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Wireless Communication Basics

In a radio receiver, which circuit is placed right after the antenna and amplifies the weak received signal while adding as little of its own noise as possible?

  1. ALNA (low-noise amplifier)
  2. BPA (power amplifier)
  3. CMixer
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Answer: A. LNA (low-noise amplifier)

The signal from the antenna is very weak, so it is amplified in the first stage. Noise added in the first stage is amplified by later stages along with the signal and cannot be told apart from it, so the noise figure of the whole receiver is set mainly by the LNA's noise figure and gain. If the LNA provides enough gain, the effect of noise from later stages becomes small. A PA is the amplifier in the final stage of a transmitter that boosts power to feed the antenna, and a mixer is a circuit that converts frequency.

  • A. Correct. It amplifies in the receiver's first stage while adding as little noise as possible.
  • B. This is the amplifier in the final stage of a transmitter that boosts the power sent from the antenna.
  • C. This is a circuit that multiplies two signals to convert frequency.
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More Wireless Communication Basics questions

  1. 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?
  2. 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?
  3. 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?
  4. 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)?
  5. 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?