PWM waveform under load

Hello, I have a basic working knowledge of electronics and am using an Uno R3 with a vma03 driver motor shield. I am using them to generate a 12v PWM signal to control a automotive valve timing solenoid that draws about 1.3 amps at 100% duty cycle. It seems to be controlling the solenoid alright but I took a screen shot of my scope and noticed a huge decrease in voltage as each pulse is turned off. It seems to slowly return to 0v before the next pulse begins. It makes a clean square wave when there is no load, but as soon as I have one it does this. Is a large voltage decrease and return to zero like this normal? If so, what is happening? Your time is much appreciated. See picture:

My guess is that it has to do with the solenoid being a huge inductor in series with the source.

You need a diode to protect against back emf when the inductor is discharged. You can put a reverse biased diode across the inductor to achieve this.

Thank you guys! You both were right. I found this site that also has more info:
http://www.rmcybernetics.com/tutorials/pwm-inductive.htm

joshyak:
Hello, I have a basic working knowledge of electronics and am using an Uno R3 with a vma03 driver motor shield. I am using them to generate a 12v PWM signal to control a automotive valve timing solenoid that draws about 1.3 amps at 100% duty cycle. It seems to be controlling the solenoid alright but I took a screen shot of my scope and noticed a huge decrease in voltage as each pulse is turned off. It seems to slowly return to 0v before the next pulse begins. It makes a clean square wave when there is no load, but as soon as I have one it does this. Is a large voltage decrease and return to zero like this normal? If so, what is happening? Your time is much appreciated. See picture:

The H-bridge is being driven in fast decay mode, so the voltage across the inductive load will simply
reverse in sign when the high-side switch goes off until the magnetic energy has been pushed back into
the power supply through the free-wheel diodes of the bridge.