How to Choose an Oscilloscope Probe to Capture High-Speed ​​Signals

This article explains how to select an oscilloscope probe to capture high-speed signals:

• Oscilloscope probe input impedance

• Probe transmission response

• Oscilloscope Probe Accessories

• Oscilloscope and probe system bandwidth

As the processing power of silicon devices has increased dramatically, engineers have to deal with increasing signal rates, whether designing computer systems, next-generation semiconductor products or communication systems. In order to enable high-speed devices to process data more efficiently, they have worked tirelessly to invent technologies that can increase the bandwidth of external buses, enabling large amounts of data to be transmitted between circuit board devices through backplanes or cables.

Increasing the signal frequency is one way to increase bus bandwidth. This is because increasing the signal frequency can reduce the rise time of digital signals. Poor performance oscilloscope probes can cause overshoot and ringing on the measured waveform of these signals with fast rise times. Engineers must determine whether these anomalies are included in the design and whether they are caused by the measurement system.

在高速传输数据时，示波器探头与被测电路接触会引起寄生效应，从而导致被测波形出现严重异常。此外，探头还有可能给电路增加负载，导致信号发生显著变化或破坏。所以，充分了解示波器探头对被测电路和被测波形有何影响，将会明显改进测量结果。

Oscilloscope probe input impedance

Oscilloscope designers have made great strides in increasing sampling rates, bandwidth, and accuracy. To achieve the best performance from an oscilloscope for a particular application, probes must be carefully selected and operated. However, the importance of accessories is often overlooked. The probe is the primary connection between the circuit under test and the oscilloscope. It can affect both the measurement results and the operation of the circuit under test.

When the probe is connected to the circuit under test, it is equivalent to adding a load to the circuit. The probe load consumes additional signal source current, changes the working condition of the circuit after the test point, and thus changes the measured signal.

To obtain accurate measurement results, the probe must capture the signal and provide the most realistic signal display without adding additional load or changing the signal source over its frequency range. In fact, all probes add a complex load to the circuit under test (see Figure 1). Therefore, when selecting a probe, limit the load value to an acceptable range.

Probe specifications list input impedance and capacitance. These two combined can change and load the circuit being measured. When a capacitor operates at low frequencies, it acts as an open circuit, and the DC resistance becomes the dominant factor in loading the circuit. Resistive loading is the least influential factor in probe loading because it does not produce nonlinear circuit characteristics.

Although excessive leakage current from low impedance probes can cause nonlinear responses or interrupt circuit operation, this problem has been largely solved by the low voltage, high speed signal probes currently used, such as Keysight InfiniiMax probes, N2795A/96A or 1156A/57A/58A.

Figure 1. A simplified input impedance model of an oscilloscope probe (consisting of resistance, capacitance, and inductance components).

Oscilloscope probe input impedance

Assuming the source impedance is a resistor, the probe's resistive component forms a voltage divider (composed of the circuit's output impedance and the probe's input impedance), which reduces the voltage amplitude of the measured signal without changing its shape (see Figure 2). The smaller the probe resistance relative to the source impedance, the more the probe loading reduces the voltage amplitude of the measured waveform. Additionally, the smaller the probe impedance relative to the circuit impedance, the greater the current that flows into the probe, and the greater the chance of adversely affecting the circuit.

As the signal frequency increases or the edge speed decreases, the probe capacitance behaves like a short circuit, causing current to flow into the low impedance probe. At high frequencies, capacitive reactance is the primary factor in circuit loading and can cause circuit failure because the circuit cannot drive enough voltage headroom.

Capacitive loading is a major source of probe-related measurement error because it affects the measurement of rise and fall times, bandwidth, and edge-to-edge time. Capacitive loading can change the shape of the measured waveform by inducing an exponential response (Figure 3), thereby removing glitches, reducing ringing and overshoot, or slowing the measured edge just enough to complete the setup and hold time violation.

Figure 2. Resistive loading reduces the amplitude of the measured signal without changing its shape.

Figure 3. Capacitive loading changes the measured signal waveform by inducing an exponential response.

