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Wireless Communication Basics

In a receiver, which circuit multiplies the received signal by the signal of the local oscillator (LO) to produce a signal at the difference (or sum) of their frequencies, thereby converting frequency?

  1. ABandpass filter
  2. BMixer
  3. CVoltage-controlled oscillator (VCO)
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Answer: B. Mixer

Multiplying two sine waves produces components at the sum and difference of their frequencies. A mixer uses this to convert the received signal to the difference from the LO frequency (an intermediate frequency or baseband). The LO signal is generated at a precise frequency by a PLL referenced to a crystal oscillator and is switched to match the channel being received. Transmitters also use mixers to convert the baseband signal to the transmit frequency.

  • A. This is a circuit that passes only a certain range of frequencies; it does not convert frequency.
  • B. Correct. Multiplication produces the sum and difference frequencies.
  • C. This is an oscillator whose frequency is set by a voltage, used inside a PLL to generate the LO signal. It is not a circuit that multiplies two signals.
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More Wireless Communication Basics questions

  1. A superheterodyne receiver receives a 900 MHz signal by setting its local oscillator (LO) to 800 MHz and converting to a 100 MHz intermediate frequency (IF). Which is the image frequency that would also be converted to the same 100 MHz IF?
  2. Which describes a direct-conversion receiver, which sets the local oscillator (LO) to the same frequency as the received signal and converts directly to baseband without an intermediate frequency, compared with a superheterodyne receiver?
  3. In a transmitter sending OFDM or multilevel QAM signals, which is the main reason for operating the power amplifier (PA) at a point below where its output saturates (with back-off)?
  4. An LNA with a noise figure of 1.5 dB and a gain of 15 dB is placed at the first stage of a receiver, followed by a circuit with a noise figure of 10 dB. Which is closest to the noise figure of the two stages together?
  5. Take the thermal noise power density at 290 K as −174 dBm/Hz. For a receiver with a bandwidth of 1 MHz and a noise figure of 6 dB that needs an S/N of 10 dB to demodulate, which is closest to the receiver sensitivity (the minimum signal power that can be received)?