Passive oscilloscope probes
The great majority of test scope probes used with
oscilloscopes are the passive variety. They enable a wide range of
measurements to be made, and cover most applications. In addition to this,
passive test probes are far cheaper than active ones as would be expected.
The most basic form of oscilloscope probe, or scope
probe, is what is often termed the 1X probe. It is so called because this
type of scope probe does not attenuate the incoming voltage as many other
probes do. It consists of a connector to interface to the oscilloscope
(generally a BNC connector), and a length of coax which is connected to the
probe itself. This comprises a mechanical clip arrangement so that the probe
can be attached to the appropriate test point, and an earth or ground clip
to be attached to the appropriate ground point on the circuit under test.
The 1X probes are suitable for many low frequency
applications. They typically offer the same input impedance of the
oscilloscope which is normally 1 M Ohm. However for applications where
better accuracy is needed and as frequencies start to rise, other test
probes are needed.
To enable better accuracy to be achieved higher levels of
impedance are required. To achieve this attenuators are built into the end
of the probe that connects with the circuit under test. The most common type
of probe with a built in attenuator gives an attenuation of ten, and it is
known as a 10X oscilloscope probe. The attenuation enables the impedance
presented to the circuit under test to be increased by a factor of ten, and
this enables more accurate measurements to be made.
As the 10X probe attenuates the signal by a factor of
ten, the signal entering the scope itself will be reduced. This has to be
taken into account. Some oscilloscopes automatically adjust the scales
according to the probe present, although not all are able to do this. It is
worth checking before making a reading.
10X oscilloscope probes also allow some compensation for
frequency variations present. A small variable capacitor enables adjustment
to be made. Most oscilloscopes have a small square wave oscillator output.
By attaching the oscilloscope probe to this a quick adjustment can be made.
As the square wave requires all the harmonics to be present in the correct
proportions to provide a "square" wave, the probe can be quickly adjusted
accordingly. If the leading edges of square wave, when viewed on the screen
has rounded corners, then the high frequency response of the probe is low
and an adjustment can be made. However if the leading edges have spikes and
rise too high, falling back to the required level, then the high frequency
response has been enhanced and this needs to be adjusted. Only when the
square wave is truly square is the frequency response correct.
Although they are not as common, 100X probes are also
available. These oscilloscope probes tend to be used where very low levels
of circuit loading are required, and where the high frequencies are present.
The difficulty using the is the fact that the signal is attenuated by a
factor of 100.
Active oscilloscope probes
Although 10X probes are widely used because of their
superior response, they are not able to provide all the performance that may
be needed for some applications. By using active electronic circuits in the
remote end of the oscilloscope probe it is possible to offer very high
levels of performance.
Active oscilloscope probes use specially developed
integrated circuits. By placing these chips right at the point at which the
signal is probed, it enables the signal to be preserved during its
transition from the point at which it is sampled to the input of the
oscilloscope, in some instances using differential techniques. In this way
the signal integrity it maintained, despite the fact that it may have a fast
rise time, may have a low signal level, or require a high input impedance at
the point at which it is sampled.
Although active probes are more expensive than their
passive cousins, they offer a better level of performance that may be
essential in some circumstances.
Summary
Oscilloscope probes are an essential addition to any
oscilloscope. In most case 10X passive scope probes may be used, but other
types of test probe need to be considered dependent upon the applications
envisaged.
