Contents
What is the impedance of a USB cable?
FS/HS USB cable differential impedance is about 90 Ohms so internal termination resistors must be 45 Ohms or so. FS/HS USB cable differential impedance is about 90 Ohms so internal termination resistors must be 45 Ohms or so.
What causes the resistance of a USB cable?
Almost. It turns out that the wire thickness used inside the cable impacts on the resistance of the cable assembly – this resistance causes energy loss inside the cable when an attached load draws a current, and causes a voltage drop which can reduce the voltage to the end device to a point where it is not possible to charge quickly or completely.
What do the numbers on a USB cable mean?
The top cable says 28AWG/1PR AND 24AWG/2C, and the bottom cable says 28AWG/1P+24AWG/2C. What this means is that the cable is made of one data pair of 28AWG thickness, and two conductors (that carry positive and negative) of 24AWG thickness. Remember, the larger the AWG number, the thinner the wire.
What is the dielectric constant of a USB cable?
Polypropylene, which is a common dielectric for high frequency cables has a dielectric constant of 2.2, meaning for a given thickness between electrodes of a capacitor it will yield 2.2 time as much capacitance as air. A USB cable is not four simple conductors, it is two conductors for power and ground and a twisted pair for the signal.
Which is twice the impedance of differential mode?
Differential impedance is twice the odd mode impedance. Odd impedance is is the impedance of a single trace when driven in differential mode. The USB standard requires 90 Ohm differential impedance as seen here, that is across both USB_P and USB_N and would equal an odd impedance of 45 Ohm.
What kind of impedance does 90 ohm coax have?
The differentilal impedance is a vector sum of single-ended impedances (one wire to ground), and coupling between two wires. Two isolated 45-Ohm coax would form a 90-Ohm impedance, but one can have two tightly-coupled wires with high impedance to ground.
What makes 90 ohms differential 90 ohm differential?
To make it 90 ohms differential two wires of certain core diameter and certain thickness of insulator are twisted together (at certain number of twists per meter). The twisting ensures that the distance between wires stays constant, because it is a major parameter that defines uniformity of the transmission line.
Do you need Impedance control on a PCB?
Owing to the prevalence of complex processors, USB devices, and antennas printed directly onto the board surface, more and more PCB designs now require impedance control and testing than ever before.
How does controlled impedance affect a fabricator?
Controlled impedance affects the combination of material types and thicknesses the fabricator will need to use in order to hit the target impedance, as well as any micro-adjustments that may be required to the Gerber files in pre-production.