Cover story
Hot wire to the measurement object “W
By Boris Adlung, Rigol Technologies Europe
hat do I need a probe for? Can the supplied measuring probe be
used for all measurements? What are the advantages of this tool?” These are questions that many developers ask themselves when putting a new oscilloscope into operation.
A logical connection Probes help to connect the oscilloscope to the development object when setting up the measurement. There are different probes for various tests. For example, currents can be measured with a current clamp, or digital signals can be recorded and displayed on an MSO with a logic probe and the necessary interface. The general goal of the tools is not only to create a simplified contact between the measuring device and the object to be measured, but also to measure with a very high signal accuracy. It is important to keep the additional influence caused by any type of probe as low as possible in order not to stress the signal source – i.e., the test object. In a measurement setup, it is also important to know what you do not want to measure, for example, the detection of undesired radiation should be minimised or completely avoided. In contrast to a measuring cable,
probes create a high-quality connection with a convenient connection option. A high measuring resistance avoids capturing unwanted interference or mains noise. The high measuring resistance enables an ideal voltage pick-up at the probe and thus an optimal connection to the oscilloscope.
Cooperative input resistance A passive probe, for example, has two contact points, the hot pin and the ground connection. For contacting the hot pin, passive probes provide a hook that can easily be attached to the circuit, and a ground connection via a short piece of wire with an alligator clip. In addition to the convenient connection
head, the probe consists of a coaxial cable with a BNC connector. Together with the oscilloscope, the probe defines the input resistance of the measurement. The oscilloscope has an input impedance of 1MΩ and a low capacitance for bandwidths of up to 350MHz or 500MHz. In the case of a direct connection, this capacitance puts a load on the DUT, which means that a probe should contribute little capacitance in order to keep this load small.
Voltage range and bandwidth A passive probe also has two settings that can be used to create a voltage divider. For the normal setting x1, the signal is applied to the oscilloscope at 1MΩ, using the probe without the voltage divider. The measured values are thus passed on to the oscilloscope 1:1. Here, not only the maximum voltage range but also the bandwidth is limited. A higher voltage and bandwidth can be achieved with a 10:1 pitch. Here, however, the oscilloscope must also be set to 10:1 in the vertical setting so that the correct amplitude is displayed. In the setting 1:1, together with the oscilloscope, the following measuring resistance ZM
is:
implemented in order to minimise these effects. The gain factor x1 can be used for measurements that require little bandwidth and little voltage to be recorded. Most measurement requirements in the normal voltage/bandwidth range therefore preferably require the gain factor of x10. Using the example of the PVP2350, the standard probe of the MSO5000 oscilloscope series, both amplification factors can be set. Here, voltages of up to 30VRMS
voltages of up to 300VRMS
can be measured at x1 and in the x10
range. So the voltage range increases by the factor 10.
The smaller capacitance in the probe also increases the bandwidth from 35MHz to 350MHz. A path for x10 is also sketched in Figure 1. Neglecting the coaxial cable resistance (since << than RO
and RP ), the
voltage divider according to equation 2 is now created as soon as the time constant according to equation 3 is taken into account:
Equation 2: Voltage divider at x10, capacitances are truncated using the time constant in equation 3
Equation 1: Measuring resistance of the probe (x1) together with the oscilloscope
It can be seen that the GND cable is inductive. The longer this cable, the more inductance it has. In the formula it can be seen that if ω increases (i.e. the bandwidth), then this resistance increases and the influence increases; see Figure 2. In addition, it is visible in equation 1 that the capacitance creates a resonator. As a result, higher-bandwidth measurements will show overshoot, which can be reduced by shortening the GND cable. Per 1 millimeter of GND cable, an induction of about 1nH is generated. The shorter the GND connection at higher frequencies, the better. With a ground spring, very short GND paths can be
06 September 2022
www.electronicsworld.co.uk
Equation 3: Time constant for voltage divider x10
Most applications can be measured with this bandwidth and the voltage values at x10. Here you also have to set x10 on the oscilloscope for the vertical setting. When using a probe with an oscilloscope, the bandwidth of both elements (probe and oscilloscope) must always be considered. The 3dB bandwidth is significantly influenced by the probe and should therefore be taken into account when purchasing the oscilloscope. If the system bandwidth is too low for the application, a probe with a higher bandwidth should be used. A compensation is integrated in Figure 1.
Here, a tunable capacity is installed in the probe, which – e.g., with the PVP2350 – can
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