Attenuating Probes

1X Probes

1X probes, also known as 1:1 (one-to-one) probes, simply connect the high-impedance input of the oscilloscope to the circuit being measured. They are designed for minimum loss and easy connection, but otherwise they are equivalent to using a cable to connect the scope. Figure 4-32 shows the circuit diagram for a high-impedance scope input connected to a circuit under test. The circuit under test is modeled as a voltage source with a series resistor. The 1X probe (or cable) will introduce a significant amount of capacitance which appears in parallel with the input of the scope. A 1X probe may have around 40 to 60 pF of capacitance, which is usually larger than the oscilloscope input capacitance.

The impedance of the circuit and the input impedance of the oscilloscope together produce a low-pass filter. For very low frequencies, the capacitor acts as an open circuit and has little or no effect on the measurement. For high frequencies, the capacitor's impedance becomes significant and loads down the voltage seen by the oscilloscope. Figure 4-33 shows this effect in the frequency domain. If the input is a sine wave, the amplitude tends to decrease with increasing frequency and the phase is shifted.


The loading also affects the oscilloscope's response to a step change in voltage. The loading due to the input impedance of the scope (and the probe capacitance) can be broken into two parts: RESISTIVE LOADING and CAPACITIVE LOADING. Figure 4-34 shows the probe and scope input loading broken into resistive and capacitive loading, which can be analyzed independently. The resistive loading is due entirely to the input resistance of the scope, while the capacitive loading is due to the probe capacitance combined with the scope input capacitance.


The resistive loading circuit of Figure 4-34 is another example of the voltage divider circuit. Thus, the voltage delivered to the scope input, VIN, is a replica of Vs but with reduced amplitude. For a voltage step from zero to VMAX at time t = 0, The effect of capacitive loading is more complex and results in an exponential response in the voltage. For a Vs voltage step which goes from zero volts to VMAX volts, VIN obeys the following equation.


The step responses due to the two loading effects are shown in Figure 4-35. The resistive loading changes the size of the voltage step, but does not change the waveform shape. Capacitive loading slows down the rise time of the step but eventually settles out to the same final value as the ideal response. As shown in Chapter 1, the bandwidth and rise time of a system are inversely related. Since the bandwidth of the instrument is effectively being decreased, the rise and fall times of pulse inputs will be increased.

The circuit model used for this analysis may not be accurate for all types of practical circuits. The output resistance (drive capability) of digital circuits may vary with the output voltage and cause the loading effect to be different. Even though this model is not 100 percent accurate for such a circuit, the basic principle of resistive and capacitive loading still applies. This means that load capacitance will slow down the rise time of the signal while resistive loading will tend to change the output amplitude. Increased rise time in a digital circuit is translated into increased delay when the signal reaches the next logic gate. This is because it will take longer for the signal to rise to the logic threshold, causing the next gate to switch later. The 1-MW input impedance of the typical oscilloscope is large enough to prevent resistive loading of most digital circuits, but the capacitive loading of a 1:1 probe will introduce significant delay into the signal.

10X probes (also called 10:1 probes, divider probes, or attenuating probes) have a resistor and capacitor (in parallel) inserted into the probe. Figure 4-36 shows the circuit for the 10X probe connected to a high-impedance input of an oscilloscope. If R₁C₁ = R₂C₂, then this circuit has the amazing result that the effect of both capacitors exactly cancel. In practice, this condition may not be met exactly but can be approximated. The capacitor is usually made adjustable and can be tweaked for a near perfect match. Under these conditions, the relationship of Vs to VIN is
which should be reminiscent of the voltage divider equation. R₂ is the input resistance of the scope's high input impedance (1 MW) and R₁ = 9R₂. From the previous equation, this results in


So the net result is a probe and scope input combination that has a much wider bandwidth than the 1 X probe, due to the effective cancellation of the two capacitors. The penalty that is incurred is the loss of voltage. The oscilloscope now sees only one-tenth of the original voltage (hence the name 10X probe). Also notice that the circuit being measured sees a load impedance of R₁ + R₂ = 10 MW, which is much higher than with the 1 X probe. Some probes are designed to be conveniently switched between IX and 10X operation.

With a 10X probe, both the resistive and capacitive loading effects are reduced (relative to a IX probe).⁴ Although the input capacitance of the scope is ideally canceled, there is a remaining capacitance due to the probe, CPROBE. This capacitance, which is specified by the manufacturer, will load the circuit under test.

The factor of 10 loss in voltage is not a problem as long as the voltage that is being measured is not so small that dividing it by 10 makes it unreadable by the scope. This means that the scope's sensitivity and the signal voltage may be factors in deciding whether to use a 10X probe. On most oscilloscopes, the user must remember that a 10X probe is being used and must multiply the resulting measurements by a factor of 10. This is a nuisance so some scopes include two scale markings: one valid for a IX probe and the other valid for a 10X probe. Other scopes have gone one step farther and automatically adjust the readings by the correct amount when an attenuating probe is used.

⁴Some 10X probes have a resistor across the probe input so that the resistive loading is 1 Mil. These probes do not represent an improvement in resistive loading over the 1 X probe, but they do have less capacitive loading.

Probe Compensation

Other types of attenuating or divider probes are available, including 50: 1 and 100: 1 probes. The general principles of these probes are the same as the 10X divider probe: voltage level and bandwidth are traded off. To obtain wider bandwidth, more loss is incurred in the probe and less voltage is supplied to the input of the scope. This may require a more sensitive scope for low-level measurements. Some divider probes use the scope's 50-W input instead of the 1-MW input.

Other Attenuating Probes

To maximize the bandwidth of the attenuating probe, the probe capacitor must be adjusted precisely such that the input capacitance of the scope is canceled. This is accomplished by a procedure known as COMPENSATION.

The scope probe is connected to a square wave source called the CALIBRATOR which is built into the scope. The probe is then adjusted to make the square wave as square and flat topped as possible. Figures 4-37a and 4-37b show the oscilloscope display during compensation with an overcompensated and undercompensated probe. Figure 4-37c shows the display when the probe is properly compensated.

The square wave is a wide bandwidth signal rich in harmonics. If the probe is adjusted so that it measures a square wave with a minimum of waveform distortion, then the probe will be correctly compensated for wide bandwidth signals in general. This concept is also used for square wave testing of amplifiers.

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Other Types of Probes

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