cable loss and output power

Discussion in 'Homework Help' started by GirishC, Jan 20, 2010.

  1. GirishC

    Thread Starter Active Member

    Jan 23, 2009
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    0
    This is a problem from Paul Claytons's Introduction to Electromagnetics.

    My question is why the cable gain or loss which is mentioned in dB gets added to input power which is dBm.

    Do you need to convert cable loss 4.5dB/100ft to dBm/100ft and then subtract from the input power to calculate the output power.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    The cable gain term is effectively a multiplier which translates to an addition when using logarithms. So no - you don't need to convert this multiplier to dBm.

    Consider the situation without using logarithms.

    The cable gain (really a loss) is -6.75dB which is an equivalent multiplying factor of 0.21135 (less than unity is a loss)

    The cable input power is -30dBm which is 1uW.

    The cable output power is 0.21135*1uW = .21135uW which translates to -36.75dBm.
     
  3. GirishC

    Thread Starter Active Member

    Jan 23, 2009
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    0
    Based on your reply I have following questions:

    1. Do you mean even if cable loss (dB/ft) can be added directly to power either dBuV or dBmV or dB?

    2. in general, if I have amplifier block with some gain lets say 32dB it can be straight away subtracted from output reading to get input reading, is it right?
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    782
    Q1. Yes - however unless the input and output signals share a common impedance base the voltage and power gains differ.

    Pgain(dB)=10log(Pout/Pin)
    Vgain(dB)=20log(Vout/Vin)

    Q2. Yes


    Consider an amplifier with fixed Voltage Gain Av=100 fed by an ideal source voltage of Vin=1mV. Say the amp input resistance is 1kΩ and output resistance is zero. The load impedance connected to the output is 100Ω.

    Av(dB)=20log(100)=40dB
    Vin=1mV=20Log(1000) dBuV=60 dBuV

    Vout(dBuV)=40+60=100 dBuV=10^5 uV = 100mV

    Pin=1nW=-60dBm
    Pout=100uW=-10dBm

    Pgain(dB)=-10-(-60)=50dB
     
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