Figure 4 shows the input impedance of the Keysight 1158A probe up to 6 GHz. You can see that at low frequencies below 1-MHz, the probe input impedance is dominated by the probe DC resistance component (100 kΩ). As the signal frequency increases, capacitive reactance becomes the dominant component affecting circuit loading. At 2-GHz, the impedance of the Keysight 1158A reaches a minimum of 165-Ω, which is dominated by the probe tip resistor.

We can use a digital signal with ultra-fast edges generated by a 25-Ω source as an example to illustrate how these probe factors affect the measured waveform and the circuit under test (see Figure 5). You can see that when the probe is connected to the circuit under test, the signal waveform changes due to the probe input impedance.

Inductive loading shows up as ringing in the measured signal (see Figure 6). The source of the ringing is an LC circuit consisting of the internal capacitance of the probe and the inductance of the ground lead and probe tip. In general, when making any type of oscilloscope measurement, keep the ground lead as short as possible. This reduces the inductance and moves the ringing frequency outside the oscilloscope and probe bandwidth, minimizing its effect on the measurement.

Figure 4. Increasing frequency changes the input impedance of the Keysight 1158A probe.

Figure 5. The probe loads the circuit at high frequencies and increases the time it takes for the circuit to produce full voltage.

Probe transmission response

The parasitic parameters that affect the probe input impedance and change the probe transfer response are usually referred to as the probe transfer response. This transfer response is defined as the ratio of the probe output voltage to the probe input voltage (Vout/Vin-), usually displayed as an amplitude (dB)/frequency graph.

The probe bandwidth is a continuous frequency band when the transmission response is below -3dB or the amplitude drops to 70.7% (Figure 6). At frequencies outside the probe bandwidth, the signal amplitude will be excessively attenuated and the measurement results will be unpredictable.

Figure 6. Bandwidth is the continuous frequency band over which the transmission response of an instrument decreases by 3 dB.

Over the probe bandwidth, you can see that the probe output signal closely matches the probe input signal with minimal deviation, so the waveform you see on the oscilloscope screen is the signal at the probe input.

In the frequency domain, the ability of a probe to transmit a signal from input to output while minimizing the effect on the signal is expressed as a very flat (0-dB) transmission response over the entire probe bandwidth. However, this is difficult to achieve in practice. When the probe is connected to the circuit under test, the parasitic effects of the physical connection and the internal components of the probe can form a resonant circuit with a resonant frequency lower than the probe bandwidth (Figure 7). This in-band resonance will cause the probe output signal to differ from the input signal, and will appear on the measured waveform in the form of overshoot and ringing.

The example in Figure 7 depicts the in-band impedance of a non-Keysight 4-GHz probe at input voltage (Vin). Note that the output voltage of the probe (Vout) does not match the input voltage. The output voltage remains flat, while the probe transmission response (Vout/Vin) peaks at 5-dB.

Figure 7. (Top) Frequency response of input voltage and output voltage for a non-Keysight 4-GHz probe in a 25-Ω system. (Bottom) Transmission response of the probe. The output voltage remains flat with a 5-dB peak in the transmission response as the input voltage resonates down to 3.5 GHz.

So what's the problem with the output voltage remaining flat when the input voltage is resonant? Isn't this the same signal as it would be without the probe connected? That's a good question, but remember that the probe transmit response will always be 5-dB peaked, causing distortion of the input signal and showing up as extra overshoot and ringing in the measured waveform.

The measurement in Figure 8 was made at the midpoint of a transmission line terminated at both ends with 50-Ω loads, the source resistance appears to be 25-Ω, and the probe response is matched to this type of circuit. If the circuit you are measuring does not provide an ideal 25-Ω source resistance, you will see signal distortion caused by the probe.

This type of distortion can be better explained with an example. Consider a 100-kΩ probe measuring the voltage of a circuit with a 100-kΩ source resistance. The probe's transfer response has been modified for this type of circuit to show the "real" output voltage of the probe. Therefore, when you connect the probe to the circuit and reduce the input voltage by half, the measured waveform shows the full voltage amplitude as if the probe was not connected.

But what happens when the probe is connected to a circuit with a 50-kΩ source resistance? The probe's transmission response still changes the measured waveform, showing the waveform voltage as 33% higher than the actual voltage at the probe input.

The best probe ensures minimal impact on the circuit being tested and transfers the voltage from the input to the output with minimal distortion, so that you can observe the signal at the probe tip.