Heterodyne vs. Homodyne Receivers: Key Differences Explained

This article explores the differences between heterodyne and homodyne receivers, two prevalent architectures in radio receiver systems. Both architectures ultimately convert modulated RF signals into baseband I/Q signals at zero IF (Intermediate Frequency). Let’s delve into how they achieve this.

Heterodyne Receiver

The heterodyne receiver employs a mixer to shift the modulated RF signal to a modulated IF signal. This IF signal is then processed by an I/Q demodulator to bring it down to baseband at zero IF.

Heterodyne receiver

In a superheterodyne receiver, the process involves two mixers. The first mixer converts the RF signal to a high IF signal, and the second mixer converts the high IF signal to a lower IF signal. This lower IF signal is then fed into the I/Q demodulator for conversion to zero IF baseband signals.

super heterodyne receiver

Homodyne Receiver

Unlike heterodyne receivers, homodyne receivers don’t require mixers at the RF stage. The modulated RF signal is directly applied to the I/Q demodulator, which outputs the baseband signals (I and Q) at zero IF.

homodyne receiver

I/Q Demodulator

The figure below illustrates a typical I/Q demodulator circuit used in many modems to convert modulated IF/RF signals to baseband at zero frequency. An appropriate frequency f0 is chosen for this purpose, where W0 = 2pif0, and f0 is the same as the RF frequency of the modulated signal.

IQ demodulator

Homodyne Receiver Principle

The principle of the homodyne receiver is illustrated in the figures above (homodyne receiver image and IQ demodulator image). The signal is initially amplified by a low-noise amplifier (LNA). After this amplification, the signal is directly converted to the baseband (DC signal).

When the RF and LO (Local Oscillator) frequencies are equal, the circuit functions as a phase detector. In other words, if the LO is phase-synchronized with the incoming carrier frequency signal, the receiver is considered a homodyne receiver.

Maximum information is extracted from the modulated I/Q signal through quadrature down-conversion. As depicted, the modulated signal is split into two channels. These two signals are then multiplied by Asin(w0t) and Acos(w0t), resulting in a complex signal (I + j*Q) comprising I and Q components. This vector signal will have a magnitude of sqrt(I^2 + Q^2) and a phase of tan^-1(Q/I).

The homodyne receiver is also referred to as a direct conversion receiver. A primary challenge with this receiver type is LO leakage. Minimizing LO leakage is crucial for efficient RF transceiver operation in delivering baseband I/Q signals.