Different Types of CRO Probes

Probes for CRO: 


CRO probe performs the important function of connecting the circuit under study to the vertical input terminals of the CRO. It helps in not loading the circuit under test. For the different types of measurements using different types of CRO probes are available. They are:

1. Passive probes.
2. Active probes.
3. Current probes.
4. High voltage probes.

(1) Passive Probes:


The passive probe is made of a length of coaxial cable with a tip at one end and a BNC connector at the other. Though the probe directly connects the tip to the CRO, the shunt capacitance of the cable is to be considered. Approximately the 50 Ω cable has a capacitance of 30 pF/0.3 m length. As this would mean a 150 pF capacitance of length of 1.5m of cable, its impedance at high frequencies will be low. Hence such a probe is useful at low frequencies only.

Compensated passive probes are used that have an attenuation of 10 to 1 over greater frequency range. Such a probe consists of an attenuating resistor shunted by a small variable capacitor of small value. 3.5



(2) Active Probes:


Active probes use FETs or BJTs. Mineature vacuum tubes were used earlier. The active probe consisting of FET is shown in block diagram below.
Active Probe using FET
The probe contains three parts. The probe head, the coaxial cable and the termination. The probe head contains the FET in a source follower form. It is followed by an emitter follower stage. This emitter follower drives the coaxial cable which has an impedance of 50 Ω and is terminated in 50 Ω active device which offers the characteristic impedance to it. Input impedance of FET circuit is 10 MΩ shunted by 5 pF capacitance.

(3) Current Probes:


This probe presents an inductive coupling of the signal to the CRO. The current probe consists of a sensor, a coaxial cable and a termination circuit.

The split core current probe arrangement is shown below in Fig.
Split core current probe
The split core passive probe can be clipped around a conductor whose current is to be measured. The current transformer is the sensing device in this probe. It consists of a stationary U-piece and a movable flat piece. A coil of several turns (around 25 turns) is wound on the leg of the ferrite core that forms the secondary of the transformer. The single turn primary is the conductor under test. Due to the flow of current through the conductor under test a voltage will be produced at the secondary of the current transformer. This output is coupled through a coaxial cable to the input of the CRO, through the termination. The termination circuit can be passive or active that provides the characteristic impedance of the cable. The current probe can sense changes in current only. Hence it is useful only to measure a.c signals. The probe sensitivity may be around 10 mA/mV.

(4) High voltage probes:


This is used to apply high voltage of the order of kilo volts to CRO. A voltage division of 1000 to 1 may be used. The probe head is made of high impact strength thermoplastic material and is of special design to protect the user from shock.

It consists of a 100 Ω resistor in the probe head that is 10 cm long. It has distributed capacitance as shown in figure. This is connected to the termination box through a special probe cable. The attenuation ratio is obtained by adjusting resistor R5, in series with R4 = 100 kΩ, and with CRO input resistance of 1 MΩ as shown. The probe compensation is done by adjusting the network consisting of R1 R2 C1 C2 and C3. The probe cable terminates in its characteristic impedance by resistors R3 and R6.

Above 100 kHz the shunting capacitance of the circuit is noticeable. At high temperature also the high voltage probe is not efficient.

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