## How does this through zero linear FM input circuit for the C

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ljwall83
Learning to Wiggle
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Joined: Tue Sep 17, 2019 2:41 am

### How does this through zero linear FM input circuit for the C

Can anyone explain how this through-zero linear FM input stage works, or at least provide a couple of pointers?

http://jhaible.com/legacy/tonline_stuff/hj2vco.gif

It's a design by Juergen Haible: the dual VCO link in the JH 3 section here: http://jhaible.com/legacy/tonline_stuff/hj_modul.html. I found this link from a post on r/synthdiy from a year or so ago.

I've redrawn what I think is just the relevant parts of the schematic here --- trying to make it more readable --- somewhat debatable, but it's a sharper pen at least:

https://imgur.com/a/AniurYO

I should have labelled the pins: 13 is the linear FM input, 6 is hard sync, 8 is triangle out, and 9 is soft sync input.

9 is being used as an output rather than an input; from the CEM3340 datasheet, if not being driven, it looks to me like pin 9 might just be sitting at the peak voltage of the triangle wave.

I've split the drawing up in to what I think are roughly the functional blocks

I've approximately bread-boarded and scoped parts (1) and (2) from my drawing --- results in the imgur link above. Except with +/- 12v rather than +15v/-5v as that's what I have handy.

Block 1 is almost, but not quite the same topology as the data-sheet implementation, but with some quite different component values. Block 2 seems to be performing a sort of inverting half-wave rectification. I think with some more thought and experimentation I should be able to get my head around that section.

I'm at a loss to understand what blocks 3,4, and 5 would do, or indeed what the ultimate aim is on the FM and hard sync pins in order to create through-zero FM, and I haven't got the components to breadboard them. Nor an actual 3340. (Soon will have a 3340 --- the cheaper Alfa AS3340.)

I have asked this same question on r/synthdiy, but it looks like there's more detailed electronic knowledge on muff wiggler:
https://www.reddit.com/r/synthdiy/comme ... _fm_input/

neil.johnson
Super Deluxe Wiggler
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Location: Cambridge, UK
Contact:
Blocks 1 and 2 make a full-wave rectifier (the linear input must be controlled by a positive current).
Blocks 3, 4 and 5 make the oscillator go backwards when the FM input is negative.

Neil
Random ramblings :: http://www.njohnson.co.uk
SSI parts and more :: https://www.cesyg.co.uk

DozenCrows
Common Wiggler
Posts: 95
Joined: Sun Jun 04, 2017 5:28 am
Track down a copy of Synthesource issue 2 (CEM's original newsletter) - there's an article on linear TZFM with the 3345 and some indication of where to go with the 3340. The circuit is very similar to the Haible one.

There's a PDF of this issue in the Synth DIY group on Facebook.

Be aware though that the engineers at CEM couldn't get glitch-free TZFM this way - there's a call for help in the article to the readers. I've also tried this circuit and variants, and run into the same issue.

Electronotes issue 129 covers TZFM, and the theory of operation described there is what is applied in these examples, adapted for the CEM chips.
Last edited by DozenCrows on Tue Sep 17, 2019 8:52 am, edited 1 time in total.

cygmu
Super Deluxe Wiggler
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Location: UK
DozenCrows wrote: Electronotes issue 129 covers TZFM, and the theory of operation described there is what is applied in these examples, adapted for the CEM chips.

There's a PDF of this issue in the Synth DIY group on Facebook.
Even better, it's on the Electronotes web site.

http://electronotes.netfirms.com/EN129.pdf

DozenCrows
Common Wiggler
Posts: 95
Joined: Sun Jun 04, 2017 5:28 am
Edit to my reply: there's a copy of Synthesource in the FB group mentioned, not EN129.

loki
Common Wiggler
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Joined: Fri Apr 25, 2014 4:20 pm

### Synthesource 2

Synthesource 2
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ljwall83
Learning to Wiggle
Posts: 9
Joined: Tue Sep 17, 2019 2:41 am
Thanks everyone here ---- this is dead handy. I was just waiting for my request to join the Synth DIY facebook group to be approved.

Going to have close read of these.

Sad to read the 3340 implementation perhaps doesn't work too well. I'm pretty sure I don't have the chops to succeed with making it work well where others have not...

That electronotes site seems like a treasure trove though.