Oscilloscope probes
Oscilloscope probes exist in many types with different applications. They provide the vital link between the measured object and the oscilloscope.
Passive probes
Fig. 1: Equivalent-circuit diagram input impedance probe.
Passive probes are the most commonly used probes for the oscilloscope. But all too often considered as ideal probes, however, nothing is less true. Usually, the input resistance is still in charge, but we forget that the input capacity plays a major role at high frequencies. At still higher frequencies is also the length of the sprunghook and ground lead play a role. This is reflected as inductors in series with the probe.
Bandwidth
Fig. 2: Measured attenunation of a 60 MHz probe at 1:1 and 1:10 settings.
Passive probes are often equipped with a switchable attenuator, for example, switchable between an attenuation of 1:1 and 1:10. It is important to realise that the specified bandwidth of the probe only is valued at the 1:10 attenuation. Not only the bandwidth but also the phase error in the 1:1 mode is a lot worse.
In Figures 2 & 3 show the results from a measurement with a switchable 60 MHz probe in the 1:1 and 1:10 position. This measurement is achieved by a comparison with a 500 MHz 1:10 probe and a 500 MHz oscilloscope. Both the tested 60 MHz as the 500 MHz reference probe had the same cable length.
Fig. 3: Measured phase shift of a 60 MHz probe at 1:1 and 1:10 settings.
The relative attenuation is shown in Figure 2. The bandwidth is usually specified by the ±3 dB limit. Shown that the probe in the 1:10 position the 3 dB level at 60 MHz reached as specified. In the 1:1 attenuation position the -3 dB level at 20 MHz exceeded a lot earlier than the specified 60 MHz.
The phase shift (Figure 3) shows a remarkable image. In the 1:10 attenuation position the graph is relatively flat with a maximum error of -7 ° in the range of DC to 60 MHz.
In 1:1 mode the phase error is significant present from about 1 MHz and at 10 MHz the phase shift is already 20 °. At 60 MHz the phase error is increased up to 90 °.
If a phase comparison is carried out between a 10 and 20 MHz signal with the probe in the 1:1 mode, then the error introduced by the probe is already about 20 °.
Impedance
Fig. 4: Impedance curve of a 1:1 probe: 1 MΩ, 45 pF, 1:10 probe: 10 MΩ, 12 pF and a 50 Ω, 8 pF scope input.
An oscilloscope probe will always form a certain load to the measured object. How great is this load is depends on the probe and the frequency of the signal. Figure 4 shows the impedance of various probes plotted against the frequency. This is the calculated impedance of a 1:1 probe (red) with an internal resistance of 1 MΩ and a parallel capacity of 45 pF. The blue line is the characteristic impedance of a 1:10 probe with a resistance of 10 MΩ and 12 pF in parallel.
Both probes have an impedance similar to the ohmic resistance of the probe in the range from DC to about 1 kHz. Above 10 kHz, the reactance of the parallel capacities becomes significant. The load that the probe forms at the measured object is increasing sharply as the frequency increases.
At 1 MHz, the impedance has already dropped to about 3.5 kΩ for the 1:1 probe and approximately 13 kΩ for the 1:10 probe.
A 1:10 probe has except a higher ohmic internal resistance also a smaller input capacity. By this is at high frequencies the impedance more favourable.
Figure 4 is also the impedance graph of the oscilloscope shown with a 50 Ω resistance and a parallel capacity of 8 pF. For wideband and high frequency measurements, this is a favourable setting. The impedance from DC to 100 MHz is in this whole area constant.
This impedance reduction vs. the frequency also has implications for the maximum permissible voltage at higher frequencies. As a result of a decrease in impedance the current will increase, which in turn affects the dissipated power.
Comparison probes and accessories
Probes and type of accessories have a strong influence on the measurement result. Here are some examples of the probe type and the measured results.
The circuit on which is measured consists of a 74F04 which conditions a 700 kHz square wave derived from a function generator.
Next to each measuring arrangement a scope screen dump is shown. The upper trace shows always a full period, and the lower trace shows a detail of the rising edge.
Probes with sprunghook and groundlead
In the first example is a 1:10 500 MHz probes applied. The probe head has a sprunghook with a 11 cm long ground lead with which it is connected to the test circuit.
Fig. 5a: Probe head with a normal sprunghook en groundlead.
Fig. 5b: The measurment results.
Clearly is an overshoot to see followed by a damped oscillation. This is caused by the self-induction of the ground lead and sprunghook in combination with the input capacity.
Probes with bayonetstyle groundlead
Better probes are supplied with a bayonet style ground lead witch can be mounted on the probe head. The ground connection is only a short pin. Here is the same 1:10 500 MHz probe applied as above.
