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Wireless Communication Basics

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?

  1. ABecause it uses no intermediate frequency, the image frequency problem becomes worse and a high-performance image-reject filter is required
  2. BBecause it converts frequency two or more times, the circuit is complex and hard to integrate on a CMOS chip
  3. CIt needs no external IF filter and is easy to integrate, but it is susceptible to a DC shift (DC offset) caused by LO leakage mixing with itself and to low-frequency 1/f noise
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Answer: C. It needs no external IF filter and is easy to integrate, but it is susceptible to a DC shift (DC offset) caused by LO leakage mixing with itself and to low-frequency 1/f noise

A direct-conversion receiver (zero-IF receiver) tunes the LO to the received frequency and converts directly to baseband with two mixers, in-phase (I) and quadrature (Q). The two I and Q paths distinguish positive and negative frequencies, so there is no problem of an image signal at another frequency getting in and no IF filter is needed, making it easy to integrate on one CMOS chip; it is widely used in transceivers for mobile phones and other devices. On the other hand, when the LO signal leaks toward the antenna and mixes with itself, a DC component (DC offset) arises that is hard to distinguish from the baseband signal. Also, because the converted signal lies near DC, it is susceptible to transistor 1/f noise and to mismatch between the I and Q paths (IQ imbalance).

  • A. The conversion uses two paths, I and Q, so there is no problem of an image signal at another frequency getting in, and no image-reject filter is needed.
  • B. This scheme converts frequency only once, so the circuit is simpler and easier to integrate in CMOS.
  • C. Correct. It is easy to integrate, but DC offset and 1/f noise need countermeasures.
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  1. 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)?
  2. 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?
  3. 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)?
  4. In an FDD mobile phone, which transmits and receives simultaneously on different frequencies, which component separates the transmit and receive signals using their frequency difference so that one antenna can be shared for transmitting and receiving?
  5. In smartphone radio circuits, the transceiver including the LNA, mixer and PLL is often integrated on a CMOS chip, while compound semiconductors such as GaAs HBTs are often used for the transmit PA. Which is the most appropriate main reason?