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TV antenna amplifier noise calculation

  1. Nov 21, 2015 #1
    1. The problem statement, all variables and given/known data

    Hello everyone. A 4-way TV antenna amplifier has the following specifications:

    Bandwidth = 40-862Mhz
    Gain = 20dB
    Noise figure = 6 dB
    Max output = 85dBuV
    Input impedance = output impedance = 75 ohm

    Question 1: Calculate the required voltage on the input of the amplifier to achieve 85dBuV max output.

    Question 2: If the aerial signal level is 5dBmV and the input noise level is 20dBuV, determine SNR (signal-to-noise ratio) on the output of the amplifier.

    2. Relevant equations

    ##SNR = 20 log (\frac{V_{signal}}{V_{noise}})##

    3. The attempt at a solution

    Question 1:

    Max output = max input neglecting noise and distortion + amplifier gain
    85dBuV = max input + 20dB
    Max input = 65dbuV

    Input voltage = ##antilog (\frac{65}{20})## = 1.778mV

    Question 2:

    5dBmV = ##antilog (\frac{5}{20})## = 1.778mV
    20dBuV = ##antilog (\frac{20}{20})## = 0.01mV

    ##SNR_{in} = 20 log (\frac{1.778}{0.01})## = 45dB

    ##SNR_{out} = SNR_{in} - noise figure## = 45 - 6 = 39dB

    Am I thinking correctly? Thank you for your comments.
     
  2. jcsd
  3. Nov 21, 2015 #2

    marcusl

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    Gold Member

    1 is correct, 2 is not--you messed up an antilog There's no need to convert to volyage for #2. it's simpler to just work in dB straight through.
     
  4. Nov 21, 2015 #3
    Right, okay. In that case:

    5dBmV + 60 = 65dBuV

    ##SNR_{in dB} = 65 - 20 = 45dB##
    ##SNR_{out dB} = SNR_{in dB} - noise figure = 45 - 6 = 39dB##

    I get the same results. Where am I mistaken?
     
  5. Nov 21, 2015 #4

    marcusl

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    Science Advisor
    Gold Member

    Sorry, I wasn't looking carefully at the units. I agree with your calculation.
     
  6. Nov 22, 2015 #5
    Thank you.
     
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