Probes

Open wire test leads (flying leads) are likely to pick up interference, so they are not suitable for low level signals. Furthermore, the leads have a high inductance, so they are not suitable for high frequencies. Hence the input signals to an oscilloscope are usually connected via coaxial cables with probes on one end [see Fig. 1]. These are normally just convenient-to-use insulated connecting clips.  As illustrated, each probe has two connections, an input and a ground.  This type of probe is usually referred to as a 1:1 (one-to-one) probe, (Fig. 10) because it does not contain resistors to attenuate the input signal.

Using a shielded cable (i.e., coaxial cable) is better for low level signals.  Coaxial cable also has lower inductance, but it has higher capacitance: a typical 50 ohm cable has about 90 pF per meter. Consequently, a one meter direct (1X) coaxial probe will load a circuit with a capacitance of about 110 pF and a resistance of 1 mega-ohm.  The coaxial cable consists of an insulated central conductor surrounde by a braided circular conductor which is covered by an outer layer of insulation as shown in Fig. 11. The central conductor carries the input signal, and the circular conductor is grounded so that it acts as a screen to help prevent unwanted signals being picked up by the oscilloscope input.  

The coaxial cable connecting the probe to the oscilloscope has a capacitance
(Ccc) which can overload a high-frequency signal source.The input impedance of
the oscilloscope at the front panel is typically 1 M
Ω
 in parallel with 30pF. The
coaxial cable can add another 100 pF to the total input capacitance. The circuit of
a signal source, probe, and oscilloscope input is illustrated in Figure 12.   

The total impedance offered by the coaxial cable and the oscilloscope input should always be much larger than the signal source impedance. Where this is not the case, the signal is attenuated and phase shifted when connected to the oscilloscope. At frequencies where the reactance of (Ccc+ Ci) is very much larger than Rs and Ri, the capacitances have a negligible effect and the oscilloscope terminal voltage is Vi = Vs * Ri / (Ri + Rs). To minimize loading, attenuator probes (e.g., 10X probes) are used. 

Active Probes

  Active probes contain electronic amplifiers that increase the probe input resistance and minimize its input capacitance. Typical active probes use FET input stages, or FET input operational amplifiers.  The circuit is connected to function as a voltage follower. The amplifier has a gain of 1 and a typical input impedance
of 1MΩ||3.5 pF. Input impedances of 10 MΩ or greater are also possible with FET input stages, and the input capacitance effect can be further reduced by resistive attenuation.  Power must be supplied to operate the amplifier.  This may be derived from the oscilloscope, or the probe may contain its own regulated power supply with a line cord. Inside the probe, a coil wound around the core provides a current into an appropriate load, and the voltage across that load is proportional to current. However, this type of probe can sense AC, only. 

Current probes

They provide a method of inductively coupling the signal to the CRO input, so that a direct electrical connection to the test circuit is not required.  The current probe consists of a sensor, a coax cable & a termination circuit as shown in Fig. 16.

SPLIT-CORE
Passive current probe, the most popular type, can be opened & clipped around a conductor (see Fig. 17) whose current is to be measured. The current sensing device of this probe is a “Current Transformer” of split core design, consisting a stationary U-piece & a Movable flat piece. A multi –turn coil of approximately 25
turns is wound on one leg of the ferrite core to form the secondary turn primary. The input signal to the probe is the current in the conductor under test; the o/p signal is the voltage developed across the transformer secondary.   This current probe senses only the changes in current & hence can be used only to measure A C Signal. When correctly terminated, the sensitivity of this probe is of the order of 10ma/mv. The transformer o/p voltage is  coupled from the probe head to the termination via a coaxial cable.  The termination circuitry
can be passive or active, depending on the kind of probe generally the termination of the coax is in its characteristics impedance. Additional circuitry to improve the response characteristics of the probe is also contained in the termination box. 

A more-sophisticated probe (originally made by Tektronix) includes a magnetic flux sensor (Hall effect sensor) in the magnetic circuit.  The probe connects to an amplifier, which feeds (low frequency) current into the coil to cancel the sensed field; the magnitude of that current provides the low-frequency art of the current waveform, right down to DC.  The coil still picks up high frequencies. 

 Attenuator probes

Attenuator probes attenuate the input signal, usually by a factor of 10.  They also normally offer a much larger input impedance than a 1:1 probe, thereby minimizing loading effects on the circuit under test. Compensation is included for oscilloscope input capacitance and coaxial cable capacitance. Because of the 10-fold attenuation, these probes are usually referred to as 10:1 probes; however, other probes are available with different attenuation factors. A typical probe uses a 9 megaohm series resistor shunted by a low-value capacitor to make an RC compensated divider with the cable capacitance and scope input. The RC time
constants are adjusted to match. 
 For example, the 9 megaohm series resistor is shunted by a 12.2 pF capacitor for a time constant of 110 microseconds. The cable capacitance of 90 pF in parallel with the scope input of 20 pF and 1 megohm (total capacitance 110 pF) also gives a time constant of 110 microseconds.   

In practice, there will be an adjustment so the operator can precisely match the low frequency time constant (called compensating the probe) as shown in Fig.13. Matching the time constants makes the attenuation independent of frequency. At low frequencies (where the resistance of R is much less than the reactance of C), the circuit looks like a resistive divider; at high frequencies (resistance much greater than reactance), the circuit looks like a capacitive divider.  The input capacitance and input resistance vary from one oscilloscope to another even for otherwise identical instruments. It is important that every probe be correctly adjusted when it is first connected for use with a particular oscilloscope. The results of the different compensations are shown in Fig.14 .

Another 10:1 probe and its equivalent circuit are shown in Figure 15. In this case capacitor C1 is a fixed quantity, and an additional variable capacitor ( C3) is included in parallel with Ci and Ccc.