Oscilloscope active probe
Adapted from information provided by Dr. Michael Lauterbach, LeCroy, Chestnut Ridge, NY. -- Test & Measurement World, 8/1/2001
At its simplest, a scope probe provides a physical and an electrical connection between a device or circuit under test and an oscilloscope's input amplifier. These days, most engineers and technicians use active probes to connect high-bandwidth signals to oscilloscopes. Unlike a passive probe that can greatly load high-speed signals, an active probe provides a smaller load to the circuit undergoing testing, which means the probe's low inductance and capacitance disturbs the original signal only slightly.
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| Figure 1 |
Until a few years ago, active probes used a high-input-impedance field-effect transistor (FET) to buffer the signal at the probe tip from the oscilloscope's input circuits. Figure 1 shows a simplified FET buffer circuit, in which the FET acts as a source follower. The stage that follows the FET provides two complementary bipolar transistors wired as emitter followers. The FET provides a high-input resistance and a low capacitance to the circuit being probed, and the bipolar transistors produce a current gain sufficient to drive a 50-V load. The probe's circuit lets the scope provide an offset voltage to the FET's input through an offset input at R2. The circuit's output resistor, R3, terminates the output signal in a 50-V resistor and protects the output transistors in the event of an accidental short circuit.
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| Figure 2 |
Today, most active probes (Figure 2) use bipolar transistors in place of the FET. These transistors don't offer the same high impedance as an FET, but they operate faster. Thus, they work well at the higher bandwidths-up to several gigahertz-over which engineers must make measurements. A bipolar-transistor probe may load a circuit with only a 0.7-pF capacitance, and it has a high-input impedance-generally in the range of 100 kV, or greater. The low capacitance is particularly important because at high frequencies, input capacitance, not input resistance, loads the test circuit the most.
The skin-loss compensation circuit in Figure 2 adds some "peaking" to compensate for current that at high frequencies flows on the outside of a conductor. Skin-loss compensation isn't perfect, though, and compensation relates directly to cable length. Therefore, if you use an active probe with a spectrum analyzer, and you power the probe with an external power supply, you must keep the "adapter" between the cable connector and the instrument as short as possible.
The DC-correction circuitry adds the right amount of bias to the input to overcome the voltage drop in the bipolar transistors, thereby restoring the input signal to its original level. An active probe usually includes temperature compensation to correct for the transistor's voltage drop, which changes as the probe warms.
