What do I need an active probe for? To answer this question one first has to look at the properties of a passive the probe. The passive probe consists of a resistor and a (trim) capacitor for balancing. The oscilloscope is composed of an input impedance of one megohm and a capacity of 20pF.
The usable frequency range of the passive 10:1 probe is up to approx. 10 MHz. It is still possible to measure at a frequency of 100MHz, the signal, however, has collapsed by a factor of 10. The limited frequency bandwidth can be bypassed by an active probe:
An active probe is based on a current supplied amplification electronics. As a rule of thumb, an active probe is based on a high speed operational amplifier. The following illustration shows a simple model of a probe. (simulation with LTSpice <http://www.linear.com/designtools/software/>`__). Download the schematics here .
The central element is the operational amplifier, which is controlled by a voltage divider coupled back to the negative input. At the output of the OP-Amp a transmission line with 50 Ohm impedance and 50ns phase delay is modeled. The oscilloscope is again modelled by a one megohm resistor and a 20pF capacitance. Here the oscilloscope is equipped with a 50 Ohm termination for measuring higher frequencies. The Megaohm resistor plays no role through the parallel-connected 50 Ohm capacity. Such termination can be either be done internally by the oscilloscope or externally by an adapter. If the oscilloscope is not terminated appropriately, strong reflections may occur which lead to a disturbed frequency response. The impedance of all components must be matched, otherwise the waves will be reflected at impedance discontinuities and run back on the line. On the input side a voltage source with 1 V amplitude and 50 Ohm series resistance to the positive input of the operational amplifier is coupled in.
The frequency response of the circuit starts to drop off slightly at approx. 40 MHz. However, the attenuation level is not high due to the operational amplifiers. The output level lies at over 20V, so the signal is amplified. At 100 MHz the signal dropps to under 1 V. With this simple circuit one can measure up to several hundred MHz. The active probe enables a higher frequency bandwidth and high output levels. With an appropriate circuit design, the characteristic curve can be further improved. Commercial probes can be used up to the GHz range.