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A few questions about using an LM13700 as an LFO

Nathan Flanigan
Prodigy 50 points
Other Parts Discussed in Thread: LM13700

Hello Engineers,

I'm hoping some one can help me sort out a plan for using the LM13700 as an LFO.  Here are my design requirements...

- Frequency controlled by 0 to +5 CV

- LFO frequency range, something like 0.1 hz to 50 hz?

- Output 0 to +5v

I'm thinking the VCO in fig 33 might work nicely.  What components should I experiment with to get where I'm going?  How do I determine Ic and Ia?  I know I have to use Ohms Law but I think I'm misunderstanding how to apply it.  I'm really looking forward to learning something about this.

Thanks friends

  • 0
    Genius 3295 points

    Hi Nathan,

    I smell music synthesizer (effect?)... The LM13700 is a fun part, but finicky. The best thing you could do for yourself is get a breadboard, build the circuit and work through the equation and the real world components. Make sure you do everything "right" ; plenty of bypass capacitors, clean supply - all the things one should do when working wit analog devices.  Also, there is a ton of good material on the 'net regarding using this device. A quick search of "LM13700 LFO" will get you enough reading material for a week. Know ahead of time; the components interact quite a bit, so don't be surprised when you're changing things. 

    I'm posting a spreadsheet with the equation taken directly from the data sheet (Fig.33). I can't tell you if the "real world" matches the results shown here; you'll have to find that out for yourself. ;)

    Note this circuit gets you triangle and square waves. This is also easily accomplished by using a couple of opamps and a digital pot... 

    Mike T.

    LM13700 CALC.xlsx

  • 0 in reply to Mike Tripoli
    Prodigy 50 points

    WOW!  Thanks for the spreadsheet, that is fantastic.  It turns out I was correctly implementing Ohm's Law, the numbers for I(A) and I(C) seemed impossibly small to me.  So a few follow up questions, it seems like the values in the equations for current are expressed in Amps (as opposed to milliamps) and capacitance is in farads (as opposed to uf or pf or whatever) how did you know what unit of measurement to use?

    Note this circuit gets you triangle and square waves. This is also easily accomplished by using a couple of opamps and a digital pot... 


    Is there a schematic or data sheet you could point me to?  The end goal is to build and LFO pedal to be used in place of an expression pedal with guitar effects.  

    Thanks for your help!

  • 0 in reply to Nathan Flanigan
    Genius 3295 points

    Hi Nathan,

    Look at just about any (analog) schematic for a phase shifter, flanger, etc. Regardless of the manufacturer, especially when it comes to phase shifters, the schematics will be virtually identical. Somewhere on the page  will be two op amps configured as a "triangle wave" generator. I'm including a link to a page that shows both using LM13700 as well as (in the lower right hand corner) the LFO with various capabilities.

    It's great that you're diving into rolling your own stuff. From the questions asked, you're just starting out. Let me suggest a couple of books:

    "The Art of Electronics" by Horowitz and Hill - The best money you will spend on an electronics book. Period.

    "Electronics for Guitarists" by Denton J. Dailey - enough stuff about guitar related electronics to keep you busy for a while, written for anyone.

    Lastly,   you will learn the most from doing. Get a copy of LTSpice (free), learn the basics and start simulating circuits.

    http://www.runoffgroove.com/tri-vibe.html

    Mike T.

  • 0 in reply to Mike Tripoli
    Prodigy 50 points

    Thanks, that's all great information!  I've been at this for a while, but like a lot of people these days I came at it from the side door.  My learning has been on a "need to know basis".  Generally speaking, I haven't been too motivated to work out my own designs simply because anytime I've thought of something I want it's already out there and well documented.  

  • 0 in reply to Nathan Flanigan
    Genius 3295 points

    Hi Nathan,

    Don't let the availability of commercial products hold you back from designing and building stuff. The amount of pride and self-accomplishment that comes from making your own stuff is immeasurable. I design and build amps and effects; here's a link to a project I'm working on, tell me if it looks familiar: https://www.dropbox.com/sh/udztudhdh5c2066/AABJAq9_Mf376Ij29gday6VVa?dl=0 . If you've been around since the 80's you might recognize the "form" as a GK250ML. That was a great amp, so I've borrowed a couple of ideas from it. However, the electronics are completely different. I have multi-fx, a dual 70W Class-D amp, the entire signal chain is TI devices. I'm also doing pedal effects using a hybrid of "real tubes" and analog/digital devices.  I'm looking to begin releasing some first "beta" products to the wild and get some feedback on them.

    Mike T.