fastcomponents
Newbie level 5
pwm jitter
Hello all.
I have a problem with a circuit I'm trying to design to control a cheap r/c hobby servo. To remind those unfamiliar with the control of these, they rely on a PWM signal of around 50Hz to control them. The limits of the servo positions correspond to a PWM on-period of between 0.8ms and 2.2ms, with the centre position being at 1.5ms. This in turn corresponds to a duty cycle of between ~3% and ~11% with the centre at about 7.5%.
The purpose of the circuit is to control the position of the servo with a control knob as cheaply as possible. Therefore, I decided to use a LM324 quad opamp. The circuit is attached, and is a miller integrator + comparitor, plus a second comparitor to generate the PWM signal.
The problem with the circuit as it stands is that there is quite a lot of jitter on the trianglewave at Vout, which in turn turns into jitter in the PWM signal - enough to really confuse some servo brands, and to overhead others. the jitter is about 0.5% - 0.75%, and I've tried many ways to reduce this. The IC is completely decoupled, and I've observed no noise on the PSU line.
Perhaps this is a limitation of the chip? from my understanding of the circuit, I would attribute the cause to be an unreliable comparitor function on succssive cycles of the oscillation. Can anyone confirm / deny? If that's the case, can anyone confirm what I should swap the LM324 for? (Cheap is favourable!).
Please find the circuit diagram attached. thanks, Tom.
Hello all.
I have a problem with a circuit I'm trying to design to control a cheap r/c hobby servo. To remind those unfamiliar with the control of these, they rely on a PWM signal of around 50Hz to control them. The limits of the servo positions correspond to a PWM on-period of between 0.8ms and 2.2ms, with the centre position being at 1.5ms. This in turn corresponds to a duty cycle of between ~3% and ~11% with the centre at about 7.5%.
The purpose of the circuit is to control the position of the servo with a control knob as cheaply as possible. Therefore, I decided to use a LM324 quad opamp. The circuit is attached, and is a miller integrator + comparitor, plus a second comparitor to generate the PWM signal.
The problem with the circuit as it stands is that there is quite a lot of jitter on the trianglewave at Vout, which in turn turns into jitter in the PWM signal - enough to really confuse some servo brands, and to overhead others. the jitter is about 0.5% - 0.75%, and I've tried many ways to reduce this. The IC is completely decoupled, and I've observed no noise on the PSU line.
Perhaps this is a limitation of the chip? from my understanding of the circuit, I would attribute the cause to be an unreliable comparitor function on succssive cycles of the oscillation. Can anyone confirm / deny? If that's the case, can anyone confirm what I should swap the LM324 for? (Cheap is favourable!).
Please find the circuit diagram attached. thanks, Tom.
