1.3 Wireless communications  (Page 4/5)

Mixers

Mixers are fundamental building blocks that translate frequencies from one band to another for further processing without changing the information content. On the transmitter side, they up-convert a baseband signal for efficient transmission over a channel. At the receiver, they down-convert to a suitable intermediate frequency for the extraction of information. The frequency translation occurs with the help of an oscillator and RF signal applied to a strong nonlinearity and then filtering of the desired frequency band.

Oscillator

Oscillators produce sinusoidal signals that up-convert or down-convert an RF signal to the required frequency, where subsequent processing might begin. They are designed to operate at a specified frequency. Generally, there is an amplifier and feedback circuit that returns a portion of the amplified signal back to the input. When feedback is aligned in phase, sustained oscillators occur. In practice, they are not perfect, and drift in frequency from time to time. They are also susceptible to phase noise. Due to this, many transceivers operate them in a phase-locked loop (PLL) that can provide frequency stability and lower phase noise. Oscillators use an external crystal to provide a reference signal to PLL-based signal sources. The accuracy of the crystal is specified in ppm. Crystal accuracy is important because the transmit or receive bandwidth may have to be changed according to the drift in the crystal frequency.

Analog-to-digital converter

Analog-to-digital converters are required to convert analog signals to digital signals for baseband processing. After digitizing, signal channel selection can occur in the digital domain, as can equalization.

Transceiver system parameters

This section identifies concepts essential to understanding and evaluating an RF system. The information on these parameters is available in the product’s data sheet. Understanding the data sheet is key to integrating efficient communication systems.

RF communication range

Receiver sensitivity, transmitter output power, signal frequency and propagation environment determine how far apart the receiver must be from the transmitter for error-free communication. A complete expression encompassing these paramaters is given by the Friis equation, Equation 1:

With isotropic antennas in free space, the signal power at distance d can be calculated with a path loss equation (Equation 2):

Where:

Pt = the signal power in dBm at distance d

L = overall system loss

λ = wavelength

Pr = the signal power at the antenna

f = the signal frequency in MHz

d = the distance in meters from the antenna

n = the path loss exponent whose value is determined experimentally and varies under different propagation environments

For example, the required sensitivity of a ZigBee device operating at 2.4 GHz is -85 dBm. Assuming isotropic antennas with unity gains and ideal line-of-sight conditions and a transmit power of 1 mW (0 dBm), the maximum separation between the radios can be approximately 175 m. Increase the power by 6 dB, however, and the range approximately doubles, to 350 m.