RF transceivers are electronic devices that receive and demodulate radio frequency (RF) signals, and then modulate and transmit new signals. They are used in many different video, voice and data applications. RF transceivers consist of an antenna to receive transmitted signals and a tuner to separate a specific signal from all of the other signals that the antenna receives. Detectors or demodulators extract information that was encoded before transmission. Radio techniques are used to limit localized interference and noise. To transmit a new signal, oscillators create sine waves which are encoded and broadcast as radio signals.
Selecting RF transceivers requires an understanding of modulation methods and radio techniques. Amplitude modulation (AM) causes the baseband signal to vary the amplitude or height of the carrier wave to create the desired information content. Frequency modulation (FM) causes the instantaneous frequency of a sine wave carrier to depart from the center frequency by an amount proportional to the instantaneous value of the modulating signal. On-off key (OOK), the simplest form of modulation, consists of turning the signal on or off. Amplitude shift key (ASK) transmits data by varying the amplitude of the transmitted signal. Frequency shift key (FSK) is a digital modulation scheme using two or more output frequencies. Phase shift key (PSK) is a digital modulation scheme in which the phase of the transmitted signal is varied in accordance with the baseband data signal. In terms of radio techniques, some RF transceivers use direct-sequence spread spectrum. Others use frequency-hopping spread spectrum.
Important specifications for RF transceivers include data rate, sensitivity, output power, communication interface, operating frequency, measurement resolution, and maximum transmission distance. Data rate is the number of bits per second that can be transmitted. Sensitivity is the minimum input signal required. Communication interface is the method used to output data to computers. General-purpose interface bus (GPIB) is the most common parallel interface. Universal serial bus (USB), RS232 and RS485 are common serial interfaces. Operating frequency is the range of signals that can be broadcast and received. Measurement resolution is the minimum digital resolution. Maximum transmission distance is the largest distance by which the transmitter and receiver can be separated. Additional considerations when selecting RF transceivers include power source, supply voltage, supply current, transmitter inputs, receiver inputs, and RF connector types.
Transmitter and Receiver
This transceiver has a transmit side (Tx) and a receive side (Rx), which are connected to the antenna through a duplexer that can be realized as a switch or a filter, depending on the communications standard being followed. The input preselection filter takes the broad spectrum of signals coming from the antenna and removes the signals not in the band of interest. This may be required to prevent overloading of the low-noise amplifier (LNA) by out-of band signals.
The LNA amplifies the input signal without adding much noise. The input signal can be very weak, so the first thing to do is strengthen the signal without corrupting it. As a result, noise added in later stages will be of less importance. The image filter that follows the LNA removes out-of-band
signals and noise
before the signal enters the mixer. The mixer translates the input RF signal down to the
intermediate frequency, since filtering, as well as circuit design, becomes much easier at lower frequencies for a multitude of reasons. The other input to the mixer is the local oscillator (LO) signal provided by a voltage-controlled oscillator inside a frequency synthesizer. The desired output of the mixer will be the difference between the LO frequency and the RF frequency.
At the input of the radio there may be many different channels or frequency bands. The LO frequency is adjusted so that the desired RF channel or frequency band is mixed down to the same intermediate frequency (IF) in all cases. The IF stage then provides channel filtering at this one frequency to remove the unwanted channels. The IF stage provides further amplification and automatic gain control (AGC) to bring the signal to a specific amplitude level before the signal is passed on to the back end of the receiver. It will ultimately be converted into bits (most modern communications systems use digital modulation schemes) that could represent, for example, voice, video, or data through the use of an analog-to-digital converter.
On the transmit side, the back-end digital signal is used to modulate the carrier in the IF stage. In the IF stage, there may be some filtering to remove unwanted signals generated by the baseband, and the signal may or may not be converted into an analog waveform before it is modulated onto the IF carrier. A mixer converts the modulated signal and IF carrier up to the desired RF frequency. A frequency synthesizer provides the other mixer input. Since the RF carrier and associated modulated data may have to be transmitted over large distances through lossy media (e.g., air, cable, and fiber), a power amplifier (PA) must be used to increase the signal power. Typically, the power level is increased from the milliwatt range to a level in the range of hundreds of milliwatts to watts, depending on the particular application. A lowpass filter after the PA removes any harmonics produced by the PA to prevent them from also being transmitted.
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