Introduction to the Signal Generator
2018-10-31 | 14 min read
Written by Tit Bin Teo, Technical Engineering Manager
What is a Signal Generator?
November 1940 News Flash. Disney releases Fantasia with “Fantasound”, a new audio stereophonic sound system.
The HP 200B audio oscillator, one of the earliest applications of a signal generator, was used to calibrate Disney’s breakthrough stereo sound system installed in theaters that showed Fantasia. This ensured audiences were able to enjoy the sounds and music as it was intended, with minimal distortions. But what is a signal generator? Simply put, a signal generator is a source that outputs a signal. This signal can be a basic sinusoidal wave, a pulse, or a modulation signal. Signal generators are also often called signal sources or simply, sources. A signal generator allows you to output signals with various frequencies, amplitudes, and time durations. Many signal generators even allow you to modulate frequency, amplitude, and phase signals.
What is a Signal Generator Used For?
A signal generator is often used to provide a stable sinewave. This stable sinewave has many uses in telecommunications. One example is as an oscillator in RF receiver testing. The purer the sine wave, the less phase noise, and distortion is injected into the RF receiver testing by the oscillator. This allows designers to see the actual performance of the RF receiver.
Another application where signal generators are used is in RF power amplifier testing. In this test, the signal generator outputs fixed RF power across a range of frequencies to the amplifier. The output of the RF power amplifier is measured to determine the output flatness.
Signal generators are also used in modern high-speed communication systems like 5G and 802.11ax. Powerful software is used with the signal generator to generate complex orthogonal frequency division multiplexing (OFDM) signals for transceiver testing.
Why are Signal Generators Important?
When testing designs, you want to have certainty in your measurements. When a signal generator outputs a sinewave, you want a sinewave that is as near to ideal as possible. A non-ideal sinewave will carry high levels of phase noise, harmonics, and spurs. High phase noise obscures low-level signals. A high-performance signal generator gives you a sinewave that is nearly ideal, with low phase noise, harmonics, and spurs. Figure 1 shows the fundamental signal (marker 1), harmonics (marker 2 and 3), spurs (marker 4 and 5).
Figure 1: Fundamental signal, harmonics, and spurs.
A signal generator also allows complex signals to be generated from a single, integrated instrument without complex hardware add-ons. Used together with powerful software, complex signals such as Orthogonal Frequency-Division Multiplexing could be generated by a single high-performance signal generator with high fidelity.
A signal generator also has the ability to output accurate power levels. This capability is often used in power amplifier, filter, and attenuator testing.
Types of Signal Generators
There are several types of signal generators; they can be classified based on their form factor and capabilities. You can save money by getting the right type of signal generator with just the right performance for your application.
The most common signal generator form factor is the benchtop form factor. These are traditional box instruments that we normally find on benches and in racks. Benchtop signal generators are well-suited for R&D, where analysis and troubleshooting benefit from direct interaction with the instrument via the front panel. Benchtop models range from RF to microwave, and from analog to vector.
More recently, signal generators using the modular PXIe form factor have become available. Modular PXIe signal generators are compact signal generators that occupy only several slots in a PXIe chassis. This compact form factor is ideal for applications that require multi-channel measurement capabilities, fast measurement speed, and a small footprint. They also offer scalability and flexibility to configure solutions with a shared processor, chassis, and other modular instruments. The PXIe vector signal generator uses the same software applications as the benchtop signal generators and provides measurement consistency and compatibility from product development to manufacturing and support.
Signal generators are also classified based on their capabilities.
Analog Signal Generators
Analog signal generators supply sinusoidal continuous wave (CW) signals with optional capability to add amplitude modulation, frequency modulation, phase modulation, and pulse modulation. The maximum frequency range for analog signal generators spans from RF to microwave. Most generators feature step/list sweep modes for passive device characterization and calibration.
Vector Signal Generators
The newer vector signal generators or digital signal generators have a built-in quadrature, also called IQ modulator, to generate complex modulation formats such as Quadrature Phase-Shift Keying (QPSK) and 1024 Quadrature Amplitude Modulation (QAM). When combined with an IQ baseband generator, virtually any signal can be emulated and transmitted within the information bandwidth supported by the system.
Agile signal generators
Agile signal generators are optimized for speed to quickly change the frequency, amplitude, and phase of the signal. They also have the unique capability to be phase-coherent at all frequencies, all the time. This attribute, along with extensive pulse modulation and wideband chirp capabilities, is ideal for electronic warfare (EW) and radar applications.
What’s the Difference between a Signal Generator and an Arbitrary Waveform Generator (AWG)?
