hi
ericgibbs
i have tested the resistances of the two potentiometers with results:
thanks have a nice day!

 

hi Matt,
You can see the 'tolerances' on the stated values, especially the left side trim pot.
If you are using the right side spindle pot in your project, it should give you the range of control required.

Thanks for the feedback.

E

 

Ok thank you eric!

 

(The oscillator on the left is not connected to the modulation input, and thus its frequency cannot be affected by that.) sorry do i need to connect them ?

 

hi Matt,
Is the project wired exactly as post #42 @crutschow and also are the voltages for the supply and the PWM control the same as that post.

E

 

Hi eric
yes i have controlled the resistors value , the supp voltage, the pwm output dutycycles but still the frequency changes with de dutys i really dont know where is the failure!

 

Hi eric
yes i have controlled the resistors value , the supp voltage, the pwm output dutycycles but still the frequency changes with de dutys i really dont know where is the failure!

Then there would seem to be a miswire in your circuit.

 

Hi crustshow:
what is the pulse( 1/4/5 ms) on Vmod to mod In?

 

hi Matt,
Try this image.
E

 

Hi Eric! sorry i am late! i will test that and give an answer! thanks for the diagram!

 

hi Matt,
No problem, lets know if any changes are required.
E

 

Hello Eric sorry i am late i have tested your circuit and here are my resutlts:
6,4%= 224,1 HZ
24%= 224,2 HZ
35%= 224,3 HZ
57%= 224,4 HZ
69%= 224,5 HZ
75%= 224,6 HZ
85%= 224,7 HZ
99,7%= 253,4 HZ
is there a solution to obtain a fixed frequency? (i have tested two circuit to be sure and the results are identical!)
thank you
have a n ice day!

 

hi Matt,
You could try buffering the connection between the Osc OPA and the PWM OPA.?
E

 

is there a solution to obtain a fixed frequency?

For most PWM applications the precise frequency is not critical. What is the PWM going to be used for, which requires a fixed frequency?
Frequency drift due to temperature changes and component tolerances is likely going to occur with any analogue approach. Duty cycle is also likely to drift and could be particularly noticeable at values close to 100% or 0%.

 

OK Eric i test that thank you!

 

hi Matt,
A couple of points , where on the circuit are you measuring the frequency and what is measurement device.?

Lets know the results of the modification.
E

 

Alec_t
I try an external pwm with fixed frequency for a push pull application that limit is 49%

 

hi matt,
Using LTSpice with the plots expanded in the time axis thse are figures for 0% and 100% PWM.
~229Hz...

E
Added: Overlay of 'tri' signal at 0% and 100%.

 

Hi Eric i measure the circuit on pin 7 lm393 with a frequency multi meter!

 

I try an external pwm with fixed frequency for a push pull application that limit is 49%

That doesn't explain why you want a precise fixed frequency.