When there is impedance discontinuity along the transmission path, reflections occur. The purpose of impedance matching is to ensure continuous impedance along the transmission line—achieved by using terminating components—thereby eliminating reflections in the transmission chain. Common impedance-matching techniques include:
A. Series Termination Method
Connect a resistor in series near the output end; to achieve impedance matching, the sum of the series resistor and the output impedance at the driving end must equal the characteristic impedance Z0 of the transmission line.
In a typical digital signal system, the output impedance of components usually ranges from about 10 to 20 ohms, while the impedance of the transmission line is typically maintained at 50 ohms. Therefore, a common choice for the series matching resistor at the source end is a 33-ohm resistor. Of course, to achieve optimal matching, it’s best to keep the transmission path—from the driver’s output through the series resistor—relatively short—short enough that the effects of this segment can be disregarded altogether.
The advantages and disadvantages of series resistors are as follows:
(1) Advantages
1. Only one resistor is needed;
2. No unnecessary DC power consumption;
3. Eliminate secondary reflections at the drive end;
4. Unaffected by changes in the receiver's load;
(2) Disadvantages 
1. The single transmission at the receiving end still exists; 
2. There will be some variations in the signal edges; 
3. The resistor should be placed close to the driving end and is not suitable for bidirectional signal transmission; 
4. The voltage transmitted online is half of the driving voltage, making it unsuitable for daisy-chain multi-load configurations. 
B. Parallel Termination Method 
Parallel termination, also known as termination matching, requires that the terminating resistor equals the characteristic impedance Z0 of the transmission line to achieve impedance matching. 
In a typical digital signal transmission system, the impedance at the receiver typically ranges from a few megohms to over ten megohms. If the terminating resistor matches the characteristic impedance of the transmission line exactly, the resulting impedance when combined in parallel with the receiver's input impedance will still be close to the line's characteristic impedance. As a result, the reflection coefficient at the termination point becomes 0, meaning no reflections occur—essentially eliminating the first-order reflection at the end of the line. 
Advantages and Disadvantages of Parallel Termination 
(1) Advantages 
1. Suitable for multiple loads 
2. Only one resistor is needed, and its resistance value is easy to select.      
(2) Disadvantages 
1. Increased DC power consumption 
2. Parallel termination can be pulled up to the power supply or pulled down to ground, effectively raising low-level signals or lowering high-level signals, thereby reducing noise margins. 
3. AC Parallel Termination 
To eliminate DC power consumption, parallel termination can be achieved using AC parallel termination (AC termination matching). To meet the matching requirement, the terminating resistor should equal the characteristic impedance Z0 of the transmission line. 
The pros and cons are described as follows: 
(1) Advantages 
1. Suitable for multiple loads 
2. No increase in DC power consumption        
(2) Drawbacks 
1. Two devices required 
2. Added capacitive loading to the terminal, increasing the delay caused by the RC circuit. 
3. Effective for periodic signals (such as clocks) but not suitable for non-periodic signals (like data). 
4. Thévenin Termination 
To achieve matching with a Thévenin equivalent at the end, the parallel resistance value of the terminating resistor must equal the characteristic impedance \( Z_0 \) of the transmission line. 
Pros and cons description: 
(1) Advantages 
1. Suitable for multiple loads 
2. Particularly suitable for situations where the output impedance is well-balanced when pulling up or down at SSTL/HSTL levels.        
(2) Drawbacks 
1. Increased DC Power Consumption 
2. Two devices required 
3. Pull-up resistors connected to the power supply or pull-down resistors connected to ground will cause the low level to rise or the high level to drop. 
4. The resistor value is relatively difficult to choose. Selecting a smaller resistor value will raise the low-level voltage while further lowering the high-level voltage, which can be even more problematic. On the other hand, choosing a larger resistor value may lead to an incomplete match, increasing signal reflections—which can be determined through simulation.