Weekly exam 2

CERTC - RFReFresher Take-Home Exam -2

EST

Instruction: Submit your answer sheet for this take home exam next meeting.

1. Calculate the noise current for a diode with a bias current of 15mA, observed over a 25 kHz bandwidth.A. 11 mAC. 11 nA *B. 11 uAD. 11 pA

2. The signal-to-noise ratio is 30 dB at the input to an amplifier and 27.3 dB at the output. What is the noise figure of the amplifier?A. 2.7 dB *C. 57.3 dBB. -2.7 dBD. -57.3 dB

3. Suppose the noise power at the input to a receiver is 1nW in a bandwidth of interest. What would be the required signal power for a signal-to- noise ratio of 25dB? A. 316 nW *C. 216 uWB. 400 nWD. 500 nW

4. An amplifier consists of two stages. Stage one has a gain of 12dB and a noise figure of 2dB. Stage two has a gain of 20 dB and a noise figure of 5 dB. Calculate the equivalent noise temperature in kelvins for the amplifier.A. 151 KC. 209.09K *B. 350 KD. 75 K5. Which of the following measures cannot be used to reduce noise?A. by increasing the channel bandwidth *B. by reducing the signaling rateC. by increasing the transmitted powerD. by using redundancy

6. In the design of a receiver, the noise figure is determined primarily by theA. first stage *C. last stageB. all of the stagesD. none of these

7. A varactor-tuned oscillator uses an inductance of 25uH with a varactor having a maximum capacitance of 45pF in the frequency-determining circuit. Calculate the range of frequencies for this oscillator, with a tuning voltage that varies from 1V to 10V. A. 5.24 MHz 9.14 MHzC. 9.82 MHz 25.98 MHzB. 6.24 MHz 10.16 MHz *D. 4.18 MHz 12.7 MHz

8. A quartz watch is guaranteed accurate to 15 seconds per month has 30 days, calculate the accuracy of the crystal oscillator in the watch in parts per million.A. 5.79 ppm *C. 3.14 ppmB. 2.14 ppmD. 4.27 ppm

9. An AM signal observed on a spectrum analyzer shows a carrier at +12dBm, with each of the sidebands 8dB below the carrier. Calculate the modulation index.A. 0.23B. 0.98 C. 0.8 * D. 0.63

10. A transmitter uses a frequency multiplication circuit consisting of two triplers and three doublers to get a frequency of 540MHz from a crystal oscillator. What frequency should the crystal have?A. 38.88 GHzC. 45 MHzB. 7.5 MHz *D. 38.88 MHz

11. An AM transmitter with a carrier power of 20kW is connected to a 50ohms antenna. In order to design a lightning arrestor for the antenna, it is necessary to know the maximum instantaneous voltage that will appear at the antenna terminals. Calculate this voltage, assuming 100% modulation for the transmitter.A. 1kV B. 1.41 kV C. 2.82 kV * D. 3.5 kV

12. An FM signal has a deviation of 10kHz and is modulated by a sine wave with a frequency of 5kHz. The carrier frequency is 150 MHz, and the signal has a total power of 12.5W, operating into an impedance of 50 ohms. What is the modulation index?A. 1.45 B. 2 * C. 2.45D.4

13. An FM receiver operates with a S/N of 30 dB at its detector input and with mf=10. If the received signal has a voltage of 10mV, what is the amplitude of the noise voltage?A. 10uV B. 1mVC. 316 uV * D. 521 uV

14. The RF power amplifier stage of a transmitter has an output of 50kW and a gain of 15dB. How much power must be supplied to this stage by the previous stage?A. 31.6 KW B. 1.58 KW *C. 1.58 MW D. 3.16 KW

15. A transmitter uses twelve modules in its solid-state output stage. Calculate the power reduction (in decibels) that would occur if one module fails.A. 0.378 dB *B. 0.5 dBC. 0.83 dB D. 1 dB

16. A transmitter power amplifier has a gain of 30dB and an efficiency of 75%. Its output power is 100 watts. How much current does the amplifier draw from a 100 volt power supply?A. 1.457 AB. 1.333 A *C. 2.4 A D. 3.6 A

17. An SSB transmitter transmits on the USB with a (suppressed) carrier frequency of 7.2MHz. It is modulated by two tones, with frequencies of 1kHz and 2.5kHz and equal amplitude. The transmitter PEP is 75W into a 50 ohms load. The carrier and unwanted sideband are each suppressed by 60 dB. Calculate the peak voltage across the load and the average power into the load.A. 86.6 V and 37.5 W *C. 1.78 V and 19.7 WB. 100 V and 27.3 WD. 27.5 V and 25 W

18. A receivers IF filter has a shape factor of 2.5 and a bandwidth of 6kHz at the 6dB down point. What is its bandwidth at the 60dB down?A. 7kHz B. 15 kHz * C. 30 kHz D. 45 kHz

19. A double-tuned IF transformer has k=1.5kc. The primary Q is 50 and the secondary Q is 40. Calculate the bandwidth of the transformer at a frequency of 10.7MHz.A. 359 KHz *C. 476 KHzB. 526 KHzD. 147 KHz

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