Hi there,
According to the JETSON TX2 OEM DG, the Jetson TX2 module has 120 ohm bead in series with the CAM0-2_MCLK signals.
(Q1) What is the use of this 120 ohm bead?
(Q2) The signal coming out of the CAM0-2M_MCLK is a CMOS 20~ MHz signal, would’nt the ferrite bead (inductor) interfere with the digital signal’s rising and falling edge (which is a bad thing)?
Thanks.
Hi, those beads are 120 ohm @ 100Mhz which are used to only filter high frequency interference. It is same as 0 ohm resistor to low speed signals.
It’s not actually a bad thing to have some filtering on a digital signal.
A “pure” square wave has infinite energy across the entire frequency spectrum, and thus can’t actually exist in physics.
If you try to drive a wire with a sharp rise time, you will use a lot of current, and generate a lot of EMI.
Meanwhile, the receiving end only has a certain dependency on rise times (slew rates) of signals. If you run your bus at 1 megabit Manchester (UART, CAM, SPI, I2C, etc) then any rise time shorter than about 0.1 microsecond will just be wasted.
Note that the receiving end will have some capacitance, and the wire will have some resistance and inductance of its own, so you will never see a perfectly sharp rising edge on an oscilloscope hooked to a physical wire.
The inductors are a common way of making sure that the wires outside the circuit board don’t have TOO much high-frequency energy, and thus radiate too much EMI into the spectrum. Doing so could interfere with all kinds of radio equipment and even with other circuits that could pick up stray EMI with traces that also work as antennas.