Series resistor in Impedance matched lines - Page 1
Forum section: electronics / beginners

author: Freshman
author_rank: Regular Contributor
timestamp: March 05, 2024, 07:42:47 am
content: I have often seen that in some high speed signals, for example, RGMII lines, which are supposed to have 50ohms single ended impedance between the signals and the ground, often have series termination resistors of small value, say, 10 ohms.Whether this 10ohms is included in the single ended impedance measurement for the trace or not?If so, would the trace impedance not be affected if I move to have higher resistor value?Please tell me how to understand this?

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2024, 05:46:50 pm
content: The series resistor slows down the edges reducing EMI, It also reduces peaking.

author: Freshman
author_rank: Regular Contributor
timestamp: March 05, 2024, 05:54:33 pm
content: Thanks. But can you comment on the impact of the series resistor on the impedance of the trace?

author: Manul
author_rank: Super Contributor
timestamp: March 05, 2024, 05:58:36 pm
content: Trace impedance does not change, it is inherent. Source (driver's) impedance changes.

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2024, 06:03:11 pm
content: Quote from: Freshman on March 05, 2024, 05:54:33 pmIt has no effect on the impedance of the trace. Why would it?

author: radiolistener
author_rank: Super Contributor
timestamp: March 05, 2024, 06:27:11 pm
content: Such resistors are used for better matching between transmission line and transmitter/receiver impedance. Also it allows to suppress reflections in transmission line.Quote from: Freshman on March 05, 2024, 05:54:33 pmThe impedance of transmission line depends on distributed capacitance and inductance along transmission line. Resistors on the end of transmission cannot affect impedance of transmission line.Such resistor changes transmitter/receiver impedance on which it is installed.

author: selcuk
author_rank: Frequent Contributor
timestamp: March 05, 2024, 07:27:16 pm
content: There are source impedance, transmission line impedance and destination impedance. You want to match those to deliver power efficiently. For those digital signals, normally source has a lower impedance and destination has a higher impedance than the transmission line. Adding series resistor alters the source impedance since transmission line impedance is already defined by track dimensions.Below link has a detailed info about matching of ferrite beads. It is the same thing as inserting a series resistor.Murata - Noise Suppression Basic Course Section 2 How to Select Ferrite Beads Considering the Characteristics of Digital Circuits

author: TimFox
author_rank: Super Contributor
timestamp: March 05, 2024, 07:30:16 pm
content: It’s important to use careful language here:“Impedance” generally is something you can measure at a circuit node, e.g. from the right end of that 10 ohm resistor to the ground plane.“Characteristic impedance” specifically is a measure of a transmission line as a circuit component and transmission line theory allows you to calculate impedance at different nodes of your circuit.In general,Zoof the line is complex (due to loss) and therefore a function of frequency.