When you are testing a signal and want to determine if it is a true signal waveform, it's essential to analyze how the oscilloscope degrades the waveform. This degradation can be evaluated from four key perspectives.
**1. The Effect of the Oscilloscope Probe on the Rising Edge of the Waveform**
The waveform displayed on the oscilloscope may not represent the actual signal, especially when using a probe that introduces significant parasitic elements. These include capacitance, inductance, and resistance, which can distort the shape of the signal.
For example, a 100 MHz signal with a 30 ns rise time may appear as 42 ns when measured with a 100 MHz bandwidth probe. This discrepancy occurs because the probe itself limits the signal's ability to transition quickly. A rule of thumb is to use a probe with at least three times the bandwidth of the signal being tested. This ensures that the probe’s rise time does not significantly affect the measurement.
Some probes are rated by their RMS bandwidth, which is calculated based on the square root of the sum of the squares of the individual bandwidths. This helps in determining the effective performance of the probe across different frequencies.
**2. The Influence of the Probe Ground Loop Inductance on the Waveform**
The ground loop of an oscilloscope probe can also introduce unwanted effects. For a typical probe, the ground wire (AWG 24) has a length of about 3 inches, leading to an inductance of approximately 200 nH. When combined with a 10 pF input capacitance, this creates an LC circuit with a time constant that affects the rise time.
In a critically damped system, the 10%-90% rise time is about 3.4 times the LC time constant. For a 300 MHz probe, the expected rise time is around 3.3 ns, but with a 3-inch ground loop, the rise time increases to 4.8 ns. This shows how even small physical characteristics of the probe can impact signal integrity.
**3. The Effect of the Probe on Signal Ringing**
Ringing is another common issue when measuring signals. It occurs due to high Q values in the circuit, which indicate the ratio of stored energy to energy dissipated per cycle. A higher Q value leads to more pronounced ringing, especially when there is impedance mismatch between the signal source and the transmission line.
If the cutoff frequency of the signal is lower than the resonant frequency, ringing may still occur even without a perfect match. This phenomenon is closely related to the concept of overshoot and undershoot in digital signals, particularly in rectangular waveforms.
**4. Crosstalk Caused by the Ground Loop**
Crosstalk is another factor that can distort measurements. When a signal loop is close to the oscilloscope probe, the changing magnetic field generated by the loop can induce a voltage in the probe’s ground loop. This is especially relevant in environments with high-speed signals, such as a 24-bit RGB signal.
For example, if the rate of change of current (di/dt) is 1×10ⷠA/s, and the loops are placed close together, the induced voltage can become significant. While this might seem negligible for low-frequency signals, it becomes problematic when dealing with high-speed or high-noise environments.
To mitigate crosstalk, PCB designs often incorporate copper plating to reduce the area of the signal return path. However, in some cases, a simple magnetic field detector can be made using the oscilloscope probe and its ground clip. By removing the ground clip and increasing the inductance, the sensitivity to external magnetic fields can be enhanced.
By understanding these four factors—probe bandwidth, ground loop inductance, signal ringing, and crosstalk—you can make more accurate and reliable measurements when using an oscilloscope.
For Honor Glass
Dongguan Jili Electronic Technology Co., Ltd. , https://www.jlglassoca.com