In the electronics domain, dominated by digital signals, synchronization is very important. This is where clock signal is very important considering the timing requirements. Whether it be any big processor, it cannot work without a clock signal. Such a critical signal has to be handled carefully and ensured that integrity of the clock is maintained. While we have been talking along on the clock signal integrity, if we want to look at what we see when design is nor proper is the ringing effect. The common problems that might cause clock signal to loose it's integrity is:
- Impedance discontinuity
- Reflections
These have to be handled by the designers during the circuit design and at the PCB engineers during the PCB Layout. One common approach seen in most of the clock circuits is having a series resistor. The most common series resistor is either 22 ohms or 33 ohms. These series actually are called the damping resistors and help eliminate reflections and there helping not to degrade the clock signal.
Why it must be 22 ohms or 33 ohms and not other value?
The resistor that is added here is called the series termination resistor. It is actually, calculated by the difference between the transmission line impedance and the driver output impedance. As we see here, the difference is 50 ohms - 25 ohms = 25 ohms
22 ohms or 33 ohms work in most cases and we see a clean clock. The approach here is that designer has to perform simulation with different resistor values and ensure that clean clock is seen at least in the simulation. Both Pre-Layout and Post-Layout simulation is performed to ensure the correct resistor is placed (post-layout must be enough though!)
These resistor are also called damping resistors. These series termination requirements are seen in the case of clock buffers, oscillators, clock generators. General PCB Layout considerations as recommended by the oscillator vendor have to be followed.
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