LM1458 Sawtooth Generator - Getting a unwanted portamento effect

Thread Starter

Jonathan Shirley

Joined Apr 9, 2015
15
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!
 

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DickCappels

Joined Aug 21, 2008
6,535
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.

 

Thread Starter

Jonathan Shirley

Joined Apr 9, 2015
15
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.

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!
 

GopherT

Joined Nov 23, 2012
8,012
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!
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.
 

Alec_t

Joined Sep 17, 2013
11,530
If you want to avoid the portamento effect and have the frequency 'jump' in pitch then the control voltage must be changed step-wise.
 

Veracohr

Joined Jan 3, 2011
721
Yes, you will get "jumping" because the 2N3904 is set up to have very high gain and will act as a switch.
The OP wants the frequency to jump but isn't getting it.

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? :)
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.
 

Thread Starter

Jonathan Shirley

Joined Apr 9, 2015
15
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.
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?
 

Thread Starter

Jonathan Shirley

Joined Apr 9, 2015
15
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.
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,
 
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