Specifications

Probes

Probes are the most common methods for connecting the oscilloscope to the circuit of interest. There

are two basic types of probes available,active and passive. An active probe contains active circuitry

(i.e.., semiconductors and perhaps an external power supply). These probes can give the highest

performance, but can be substantially more expensive than passive probes. By far the most commonly

used probes are passive probes, which contain no active circuitry and use only passive components in

their construction. The majority of the following material will focus on passive probes.

The circuits that need to be probed have a variety of effective input impedances. The best strategy for

the scope and probe is to appear as a high impedance because this minimizes the loading of the

circuit. While this is easy to say, it may not be easy to do in a high-frequency circuit.

A user’s first pass at a probe design would be to connect a bare wire to the circuit to be tested from the

scope's input. This works fine for DC and low frequencies (say, audio), but suffers from the problem of

picking up substantial noise. Next, the user would think to use a shielded coaxial cable, as this can cut

down on picked-up noise, but the coaxial cable introduces a new problem - high distributed

capacitance. This distributed capacitance can be significant with respect to the scope's input

capacitance. The popular RG-58 coaxial cable has a distributed capacitance of about 80 to 100 pF per

meter. This distributed cable capacitance leads to signal degradation as the frequency is increased

because the capacitance shunts the scope's input resistance (see the right-hand side of Figure 36

below).

Probe designers use coaxial cables that have substantially smaller distributed capacitance. An

additional method of reducing this degradation is to insert a resistor in series with the probe cable. This

offsets the degradation due to the cable capacitance, but at the cost of a reduced signal at the scope.

Below is a schematic of a typical 10X probe. Note the distributed capacitance of the cable C

is in

parallel with the scope's input capacitance C

Figure 36

The resistor R

is typically 9 MΩ. At DC, this means the input to the scope is through a voltage divider

made up of R

and R

. At DC, the capacitors are effectively open circuits. Oscilloscope manufacturers

have standardized on using 1 MΩ input resistances for their scopes. This standardization is important

because it allows other manufacturer's probes that are designed for 1 MΩ inputs to be used, giving the

consumer more choices. The scope's input also has a capacitance of roughly 10 to 30 pF, depending

on the scope. Since R

is 1 MΩ, the total input resistance is 10 MΩ and the voltage divider ratio is:

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