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Attenuators and Amplifiers

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Lord Loh.

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In the block diagram of most instruments shown in my text books, I see an attenuator followed by an amplifier.

What is the need for this? Why not provide a lower gain amplifier without a preceding attenuator or a lower attenuation attenuator without a succeeding amplifier?

I have not found any satisfactory explanation for this anywhere.

Thanks in advance.
 

Lord Loh.

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Many equipments... Oscilloscopes and wave analysers to begin with...

Yes both are variable in oscilloscopes and there was no mention of variablity in wave analysers.
 

Borber

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Oscilloscopes or wave analyzers are universal instruments. They are build for certain maximal sensitivity, for example 5mV/div and it has 8 div for vertical display. If you want to observe signal with 5Vpp amplitude you need attenuator in front of such instrument to be able to see the signal. Without the attenuator you will be able to observe only signals which has amplitude smaller than 40mVpp.
 

Lord Loh.

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Does that sensitivity not depend on the accelration of the electrons in the CRT.

I was never able to figure out the approximate value that is needed to be fed into the CRT to create a unit deflection. is it in the range of mv or does it require several volts...
 

xxargs

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Lord Loh. said:
Does that sensitivity not depend on the accelration of the electrons in the CRT.

I was never able to figure out the approximate value that is needed to be fed into the CRT to create a unit deflection. is it in the range of mv or does it require several volts...

it's range of couple of volt to deflect in tube (guess 10-100 volt), also need high gain amplifier if you will observe 5 mV...


Mostly RF-instrument have attenuator on input to protect sensivity reciver (listen around -100 - -120 dBm (0.1 - 3 uV)), reciver itself have dynamic range around 80 - 100 dB and with internal and externa attenuator you can incrace measure range from listen so low at -120 dBm to measure heavy transmitter in range of sereval watt to kiloWatt - but still only measure 70 -100 dB lower than incoming maximum power.


Intruments using attenuator to 'moving' measure window to fit incoming signal strength for best possible view or measure and without burning input reciver/amplifier or make intermodulation product (clipping in hard overdrive situation) ruins measure.


Using attenuator right is very importent know how for skilled measure engineer, and every engineer working couple of year, have burn least 1-2 attenuator and least one reciver input (at > 4000 € in repair cost) or, is not can called for experienced.... or if burn at > 7 attenuator and 2 reciver input in few years without good reason, is not can calling for skilled measure engineer....

/xxargs
 

E-design

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It is often good Engineering practice (for stability) to have amplifier in/outputs look into stable well defined impedances that does not change (attenuation may change, but impedance stay the same) with operating or bias conditions. Also by designing these to be 50 or 75 ohm makes it convenient to use standard RF test equipment such as VNA, spectrum analyzer etc.
 

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