Sample Problem about Noise

1. The noise output of a resistor is amplified by a noiseless amplifier having a gain of 60 and a bandwidth of 20 kHz. A meter connected to the output of the amplifier reads 1 nVrms (a.) The bandwidth of the amplifier is reduced to 5 kHz, its gain remaining constant. What does the meter read now?(b.) If the resistor is operated at 80 degrees C, what is its resistance?

2. A parallel-tuned circuit having a Q of 20, resonated to 200 MHz with a 10-pF capacitor. If this circuit is maintained at 17 degree C, what noise voltage will a wideband voltmeter measure when placed across it?

3. The RF amplifier of a receiver has an input resistance of 1000 ohms, and equivalent shot-noise resistance of 2000 ohms, a gain of 25, and a load resistance of 125 kilo-ohms. Given that the bandwidth is 1 MHz and the temperature is 20 degree C, calculate the equivalent noise voltage at the input to this RF amplifier. If this receiver is connected to an antenna with an impedance of 75 ohms, calculate the noise figure.

4. A 460 MHz communication receiver has a NF of 2 dB. A 100-meter length of coaxial cable transmission line connects the receiver to an antenna. The loss of the transmission line at 460 MHz is measured at 6 dB. The temperature of the transmission line is 20 degree C. Calculate the over-all noise figure of this communication system.

5. A standard telephone channel having a frequency bandwidth of 300-3400 Hz can carry 30,901 bps information when S/N is 30 dB. (a.) By how much will the channel capacity increase when the signal power is doubled (noise held constant)? (b.)What can be concluded from the result?