multiposition switch be preferable? The continuous variability doesn't really seem necessary, although it works fine. Fixed levels would make it easier to to explore different modulation depths and to get back to previously dialed-in sounds, but It would be an additional expense.

Opinions?

Very Happy

Ian


this seems to be the case when and extra option means less options... which i couldn't agree with more... i especially like the thought of being able to explore more in detail the fewer options that will result from such a switch Wink

the previous fm 'bell' sounds sounded fantastic... look forward to this

aero
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frijitz



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PostPosted: Fri Oct 17, 2008 9:18 am    Post subject: Re: Don't quite comprehend... Reply with quote  Mark this post and the followings unread

phdinfunk wrote:
...I'm not exactly sure what you mean by "bias"... is that the initial frequency of the osc, as in "frequency" or "tune" or is it the modulation amount?
Jonathan -- You are asking a good question. I believe the analog setup may be a bit different from what you are used to, and can be a bit confusing at first. I'll try to explain it better.

In an analog TZ VCO the frequency is determined by two different input voltages. This is a bit awkward to use, which is one of the reasons I am thinking of using fixed switchable bias levels.

The base frequency of the oscillator is determined by the product of the usual expo current and the dc level at the FM input (called "bias", or "initial"). So lower bias levels require more expo current to get to the same base frequency.

Mathematically, the frequency is given by:

F ~ (Vbias + Vmod*Sin(2*pi*Fmod))*Iexpo

(I hope the notation is self-explanatory).

So what you would call "frequency" or "tune" comes from Vbias*Iexpo. And the modulation amount, expressed as the FM index "m" is given by

m = deltaF/Fmod = (Vmod/Vbias)*(Finit/Fmod).

(Be careful on this one. Naively, you might think "m" is just the first factor. This had me confused for a while.)

But please don't anyone be put off by all this confusing math. Using the unit is quite easy. If you want really deep modulation (and don't care about super accurate tracking) then you set the bias control near the low end and increase the regular frequency control to get the base frequency you want. Then you bring up the FM amount control to get the depth of modulation you want. If you want a harmonic waveform, then you additionally bring up the sync control and adjust the fine frequency control to lock in the waveform.

It's lots of fun to use! But if you want to make a one-to-one correspondence with your Yammy settings, there is a bit of work involved.

Please let me know if any of this is unclear and I'll try to do better.

Very Happy

Ian
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