Hello!
I have been trying to build a generator, which will output a triangle/sawtooth wave with variable frequency and duty cycle. I came up with the circuit attached to the post. It works quite nice up to around 500kHz (my aim is 1Mhz) but then the problems begin. Actually the most disturbing problem is the distortion seen on the attached images. I have traced the problem to the junction capacitance of the two diodes (decreasing the value of this parameter in the simulation reduces the distortion). I've been searching the web for diodes with small junction capacitance, but was unable to find an adequate one (according to the simulation Cj would have to be lower than 300fF - the lowest I could find has 1,5pF). One solution is adding diodes in series and lowering the capacitance that way, but this solution is by no means an elegant one and brings with it other problems (bigger voltage drops->smaller current and frequency, larger variation in the combined voltage drop which is dependent on the current,...).
My question therefore is: how to solve this problem? Any ideas for circuit improvement which resolves the problem, or a completely different design which satisfies the mentioned requirements (variable frequency up to 1Mhz, variable duty cycle-preferably from 1% to 99%) would be greatly appreciated. I would however like to avoid the use of OTAs as in this idea: http://www.till.com/articles/VariableSaw/index.html
The second problem I encountered presents itself already at frequencies at which the above described problem does not yet cause noticeable distortion. This second problem is the changing of frequency with the change of duty cycle (rotation of potentiometer P2). At 50% duty cycle the frequency is largest. Then at 1% and 99% duty cycle, the frequency drops.For example: at 113kHz with 50% DC, it drops to about 110kHz at 1% and 99% DC.
So my second question is: where is the reason for that? When tossing around the equations I came to the conclusion, that the frequency should stay the same for the given circuit. Maybe some parasitic effects are causing the problem again? One reason might be the exponent-like changing of the diodes forward voltage with current, but I also simulated the circuit with ideal diodes and the problem remains. Any ideas?
I am fairly new to electronics so I am hoping that the solutions will be quite obvious to some of you more experienced members of the forum and wil therefore not take too much of your time.
I thank you in advance for all your answers and help.
Cheers,
Nejc
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I have been trying to build a generator, which will output a triangle/sawtooth wave with variable frequency and duty cycle. I came up with the circuit attached to the post. It works quite nice up to around 500kHz (my aim is 1Mhz) but then the problems begin. Actually the most disturbing problem is the distortion seen on the attached images. I have traced the problem to the junction capacitance of the two diodes (decreasing the value of this parameter in the simulation reduces the distortion). I've been searching the web for diodes with small junction capacitance, but was unable to find an adequate one (according to the simulation Cj would have to be lower than 300fF - the lowest I could find has 1,5pF). One solution is adding diodes in series and lowering the capacitance that way, but this solution is by no means an elegant one and brings with it other problems (bigger voltage drops->smaller current and frequency, larger variation in the combined voltage drop which is dependent on the current,...).
My question therefore is: how to solve this problem? Any ideas for circuit improvement which resolves the problem, or a completely different design which satisfies the mentioned requirements (variable frequency up to 1Mhz, variable duty cycle-preferably from 1% to 99%) would be greatly appreciated. I would however like to avoid the use of OTAs as in this idea: http://www.till.com/articles/VariableSaw/index.html
The second problem I encountered presents itself already at frequencies at which the above described problem does not yet cause noticeable distortion. This second problem is the changing of frequency with the change of duty cycle (rotation of potentiometer P2). At 50% duty cycle the frequency is largest. Then at 1% and 99% duty cycle, the frequency drops.For example: at 113kHz with 50% DC, it drops to about 110kHz at 1% and 99% DC.
So my second question is: where is the reason for that? When tossing around the equations I came to the conclusion, that the frequency should stay the same for the given circuit. Maybe some parasitic effects are causing the problem again? One reason might be the exponent-like changing of the diodes forward voltage with current, but I also simulated the circuit with ideal diodes and the problem remains. Any ideas?
I am fairly new to electronics so I am hoping that the solutions will be quite obvious to some of you more experienced members of the forum and wil therefore not take too much of your time.
I thank you in advance for all your answers and help.
Cheers,
Nejc
View attachment 40921
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