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What is Superheterodyne receiver?

A receiver architecture that mixes the received signal with the local oscillator (LO) signal in a mixer to convert it to a fixed intermediate frequency (IF) before amplifying and demodulating it. Highly selective filters at the IF remove adjacent channels. Signals at the image frequency, on the opposite side of the LO from the desired signal, are converted to the same IF, so they must be removed before the mixer.

Japanese name: スーパーヘテロダイン方式

Superheterodyne receiver appears in 2 practice questions across 1 topic, including Wireless Communication Basics.

Field: Wireless Communication Basics

Try it: questions about Superheterodyne receiver

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?

  1. A700 MHz
  2. B1000 MHz
  3. C1700 MHz
Show answer and explanation

Answer: A. 700 MHz

The difference frequency of the mixer output is used, so any signal that differs from the LO by 100 MHz is converted to the 100 MHz IF. This is satisfied by the desired 900 MHz and by 700 MHz on the opposite side of the LO, and the latter is called the image frequency. A signal at the image frequency cannot be distinguished after conversion to IF, so it is removed by a filter before the mixer (an image-reject filter) or by an image-reject mixer. A higher IF moves the image frequency farther from the desired signal, making it easier to remove with the filter before the mixer.

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
Show answer and explanation

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).

Questions that mention Superheterodyne receiver (2)

  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?

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Related terms

Direct-conversion receiver CMOS 1/f noise (flicker noise) Beamforming Bluetooth Low Energy (BLE) Fading Free-space path loss (FSPL) Hidden node problem Link budget LPWA (low-power wide-area)

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