Radio transmitter

(electronics) The equipment used for generating and amplifying a radio-frequency carrier signal, modulating the carrier signal with intelligence, and feeding the modulated carrier to an antenna for radiation into space as electromagnetic waves. Also known as radio set; transmitter.

A generator of radio-frequency (rf) signals for wireless communication over some distance, which can vary from the short ranges within a building to intercontinental distances. Most applications utilize signals from very low frequencies (VLF) to extremely high frequencies (EHF); some applications require frequencies as low as 45 Hz or as high as 100 GHz. The radio-frequency output power varies from a fraction of a watt in emergency beacons and portable equipment to several megawatts in long-range, low-frequency transmitters. See also Radio spectrum allocations.

The architecture (organization) of a radio transmitter is determined by the type of signal it is intended to produce. The four basic architectures are those used for continuous-wave, frequency-modulation, amplitude-modulation, and single-sideband signals. Transmitters for some applications (for example, television) use a combination of these architectures (for example, frequency modulation for sound and single sideband for video), while others (for example, Loran C) use unique architectures. Alternative architectures such as envelope elimination and restoration or outphasing can be used to improve efficiency. See also Electrical communications; Modulation.

Continuous-wave (CW) transmitter

The most basic type of radio transmitter produces only a continuous-wave signal. Such transmitters are often switched on and off (keyed) to produce telegraph signals. The block diagram of a simple continuous-wave transmitter is shown in Fig. 1. The oscillator G1 produces a low-power signal, which is boosted to the final output power by a series of progressively larger power amplifiers. The optional inclusion of a frequency multiplier improves stability by allowing the frequencies of the oscillator and high-power amplifiers to be different. See also Frequency multiplier; Oscillator; Power amplifier.

Basic continuous-wave (CW) transmitter.

The architecture includes both frequency translation and power splitting, which makes it more suitable for generating high-power signals at various frequencies. While at a relatively low level, the signal is translated by a mixer to the desired output frequency. After amplification by a chain of power amplifiers, it is split into two parts to drive two final power amplifiers whose outputs are combined to produce the transmitter output. See also Mixer.

Frequency-modulation (FM) transmitter

Analog frequency modulation is widely used for voice communication, high-quality audio broadcasting, and television audio. Frequency-shift keying (FSK) and phase-shift keying (PSK) are widely used for transmission of digital data via radio-frequency signals. See also Frequency modulation; Frequency-modulation radio; Mobile radio; Radio broadcasting.

Frequency-modulated and phase-modulated (PM) signals have constant amplitudes and are therefore produced by transmitters with architectures similar to those of the continuous-wave transmitter (Fig. 1). The principal change is the replacement of oscillator G1 by a frequency or phase modulator. In frequency-modulation transmitters, the frequency multiplier increases the frequency deviation as well as the carrier frequency of the frequency-modulated signal. See also Phase modulation.

In communication applications, the frequency modulator is typically a voltage-controlled crystal oscillator (VCXO) in which the capacitance of a varactor diode is used to vary slightly the frequency of a crystal oscillator. Other applications employ various types of modulators, including phase-shift, phase-locked-loop, comparator, and Armstrong. See also Frequency modulator.

Amplitude-modulation transmitter

Full-carrier amplitude modulation is used in medium-frequency (MF) broadcasting, high-frequency (HF) international broadcasting, citizen-band communication, aircraft communication, and nondirectional navigation beacons. See also Amplitude modulation; Electronic navigation systems.

Most modern full-carrier amplitude-modulation transmitters produce the output signal by amplitude modulation of the final radio-frequency power amplifier. Generally, the modulation is accomplished by varying the supply voltage of the radio-frequency power amplifier with a high-power radio-frequency amplifier. Since the radio-frequency carrier has constant amplitude until the final power amplifier, the architecture of the radio-frequency chain (Fig. 2) is similar to that of a continuous-wave or frequency-modulation transmitter. See also Audio amplifier.

Amplitude-modulation (AM) transmitter.

Single-sideband (SSB) transmitter

Single-sideband amplitude modulation is widely used for high-frequency voice communications, including military, marine, aeronautical, diplomatic, and amateur. It also finds use (as amplitude-compandored single-sideband, or ACSB) at very high frequencies (VHF) and ultrahigh frequencies (UHF).

Although single sideband is technically a form of amplitude modulation, the single-sideband signal itself has variations of both amplitude and phase. Signals such as multitone, independent sideband (ISB), and vestigial sideband (VSB), used for video, also possess such characteristics. Consequently, these signals are traditionally amplified by a chain of linear radio-frequency power amplifiers operating in class B. See also Single sideband.

The low-level output of the single-sideband modulator is first shifted by the local oscillator G2 to an intermediate frequency (i-f) that is at least twice the highest output frequency. The intermediate-frequency signal is then shifted downward to the desired output frequency by the variable-frequency oscillator (VFO) G3. The mixer output is low-pass-filtered and then amplified to the desired power. See also Electric filter.