LM1458 Sawtooth Generator - Getting a unwanted portamento effect

Discussion in 'General Electronics Chat' started by Jonathan Shirley, Jun 19, 2016.

  1. Jonathan Shirley

    Thread Starter New Member

    Apr 9, 2015
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    Hey!

    Here is a sawtooth generator circuit i got from a friend, It appears to be working well and producing a nice looking waveform. However, when i change the input C.V (between 0 - 5v) it 'slides' between different audible frequencies instead of jumping. Im unsure what may be causing this but i would like to use it with a C.V sequencer so it plays patterns rather then sliding everywhere between different freqs. . Any ideas? :)

    Cheers!

    P.s i find that when i change the circled cap value i get a different audible frequency range.

    P.ss Im rather amateur at all of this stuff, but learning!
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    Yes, you will get "jumping" because the 2N3904 is set up to have very high gain and will act as a switch. An N-Channel MOSFET might work the way you want it to, or you could try this Voltage Controlled Oscillator circuit from a National Semiconductor datasheet.

    [​IMG]
     
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  3. Jonathan Shirley

    Thread Starter New Member

    Apr 9, 2015
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    Thanks for the reply Dick! i am trying to give this Voltage Controlled Oscillator circuit but struggling a little bit - what exactly is the V+ /2 symbol asking for? + Vc would be the control voltage. Also, what transistor would you recommend?
    cheers!
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    V/2 is simply one-half if the supply voltage used for the op amps (known as a virtual ground on a single supply circuit). Use a pair of 4.7k or 10k resistors in series from supply to ground. The node between the two resistors is v/2. A regulated voltage source may be better but ~10x current supplying the virtual ground is usually good enough. Note the 50k and 100k resistors are feeding off of this v/2 node.

    Any small signal transistor like the 2n3904 should be fine. It is not specified on the datasheet because almost any small signal transistor will work. Small signal usually means under 750 mA and Gain (Hfe) > 100.
     
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  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you want to avoid the portamento effect and have the frequency 'jump' in pitch then the control voltage must be changed step-wise.
     
  6. Veracohr

    Well-Known Member

    Jan 3, 2011
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    The OP wants the frequency to jump but isn't getting it.

    In what way did you change the CV input? From a sequencer? Like Alec_t said, the CV must jump when changing frequencies, rather than slide, in order for the frequency to do so. I simulated your circuit and the frequency jumped just fine when the CV input did.
     
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  7. Jonathan Shirley

    Thread Starter New Member

    Apr 9, 2015
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    Thanks GopherT!, Ok, so one half of the supply voltage for the OpAmp goes to v/2 and v/2 is also connected to ground by a pair of 4.7k or 10k in series? Are you apply to supply me with a image or photo of the circuit for me to compare to?
     
  8. Jonathan Shirley

    Thread Starter New Member

    Apr 9, 2015
    15
    0
    Cheers for the reply and for testing out the circuit! i'm wanting to try from a sequencer eventually. I find at the moment that ,when i jump from two distance voltage variable with 0 - 5v in the c.v , that it kind of sweeps up or down to that C.V audible frequency, instead of immediately jumping to that audible frequency, but i suppose this is meant to happen.
    Its more noticeable when i change the circled cap. in the schematic to a smaller nf,
     
  9. bertus

    Administrator

    Apr 5, 2008
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