Distortion of pwm signals

Thread Starter

yassser

Joined Jul 25, 2011
91
Hello ,

I'm making a dc motor control system , I have a control ct. and a power ct (H-bridge configuration) , the PWM control signals need to be transferred a long way to the power ct. in a noisy environment , when I used the oscilloscope to view the signals , I found the signals to be very distorted as shown in the figure when it reached the power ct. which caused fatal results .


I understand this is because the inductance of the cable , but I can't understand it mathematically and physically , can someone please illustrate it for me , and is there some way to prevent this kind of distortion ?

note: i use a shielded cable to transfer the signal
 

#12

Joined Nov 30, 2010
18,076
I need to see a drawing of the driver circuitry to figure out why you aren't getting a fast enough move in the positive direction.

Schematic, please.
 
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#12

Joined Nov 30, 2010
18,076
That's an open collector driver...it has no positive drive. You must provide more pull up current with a resistor.
 

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Thread Starter

yassser

Joined Jul 25, 2011
91
i do have a 10K pull-up resistor , may be I need to reduce it .

I checked the max232 line driver , the problem is that its delay is 500ns , these pwm signals have to be synchronized and these high delay times isn't acceptable , I could use a transistor to level shift the signals to high voltage to reduce noise effect and use a smaller pull up resistance , would that work?
 

#12

Joined Nov 30, 2010
18,076
I see the 10k pull up resistor. It is too much resistance.
This driver can sink as much as 50 ma. The smallest possible resistance is 100 ohms. Try something around 1k ohms.
 

Thread Starter

yassser

Joined Jul 25, 2011
91
okay , i will .

but can you please explain to me why does the signal look like if a cap is being charged , why does it have this shape ? and would reducing the resistance reduce the time constant ? i mean what is the mathematical reason ?
 

#12

Joined Nov 30, 2010
18,076
The shielded cable is a capacitor. It has picofarads per meter. You can treat it as if the driver only sees a capacitor.
 

Audioguru

Joined Dec 20, 2007
11,251
can you please explain to me why does the signal look like if a cap is being charged , why does it have this shape ? and would reducing the resistance reduce the time constant ? i mean what is the mathematical reason ?
The 10k resistance and the high capacitance of the long cable slow down the risetime.
 

crutschow

Joined Mar 14, 2008
23,158
but can you please explain to me why does the signal look like if a cap is being charged , why does it have this shape ? and would reducing the resistance reduce the time constant ? i mean what is the mathematical reason ?
The signal looks like a cap being charged because it is. Look up the capacitance of the shielded cable. It's likely somewhere around 50pF/ft. For 10 meters that would be 1.6nF. (You can also measure it with a capacitance meter). So you need to reduce the driver impedance to charge that capacitance and provide the RC risetime you want. Remember that the signal rises to 63% of its final value in one RC time-constant and will go to 98% of its final value in 4 time-constants.
 
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