Physics and Engineering of Radiation Detection

746 Appendix-A A. Essential Electronic Measuring Devices

A typical passive probe is shown in Fig.A.2.3. This is the so called high impedance
compensated probe. Due to its high dynamic range, it can be used in a variety of
circuits. In fact, due to its versatility, most oscilloscope manufactures include this
in the standard oscilloscope package. The most problematic thing about this probe
is its very low bandwidth, which does not allow it to be used for highly sensitive
measurements. As shown in Fig.A.2.3, this probe has a variable capacitor. This
allows the user to match the capacitance to the input capacitance of the oscilloscope.
The compensation unit of the probe is provided in the termination box, which is
usually at the other end of the probe, that is, at the oscilloscope side. The input
impedance of the probe is very high, on the order of a few mega ohms. However
probes having input impedances of about 1MΩ are also not uncommon. For typical
probes the input resistance is chosen such that the probe provides a 10:1 attenuation.
Probes with other attenuation factors are also available.

Cable

Tip To Scope

Termination Box

Figure A.2.3: A shigh impedance compensative passive probe.

The probe shown in Fig.A.2.3 does not have high bandwidth. This shortcoming
does not allow it to be used in highly sensitive measurements, specially when the
signal has a high or wide bandwidth. A much simpler probe, as the one shown
in Fig.A.2.4 can solve this problem. Such a probe consists of a resistor in series
with a 50 Ω signal carrying cable. Typically the value of the resistor is chosen such
that it ensures a signal attenuation of 10:1. However higher attenuation probes
are also available. This kind of probe is good for probing low level voltage signals
specially when timing information is important. Such probes offer excellent time
measurements due to their very wide bandwidths.

Cable (50 )

Resistor (usually < 1k )Ω

Tip To Scope

Ω

Figure A.2.4: A simple low impedance passive divider probe.

The two probes we just discussed are not the only passive probes available in the
market. There is a variety of probes available and the user should carefully chose
one according to the requirements.

C.2 Active Probes

A simple active probe is shown in Fig.A.2.5. As the name suggests this probe has an
active electronic component, which is usually an operational amplifier. Such a probe
offers excellent resistive and capacitive loading together with wide bandwidth. The