Probe Selection: Understanding the Signal Source

When selecting a probe, four basic signal source issues must be considered: signal type, signal frequency composition, signal source impedance and physical properties of test points. Each issue is discussed below

signal type

image 1 Probes classified by the type of signal being tested

The first step in selecting a probe is to evaluate the type of signal to be detected. To do this, the signals can be divided into:

• Voltage signal

• Current signal

• Logical signal

• Other signals

Voltage signals are the most frequently encountered type in electronic device measurement, which is why voltage sensing probes are the most commonly used type of oscilloscope probe. Additionally, it should be noted that since oscilloscopes require voltage signals at the input, other types of oscilloscope probes essentially convert the sensed phenomena into corresponding voltage signals. A common example is a current probe, which converts current signals into voltage signals for viewing on the oscilloscope

logic signals are actually a special type of voltage signal. A standard voltage probe can be used to view logic signals, but more commonly, it is necessary to view specific logic events. This function can be achieved by setting logic probes to trigger signals to the oscilloscope after a specified logic combination occurs

This allows you to view specific logic events on the oscilloscope display

Besides voltage signals, current signals, and logic signals, there may also be many other types of signals that users care about, such as signals from light sources, mechanical sources, heat sources, sound sources, and other sources. Various frequency converters can be used to convert these signals into corresponding voltage signals for display and measurement on the oscilloscope. During this conversion, the frequency converter acts as the signal source, selecting probes to transmit the frequency converter signal to the oscilloscope

Figure 1 graphically classifies probes according to the type of signal to be measured. Note that under each category, there are various probe subclasses, and additional signal properties and oscilloscope requirements further determine these probe subclasses

signal frequency components

regardless of type, all signals have frequency components. The frequency of a DC signal is 0 Hz, and a pure sine curve has a single frequency, which is the reciprocal of the sine curve's period. All other signals contain multiple frequencies, and the frequency value depends on the signal waveform. For example, the fundamental frequency (fo) of a symmetrical square wave is the reciprocal of the square wave period, and there are other harmonic frequencies that are odd multiples of the fundamental frequency (3fo, 5fo, 7fo, ...). The fundamental frequency is the basis of the waveform. Combining harmonic frequency with the fundamental frequency adds structural details such as waveform transformation and corners

To allow the probe to transmit the signal to the oscilloscope while maintaining sufficient signal fidelity, the probe must have sufficient bandwidth to transmit the main frequency components of the signal with minimal interference. For square waves and other periodic signals, this generally means the probe bandwidth must be 3-5 times higher than the signal's fundamental frequency. This allows the transmission of the base frequency and the first few harmonics without improperly attenuating their relative amplitude. Additionally, higher harmonics will be transmitted, but the amount of attenuation will increase, as these higher harmonics exceed the probe's 3-dB bandwidth point. However, since at least some higher harmonics still exist, they still affect the waveform structure to some extent

Image 2 When the main frequency components of the signal exceed the measurement system bandwidth (a), they experience a high degree of attenuation. As a result, waveform details are lost through rounded corners and elongation conversions. (b)

The main effect of bandwidth limitation is reducing signal amplitude. The closer the signal base frequency is to the probe's 3-dB bandwidth, the lower the overall signal amplitude seen at the probe output. At the 3-dB point, the amplitude decreases by 30%. Additionally, due to bandwidth roll-off, harmonics or other frequency components extending above the probe's bandwidth experience a higher degree of attenuation. Higher attenuation at higher frequency components can be seen by sharp rounding and slow rapid waveform transitions (see Figure 2).

It should also be noted that the capacitance at the probe tip also limits the rise time of signal conversion. However, this is related to the signal source impedance and load, which we will discuss later

signal source impedance

discussion of signal source impedance can be summarized into the following key points:

1. The probe's impedance combined with the signal source impedance generates a new signal load impedance, which to some extent affects the signal amplitude and rise time

2. When the probe impedance is significantly higher than the signal source impedance, the effect of the probe on signal amplitude can be neglected

3. The probe tip capacitor, also called input capacitance, affects the signal rise time and widen. This is due to the time required to increase the probe's input capacitance from 10% to 90%, with the formula as follows:

tr = 2.2 x Rsource x C

From the above points, it is clear that it is best to choose high-impedance, low-capacitance probes to minimize the probe load on the signal source. Additionally, by selecting low-impedance signal test points wherever possible, the impact of probe load can be further reduced

Physical connection considerations

The location and shape of the signal test point are also key considerations when selecting a probe. Is it sufficient to just touch the probe to the test point and observe the signal on the oscilloscope? Or is it necessary to connect the probe to the test point to monitor the signal and simultaneously adjust various circuits? For the former case, a pin-type probe tip is suitable; For the latter, a certain type of retractable hook probe tip is required

The specifications of the test points also influence probe selection. Standard-sized probes and accessories are especially suitable for detecting connector pins, resistor leads, and backplanes. However, for probing surface-mounted circuits, we recommend using small probes with accessories specifically designed for surface-mounted applications

The goal is to select the probe specifications, shapes, and accessories most suitable for a specific application, so that the probe can be quickly, easily, and securely connected to the test point for reliable measurement.

Reprinted from Tektronix