A signal generator is a testing instrument that produces and transmits electrical signals with known waveform, frequency and amplitude. Testers use oscilloscopes to analyze electric waveforms as they travel through various electronic devices. A variety of signal generators are available on the market, ranging from radio frequency – up to 10 gigahertz – to microwave frequency – surpassing 70 gigahertz. Previously, only analog signal generators were available. In recent years, vector, or digital signal generators are more commonplace, producing waveforms digitally and synthesizing frequency.
Amplitude-shift keying (ASK) modulation is a digital modulation method in which an analog carrier signal is modulated by two or more discrete amplitude levels. ASK modulation employs simple and energy efficient transmitters and uses minimal bandwidth, but it is less prevalent than FSK and PSK modulation.
Continuous wave (CW) modulation is the simplest form of modulation, and is also known as analog modulation. In a CW scheme, signal generators will produce sinusoidal, fixed-frequency output signals.
Direct Digital Synthesis (DDS)
Direct digital synthesis (DDS) is an electronic technique for digitally generating arbitrary waveforms and frequencies from a solitary, fixed source frequency. Highlights of DDS include operation over a wide spectrum of frequencies, fine frequency resolution, and rapid exchange of output frequencies. DDS circuits generally include a random-access memory, an electronic controller, a frequency reference, a digital-to-analog converter (DAC), and a counter. Nearly all contemporary RF signal generators employ frequency synthesizers, many of which contain DDS architecture.
Frequency-shift keying (FSK) modulation is a digital modulation method in which an analog carrier signal is modulated by two or more discrete frequency levels. FSK is less efficient in both power and bandwidth than most other modulation modes. Applications include Caller ID, amateur radio, emergency warning systems and remote metering.
I/Q modulation, also known as quadrature amplitude modulation (QAM), is a modulation technique that transmits two signals, namely I, in phase, and Q, quadrature. The Q signal is phase shifted 90 degrees from the I signal. Both signals are amplitude modulated, algebraically summed, and transmitted across one channel as a single signal. Employed extensively as a modulation technique for digital telecommunication systems, the advantages of I/Q modulation are minimization or elimination of intermodulation interference, and effective encryption.
A low frequency (LF) signal is any signal characterized by frequencies in the range of 30 – 300 kHz. It is also known also as the kilometer wave signal for its lengthy wavelengths ranging from 10 to 1 km. Low frequency signal generators sustain stable frequencies and can be continuously controlled over their entire range. Low-frequency generators are used principally for tuning and testing of radio receiving and transmitting devices, as well as for calibrating devices operating in the frequency range of 20 Hz to 200 kHz.
Modulation refers to the process of altering one or more characteristics of a high frequency periodic waveform known as the carrier signal, in accordance with the characteristics of a modulation signal for the purpose of data transmission. The three basic types of modulation consist of amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM), in which the amplitude, frequency, or phase are correspondingly altered. Sophisticated signal generator models include several modulation methods, increasing accuracy and sensitivity of measurements.
Phase-shift keying (PSK) modulation is a digital modulation method in which an analog carrier signal is modulated by two or more discrete phase levels. PSK’s widespread use in present-day technologies, particularly in the developing field of data communications, stems from its inherent simplicity.
Single-Sideband (SSB) Phase Noise
Single-sideband (SSB) phase noise refers to the measurement specified in dBc/Hz of the ratio of the power at a given frequency offset from the carrier signal to the power of the carrier in a 1-Hz bandwidth. A significant factor in spectral purity determination, low phase noise is essential for construction and control of accurate waveforms. The commonplace synthesized signal generator offers many advantages, but high phase noise is often problematic, necessitating careful consideration of the phase noise specification.
Spectral purity represents the inherent frequency stability of a signal. Short term and long term stability are defined as the changes in frequency in less than or more than one second respectively. Spectral Purity is an essential component of signal generators for accurate testing of receiver selectivity; this is vital as the available spectrum for radio channels decreases and designers are constructing more selective receivers.
Voltage Standing Wave Ratio (VSWR)
Voltage standing wave ratio (VSWR) is defined as the ratio of the amplitudes of partial standing waves from a maximum node to an adjacent minimum node, and is essentially a measure of how precisely an electrical load is impedance-matched to a signal source, or a measure of communication line efficiency. Partial standing waves are the result of impedance mismatches, or the opposition presented by a route to a signal. Decent VSWR values imply a maximization of power transfer, and minimization of signal reflection back to the source.
Wideband frequency modulation (FM) is a form of modulation in which the modification of the carrier frequency is significantly higher – modulation index >1 – than the signal frequency. Requiring a wider signal bandwidth than other modulation techniques, wideband FM is resistant to changes in signal strength, considerably improves signal-to-noise ratio, and is resistant to interference. Wideband FM is used extensively in communications, particularly broadcasting.