ATTENUATORS

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  ATTENUATORS:

The attenuators are designed to change the magnitude of the input signal as seen at input stage, while introducing constant impedance on all ranges of the attenuator input.

  RC attenuator(compensated) :- 

It is required to attenuate all frequencies equal without as this compensation and  signal of HF measurements would always have to take the input circuit RC time constant into account of that.

The input attenuator must provide the correct 1 -2-5 sequence while maintaining  constant

input impedance, as well as maintain both the input impedance a attenuation above the frequency range for which the oscilloscope is designed.

Attenuators (Uncompensated ) :- as found in Fig. ( circuit diagram )it gives a resistive divider attenuator connected to an amplifier with a 10 pf input capacitance. If the input impedance of the amplifier is high, the input impedance of the attenuator is relatively constant negligible of the switch setting of the attenuator.

The input impedance, as found by the amplifier, variations or changes greatly depending on the setting of the attenuator. Because of this, the RC time constant and frequency response of the amplifier are based mostly on the setting of the attenuator, which an unfavorable feature.

Basic and Simple Compensated Attenuator :-

The capacitive voltage dividers enhance the HF response of the attenuator. This combination of capacitive and combination of resistive voltage dividers is known as a compensated attenuator. For oscilloscopes where the frequency range increases to 100 MHz and above, more complex dividers are used.

indicates an attenuator divider between the input and output of the vertical deflection preamplifier.

The input attenuator gives switching powers of 10, while attenuator at the output

of the vertical preamplifier provides 1-2-5 attenuation.

Almost all oscilloscopes provide a switchable input coupling capacitor, as shown in Fig. 

 The input impedance of an oscilloscope is 1 MΩ which is shunted with an input capacitance of 10-30 pf. If a probe were connected to the oscilloscope, the input impedance at the probe tip would have a greater capacitance due to the fact of the included capacitance of the probe assembly and of the connecting shielded cable. If it is desired for HF oscilloscopes to have an input capacitance of much less than 20-30 pf, an attenuator Figure 8.1  probe is used ,as shows in fig 8.1a 10 to 1 attenuator probe connected to the input of the oscilloscope.

Within the probe tip is a 9MΩ resistor and shunted across this resistor is a

capacitor .This capacitor is fine-tuned( adjusted) so that the ratio of the shunt capacitance to the series capacitance is exactly 10 to 1.

The attenuator probe generally called a 10 to 1 probe provides an approximately 10 to

1 reduction in the input capacitance. On the other hand, it is also gives a 10 to 1 reduction in overall oscilloscope sensitivity.

The input capacitance is not constant from one oscilloscope to another as a result the

probe is provided with an adjustable compensating capacitor .If the ratio of the series to shunt is not adjusted specifically(precisely) to 10 to 1,and the frequency response of the oscilloscope will be flat.

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