CRO Probes

CRO Probes

We can connect any test circuit to an oscilloscope through a probe. As CRO is a basic oscilloscope, the probe which is connected to it is also called CRO probe.

We should select the probe in such a way that it should not create any loading issues with the test circuit. So that we can analyze the test circuit with the signals properly on CRO screen.

CRO probes should have the following characteristics.

  • High impedance
  • High bandwidth

The block diagram of CRO probe is shown in below figure.

CRO Three Blocks

As shown in the figure, CRO probe mainly consists of three blocks. Those are probe head, co-axial cable and termination circuit. Co-axial cable simply connects the probe head and termination circuit.

Types of CRO Probes

CRO probes can be classified into the following two types.

  • Passive Probes
  • Active Probes

Now, let us discuss about these two types of probes one by one.

Passive Probes

If the probe head consists of passive elements, then it is called passive probe. The circuit diagram of passive probe is shown in below figure.

Passive Probes

As shown in the figure, the probe head consists of a parallel combination of resistor, R1R1 and a variable capacitor, C1C1. Similarly, the termination circuit consists of a parallel combination of resistor, R2R2 and capacitor, C2C2.

The above circuit diagram is modified in the form of bridge circuit and it is shown in below figure.

Bridge Circuit

We can balance the bridge, by adjusting the value of variable capacitor, c1c1. We will discuss the concept of bridges in the following chapters. For the time being, consider the following balancing condition of AC bridge.

Z1Z4=Z2Z3Z1Z4=Z2Z3

Substitute, the impedances Z1,Z2,Z3Z1,Z2,Z3 and Z4Z4 as R1,1jωC1,R2R1,1jωC1,R2 and 1jωC21jωC2 respectively in above equation.

R1(1jωC2)=(1jωC1)R2R1(1jωC2)=(1jωC1)R2

R1C1=R2C2⇒R1C1=R2C2Equation 1

By voltage division principle, we will get the voltage across resistor, R2R2 as

V0=Vi(R2R1+R2)V0=Vi(R2R1+R2)

attenuation factor is the ratio of input voltage, ViVi and output voltage, V0V0. So, from above equation we will get the attenuation factor, αα as

α=ViV0=R1+R2R2α=ViV0=R1+R2R2

α=1+R1R2⇒α=1+R1R2

α1=R1R2⇒α−1=R1R2

R1=(α1)R2⇒R1=(α−1)R2Equation 2

From Equation 2, we can conclude that the value of R1R1 is greater than or equal to the value of 2 for integer values ofα>1α>1.

Substitute Equation 2 in Equation 1.

(α1)R2C1=R2C2(α−1)R2C1=R2C2

(α1)C1=C2⇒(α−1)C1=C2

C1=C2(α1)⇒C1=C2(α−1)Equation 3

From Equation 3, we can conclude that the value of C1C1 is less than or equal to the value of C2C2 for integer values of α>1α>1

Example

Let us find the values of R1R1 and C1C1 of a probe having an attenuation factor,αα as 10. Assume, R2=1MΩR2=1MΩ and C2=18pFC2=18pF.

  • Step1 − We will get the value of R1R1 by substituting the values of αα and R2R2 in Equation 2.
R1=(101)×1×106R1=(10−1)×1×106
R1=9×106⇒R1=9×106
R1=9MΩ⇒R1=9MΩ

Step 2 − We will get the value of C1C1 by substituting the values of αα and C2C2 in Equation 3.

C1=18×1012(101)C1=18×10−12(10−1)
C1=2×1012⇒C1=2×10−12
C1=2pF⇒C1=2pF

Therefore, the values of R1R1 and C1C1 of a probe will be 9MΩ9MΩ and 2pF2pF respectively for the given specifications.

Active Probes

If the probe head consists of active electronic components, then it is called active probe. The block diagram of active probe is shown in below figure.

Active Probe

As shown in the figure, the probe head consists of a FET source follower in cascade with BJT emitter follower. The FET source follower provides high input impedance and low output impedance. Whereas, the purpose of BJT emitter follower is that it avoids or eliminates the impedance mismatching.

The other two parts, such as co-axial cable and termination circuit remain same in both active and passive probes.