Fig. 6a: Probe head with bayonet style groundlead.
Fig. 6b: The measurment results.
Still an overshoot is visible, but it is much smaller than in the example above. The ringing is almost gone.
Also probes must have short as possible connections where higher harmonics signals are measured.
Standard test leads
Simple measurement leads are unsuitable for oscilloscope measurements. As an example a measuring arrangement with one meter long lead with banana plugs. Those are connected with a BNC=>banana adapter to the oscilloscope. With crocodile clips at the other end of the measurement leads a connection is made with the test circuit.
Fig. 7a: Measuring arrangement with simple measurement leads.
Fig. 7b: The measurment results.
The measuring result is a mess. By measuring with long lines and thus relatively large inductors in combination with the input capacity of the oscilloscope created additional oscillations. Only on low-frequency signals where higher harmonics would lack is it possible to use standard cables.
Differential probes
Fig. 8: High-voltage differential probe.
A normal probe measures the voltage relative to the ground. If multiple ground leads of different probes are connected to various mass points they will be create ground loops. This can seriously distort the measurement and lead to incorrect measurements. This can be prevented by a differentialprobe. This type of probe measures the voltage difference between the two terminals of the probe.
Fig. 9: Principle differential probe.
Some types of differentialprobes are specially designed to measure high voltages. It often happens that when measuring under voltage components standard probes are being used. To avoid short ciruits between the power and ground of the oscilloscope often the oscilloscope earth connection are disconnected. This will results in very dangerous situations. The complete oscilloscope is so connected to the power grid. Also passive probes are not designed for these high voltages. With a high voltage differentialprobe can without danger be measured on voltages connected to the power grid and scope can remain grounded.
Current probes
Fig. 10: Example of a DC current probe placed around a conducting wire.
Currents can be measured by measuring the voltage over a known resistance. A major disadvantage is that the circuit has to be opened to add the measure. Also can this additional resistance affect the measurement. In this way there is still a galvanic coupling that is often undesirable. This can cause problems at high voltages and there are limited places where the flow can be measured.
Fig. 11: Principle DC current probe.
Currents can also be measured with a current probe, also known as current clamp. These disadvantages do not know the above, or to a much lesser extent. A current probe is simply clamped on the wire and connected to the oscilloscope.
Current probes are roughly divided into two types: AC and DC current clamps.
The AC current clamps is not much more than a transformer where the conductor who conducts the current to be measured is the primary winding, and the second winding is fitted onto the core and is connected to the oscilloscope. This is a passive probe.
Fig. 12: Construction DC current probe.
The DC current probe is an active probe. Here is the current carrying conductor inserted through the core opening and is the primary winding. The secondary winding is a compensation coil. The core has an air gap that holds a sensor that measured the magnetic flux in the core. The current carrying wire will magnetise the core. This is measured and as a result of this the control circuit runs a current true the compensation winding in a way that the magnetic flux in the core is kept zero. As a result the core will never be magnetised. The advantage is that the non-linear properties and hysteresis of both the core and the magnetic sensor have little influence on the measurement result.
Except the measurement circuit includes the electronics also a demagnetisation circuit. Before using the current probe the core must be degaussed.
Tips
If the measured current is small compared to the range, the sensitivity can be increase. The wire where the measurand flows through must several times led though the current probe. The sensitivity is increased in proportion to the number of times that the wire runs through the probe.
Keep in mind that by increasing the sensitivity in the manner described reduces the bandwidth.
Currents can be added by leading all the wires of which is the total current must be know though the probe. Caution must be made on the current direction in each of the wires.
Similarly, a differential measurement can be performed. The two wires should be led in reverse directions through the probe.
Delay time probes and cables
Signals are not infinitely fast transported by cables. The speed with which this is done depends on the species, and most of all the length of the cable. If exactly the same cables or probes are used, there is no problem. However, if different cables are used then signals will not arrive at the same time at the oscilloscope. By this time difference, phase differences will be notified. These phase differences are noticeable at higher frequencies.
Also signals that are processed by electronics in differential and flow probes undergo a certain delay.
Fig. 13: Difference delay time in cables occur phase shifts.
The image above shows the difference in delay between two cables connected to the same voltage source. Channel 1 measured the signal by a 500 MHz passive probe with a cable length of 1.2 m. Channel 2 measured the same signal via a 0.5 m RG58 cable ends with 50 Ω.
The difference in delay time between these two cables is 2.8 ns. In this case of a signal of 20 MHz, the phase shift tdelay*frequency*360degº = 2.8*10-9*20*106*360º = 20.16°.
It is therefore recommendable to use the same cables for measurement, or calculate the delay time of the cables and/or probes in effect.
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