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Scope probes include: Oscilloscope probes Probe compensation Oscilloscope probe specifications
Oscilloscope users often use X10 oscilloscope probes to increase the bandwidth available and decrease the loading on the circuit under test.
On most scope probes there is an adjustment to compensate the probe to improve its frequency response. Most scope users are familiar with compensating the probe, but some of the reasons and circuits behind it may not always be obvious at first sight.
It is often easy to forget to compensate a probe and this can lead to signals having the wrong amplitude, or becoming distorted because the frequency response of the probes is incorrect. Compensating the probe enables reliable accurate measurements to be made.
Basic scope probe compensation
Frequency compensation for oscilloscope probes is required when using a voltage type probe that includes an attenuator, i.e. X10, X100 or other values. Scope probe compensation is not required for X1 probes.
For many probes there is only one adjustment and this is easily compensated by simply connecting the probe to the a square wave generator in the scope and the compensation trimmer is adjusted for the required response - a square wave.
As can be seen, the adjustment is quite obvious and it is quick and easy to undertake. It should be done each time the probe is moved from one input to another, or one scope to another. It does not hurt to check it from time to time, even if it remains on the same input. As in most laboratories, things get borrowed and a different probe may be returned, etc . .
Scope probe compensation: basic theory
The aim of a X10 scope probe is to present a ten fold increase in impedance to circuit under test. To achieve this a 9 MΩ resistor is placed at the tip of the probe and this forms a 10:1 potential divider with the internal 1MΩ of the oscilloscope (assuming the high impedance input is used as some scopes also have a 50Ω input for RF measurements).
Apart from the 1MΩ the scope also has a level of capacitance in parallel with this. It may be of the order of 15 to 30pF or so and it is normally quoted on the scope itself by the input connector.
If the probe did not accommodate this, then the bandwidth of the probe would be very low. The internal capacitance would work with the 9MΩ resistor to form a low pass filter.
To overcome this, a capacitor is placed across the 9MΩ resistor to create a capacitive potential divider. When the two components have the same division ratio, the response of the probe will be flat.
The circuit for a basic probe with compensation is shown above, and below it the circuit. It should be stressed that this is a first order representation as there are many small spurious levels of capacitance, resistance and inductance.
For the circuit to provide a flat response the potential diver effect of resistors and capacitors must be the same.
For this to be true, the following equation must hold:
Note that Ct is on the top of the equation and the 9MΩ is on the bottom. This “inversion” is because the capacitive reactance is equal to 1 / 2 π f C, i.e. it is inversely proportional to the capacitance.
As the value of the input capacitance, Cin varies from one type of scope to another, and there are small variations from one scope to another of the same type, it is necessary to adjust the balance in a process called compensation.
To achieve the scope probe compensation either Ct at the tip of the probe, or Cp is made variable. On some scope probes the compensation adjuster is at the tip and others it is in a small box by the connector that mates with the oscilloscope.
It is also surprising to note the frequencies at which compensation starts to be required. With a 9MΩ resistor in the tip, the 25pF or so capacitance starts to be noticed at a very low frequency.
The reactance of just 15pF is 1MΩ at 10 kHz, so it can be seen, that even at very low frequencies, the compensation of the scope probe is very important.
Types of scope probe compensation
The basic compensation described above is that which is used for most standard scope probes. But more advanced probes can have two types of compensation to ensure that their performance is optimum over as wide a bandwidth as possible:
- LF probe compensation: This is the type of scope probe compensation that is seen on all X10 scope probes. It compensates the probe for comparatively low frequencies, but gives a good level of compensation. Typically the circuit shown for the compensation is like that below.
The adjustable capacitor may be located in the connector area of the probe, i.e. where it is connected to the scope. Often, though it is located in the tip where the capacitor in the tip is the one that is adjusted. It typically does not matter which one is adjusted.
Lower cost scope probes may only have the LF compensation added. This is quite acceptable where very high frequency performance is not required.
- HF probe compensation: This type of compensation is used in many scope probes. Occasionally two adjusters may be provided, but often it is set by the design, or during manufacture. It tends not to vary much when the probe is connected to different scopes and therefore does not need to be adjusted in the same way.
There are two main variable factors that affect the high-frequency response of the probe:
- Probe cable impedance: The scope probe cable needs to be accounted for and this affects the HF performance. There is typically a small amount of capacitance at the tip of the probe. This may be 2 - 5pF, and also the cable itself introduces capacitance. This may be of the order of 40 to 50 pF.
- Scope input impedance: The scope input is not usually a perfect resistive and capacitive input. There is also some series inductance as well as some nonlinearity. Even chip capacitors which have very good levels of performance to frequencies of 1 GHz and more have some inductance. This gives rise to a self resonance that gives a dip in the series impedance.
The input arrangement for high-frequency oscilloscope scopes consists of a 1 MΩ resistance to ground. On top of this there are some stray capacitances and inductances. Each of these has its own series and parallel inductive and capacitive components which have a nonlinear characteristic at frequencies of a few hundred MHz and more and this adds an extra layer of complexity to the response of the input.
To compensate for these nonlinearities, HF probes tend to shunt the input of a scope with a very small capacitor and a series resistor right at the BNC. This serves to move any nonlinearity into a higher frequency region, outside the intended range of the probe, without causing severe overshoot.
To compensate the scope probe for the high frequency operation, the LF compensation is normally located at the tip as shown. HF compensation is normally provided by components located at the connector end of the probe. Typically there is a small screened box located virtually on the connector.
The HF compensation consists of a series resistor capacitor network. There may be two of these series RC networks to give compensation over the whole required band - possibly one for mid band and another for higher frequencies. These are adjusted either in design, production, or occasionally by the user before usage.
Using this compensation it is possible to ensure that the fastest edges of square waves and the like are reproduced as faithfully as possible.
Scope probe compensation range
One point to note when selecting a scope probe is to ensure that it has sufficient compensation range for the scope with which it will be used. Issues can occur when a high bandwidth probe is used with a low bandwidth scope.
The reason is here: typically a low bandwidth scope may have a high input capacitance. The standard input resistance for the high impedance input of a scope is 1MΩ but the capacitance may vary. It may be 15pF or it may be 25pF, etc . . Typically low bandwidth scopes tend to have a higher input capacitance.
When selecting a scope probe, check the compensation range. This is the range of input capacitance levels for which the probe can apply the correct compensation. High bandwidth probes tend to only compensate for lower input capacitance levels - as this is what a high bandwidth scope may have.
It is no use trying to use a scope probe with a compensation range of 8 - 18pF for example with a scope that has an input capacitance of 25 pF.
Accordingly it is imperative to ensure the probe can compensate the input capacitance for the scope with which it will be used.
As the performance of the scope is only as good as the scope probe, it is necessary to ensure that the good probes are used, typically the bandwidth of the probe should be around 1.5 times that of the scope, and then it should be correctly compensated. In this way the scope performance can be maximised and the waveforms on the circuit under test can be maximised.
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