An AWG is used to output any arbitrarily defined waveform. These arbitrary waveforms include cardiac, pulse, sawtooth and other real-life signals. An AWG can also generate sinewaves. Waveforms in an AWG are usually defined as a series of waypoints across time. Therefore, there is no limit to the type of waveforms an AWG can generate.
If that is the case, what good is a signal generator if an AWG can generate all the types of waveforms that a signal generator can? A signal generator is used when you need low phase noise or accurate power level in your signal. Low phase noise is critical especially when you have dense constellations in your modulation scheme or if you need to work with low-level signals. Accurate power is critical when testing RF power amplifiers and receiver.
As digital technologies improve, the spurious-free dynamic range (SFDR) and phase noise of modern AWGs is coming close to that of signal generators. This has allowed AWGs to replace signal generators in certain RF applications.
Another key advantage a signal generator has over an AWG is the ease of changing modulation carrier frequency with the turn of a knob. An AWG requires a re-calculation of the waveform if there are changes to the carrier frequency.
We will discuss, in-depth, the differences between AWGs, signal generators, pulse generators, and function generators in an upcoming post. Stay tuned.
Key Specifications of a Signal Generator
Understanding signal generator specifications is critical when determining the right type of source for an application. Specifications are generally divided into three broad categories – frequency, amplitude, and spectral purity.
Range, resolution, accuracy, and switching speed are the main frequency specifications. Range specifies the range of output frequencies that the source can produce. Resolution is the smallest frequency increment of the source. Accuracy represents how close the source’s output frequency is to the set frequency. Switching speed is how fast the output settles down to the new frequency. Switching speed is an important specification for manufacturing because time is cost.
The frequency accuracy of a source is affected by two factors; they are the aging rate of the time base reference oscillator, and the amount of time since the source was last calibrated. The aging rate indicates how fast the reference will drift from its specified value. Let’s say, for example, a signal generator’s 1 GHz reference oscillator has an aging rate of 0.152 ppm (parts-per-million) per year. If this oscillator has not been calibrated for one year, the signal generator’s output frequency will be within 152 Hz off its set frequency. The calculation is shown below.
Frequency Accuracy (Hz) = Output Frequency (Hz) x Aging Rate (ppm/year) x Time since last calibration
= 10 GHz x 0.152 ppm/year x 1 (year)
= 152 Hz
Figure 2: Calculating the accuracy of a typical oscillator
Dynamic range, accuracy, resolution, and switching speed are the main amplitude specifications. The dynamic range of a signal generator is the difference between the maximum output power capability and minimum output power capability. The resolution of a source indicates the smallest possible amplitude increment. Switching speed is a measure of how fast the source can change from one amplitude level to another.
Figure 3: Power output accuracy
The specifications associated with spectral purity are often the most difficult to understand. These specifications include phase noise, harmonics, and spurious. Harmonics occur at integer multiples of the output frequency. Spurious signals, also known as non-harmonics, can occur at any frequency. The ideal output is a sine wave at a single frequency. Unfortunately, there are no ideal sources. All signal generators are made with non-ideal components which introduce phase noise and unwanted distortions. Phase noise is a frequency-domain view of the noise spectrum around the oscillator signal; it describes the frequency stability of an oscillator. Phase noise is expressed in dBc/Hz at a certain frequency offset. The unit dBc/Hz represents the noise power contained a 1 Hz bandwidth relative to the power contained in the fundamental frequency. For example, the phase noise of M5182B at 1GHz is <-145 dBc/Hz at 20 kHz offset as shown in Figure 4.
Figure 4: Measured phase noise performance for N5182B
Harmonic spurs are spurious with frequencies that are integer multiples of the fundamental frequency (fo) output. These harmonics are caused by non-linear characteristics of components used in the signal generator. These non-linear components are needed to provide a broad range of output frequencies and power. Harmonics are expressed in dBc. For example, when the second harmonic is specified at less than -30 dBc, this means the second harmonic is at least 30-dB below the output level of the fundamental frequency.
Non-harmonic spurs come from a variety of sources, such as power supply and are typically are lower than -65 dBc. Multipliers are often used in sources to extend the output frequency. This can result in the presence of sub-harmonics, which appears as spurs with frequencies less than the fundamental frequency. Figure 5 shows how each spectral purity components relate to each other.
Figure 5: Spectral purity specifications
Signal generators are versatile instruments capable of providing complex and accurate signals for your RF test needs. Building a solid foundation in signal generator know-how will help you design more effective and efficient test strategies.
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