Cross talk

Cross talk is a common feature that probably originated from phone lines carrying noise from an active conversation across its wires. If you have a piece of electrical wire close to another one, then the change of the current on one wire will induce change of current of the other, simply by the law of induction in physics. If you have strange noise in your project such as ultrasonic ranger, motors, etc., you may want to see if your power lines are cross talking with your control or sensor signal lines.

The ultrasonic transducer for example is a big power sucker. It’s turned on only momentarily and then turned off quickly, and then repeat for 7 to 8 times to generate the necessary sonic pulse for distance detection. The change in current within a unit amount of time, or di/dt, is big on the power lines and that could couple to their neighbors via induction with dB/dt (magnetic field changes with time), AKA cross talk. To eliminate this noise, you may add a capacitor between power and ground near the transducer to reduce the need to change current on the wire that passes the current. Charges on the capacitor simply flow to the device as needed to reduce the need to pull additional currents along the power line. This capacitor is called a decoupling capacitor. Or you can also shield your control or signal line and connect it to the ground. For a mobile device, a decoupling capacitor is most convenient.

When I was a grad student, I observed this strange but straightforward physics law of induction first hand. I was trying to control a power supply with a PID temperature controller. The power supply was connected to resistive heaters so was a thermistor. They went down the same cable, which was not properly shielded. The thermistor was used as feedback for tjhe PID temperature controller. I got the strange jump of power every now and then and the temperature was not stable. Then I realized the problem and used the shield inside the cable to shield the signal from the power lines. It worked nicely afterward. It turned out, the thermistor reading was strongly affected by the power supply suddenly turning on or off and that affected the temperature reading and knocked the PID off balance. My experience, either use a decoupling capacitor or use shield around your sensors. The shield needs to be grounded.

One Response to Cross talk

  1. great post thank you.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: