Input Attenuator in CRO

Input Attenuator in Cathode Ray Oscilloscope

The input attenuator consists of number of RC potential dividers controlled on the CRO front panel. This control is done by the VOLTS/DIV selector. This selector will be calibrated in terms of the deflection factor (V/div). The sequence of attenuation commonly used with the CROs is 1-2-5. For example the range of the attenuator setting can be 0.1. 02, 0.5, 1, 2, 5, 10, 20 and 50 volts per division with a maximum attenuation of 50 v/div setting. There are attenuators that have 12 settings.

To ensure linear operation of the CRO over its specified frequency range the attenuator must be designed to work independently for any frequency uneffected by the frequency of the input signal. To obtain independent operation of the attenuator compensation technique is used. The input attenuator is shown in Figure along with the amplifiers input circuit. From the figure showing only two positions of the attenuator, it can be seen that in the first position of the attenuator the input signal is directly connected to the input of the vertical amplifier. The input signals appears directly without attenuation. In this position it corresponds to the minimum attenuation and from the above example will be 0.1 V/div setting. This discloses the maximum sensitivity of the vertical deflecting system. In the second position of the switch it can be observed that the resistor R₁, C_v, the resistor R, and capacitor C form a voltage divider consisting of R₁ C_v R_i C_i parallel combination in them. The magnitude of the input voltage now will be depending on the values of resistances R₁ and R_i. The input to the amplifier is given by

V_a = (R_i/R₁ + R_i)V_s

Input Attenuator in CRO

It can also be seen that two capacitors and the two resistors form a bridge as shown in Figure. At balance the product of the resistance and capacitance of R_iC_i and R₁. C_v will he equal. Also at balance there will be no current in branch PQ. Therefore the PQ connection can be omitted from the circuit. Thus the voltage at the amplifiers input is v_a as given above. As no reactive terms are there in the above expression for the input voltage to the amplifier, the voltage is independent of frequency of the signal. However this is valid only if the bridge is balanced. In other words the attenuator works as a compensated attenuator only when the balance condition is satisfied.

To balance the bridge and hence to compensate the attenuator the following procedure is employed. A square wave test signal will be applied to the attenuator input. The waveform obtained on the screen will be continuously observed adjusting the value of C_v. The value of the capacitor will be adjusted until the true waveform of the applied signal is observed on the CRO. If too large a value of C_v is offered over compensation results giving a waveform with over shoot. Too small a value will give under compensation rounding off the corners of the waveform observed. These two effects are noticed when high frequency signal is observed. With overdamping a pure sine wave appears with enhanced amplitude of positive half cycle. Under underdamped conditions the sine wave appears nearer to a rectified voltage waveform with inversion. Therefore the attenuator must be correctly compensated to prevent distortion of the waveform. For the different attenuator settings different RC combinations with the value of C adjusted for proper compensation are used.

The selection of the resistances and capacitances of the attenuator are such that the CRO vertical input always presents the same impedance to the circuit under consideration irrespective of the V/div setting. Cathode ray oscilloscopes have input parameters typically of 1 MΩ shunted by capacitance ranging from 20 pF to 35 pF.