Producing multitone test signals can be simplified by combining multiple modular frequency synthesizers.
Modern appetites for increased information from wireless devices has driven the complexity of communications modulation formats, as well as the complexity of the signal sources needed to test those communications systems. Advanced modulation formats often cannot tolerate linearity shortcomings of components in those systems, often visible as unwanted intermodulation distortion (IMD). Testing active and some passive components for susceptibility to IMD usually requires multitone test signals. While securing a rack of laboratory-grade signal generators can be expensive, multitone test signals can be generated cost effectively. Doing so requires a proper review of an application’s requirements and assembling a multitone test source that is flexible, practical, and accurate.
As noted in an earlier report (see “Generating Realistic Signals For Testing”), a host of new, compact, low-power signal generators from various suppliers make it easier to create the signals needed for multitone RF/microwave testing. Some of these signal generators are capable of operating on battery power to simplify R&D laboratory, production, and on-site multitone testing.
The aforementioned multitone testing (Fig. 1) is usually required in every terrestrial and satellite-communications (satcom) application. Multitone test signals are created by combining two or more single-frequency signals according to a frequency plan, such as using differences in frequencies between the tones that mimic the content of a carrier signal from a cellular base station. Multitone testing helps reveal when a communications component or system produces intermodulation signals that fall outside the system’s designated operating frequency range; these unwanted signals act as interference for other communications systems.
1. Two possible approaches for producing multitone test signals are the vector signal generator and multiple modular frequency synthesizers.
For broadband component testing, multitone signals may cover an octave or more and may be performed with fixed test tones or on a swept-frequency basis. In addition to testing for intermodulation, multitone signals are effectively used for radiated immunity testing. The primary considerations when setting up multitone test solutions include the number of tones; their frequency separation and frequency range; the power levels of the input and output signals during testing; any required modulation; and, of course, the size and cost of the hardware and software needed for multitone testing.
A number of approaches can be used to generate multitone test signals. When considering signal-source candidates, it can be helpful to understand how they can differ and how to use key performance parameters to compare different multitone signal sources. First and foremost, multitone test signals must be high quality, with well-behaved spectral characteristics in terms of harmonics, spurious, and phase noise. For example, the relative phase between different tones can influence the IMD produced by a device under test (DUT). When a DUT, such as a high-gain amplifier, is known to have this sensitivity to test-signal phase, the resulting IMD may need to be measured not only as a function of frequency spacing between the tones but also by the phase differences between tones. Amplitude differences between the tones can also affect measured IMD results, so that signal sources for multitone testing should be stable in terms of frequency, phase, and amplitude.
The two traditional methods for generating multitone signals are vector signal generators (VSGs) and multiple combined signal generators. These are both shown in Figure 1, where either acts as the input to the DUT and the output is monitored on a spectrum analyzer. Multitone test signals are traditionally produced by combining the outputs of separate test sources; depending upon the cost of each source, however, this can be an expensive solution. Multitone test signals can be represented as time functions of multiple sinusoidal signals. In the case of n sinusoidal signals with associated voltages of V1, V2, V3 to Vn, the total voltage waveform, Vi(t) as a periodic function of time can be written as:
Vi(t) = V1 sin(ω1t + φ1) + V2 sin(ω2t + φ2) + V3 sin(ω3t + φ3) + Vn sin(ωnt + φn)
where ω1, ω2, ω3, and ωn are the frequencies (2πf) of the n signals and φ1, φ2, φ3, and φn, are the phases of the n signals. The combined multitone signal is fed to the input of the DUT and is responsible for the various output products from the DUT, including harmonics, mixing products, and other nonlinear products. This is monitored on the spectrum analyzer as shown in Fig. 1, and the results are fed back to the controller for data gathering and analysis.
VSGs produce multitone signals by modulating a single synthesized carrier frequency with a complex waveform to create the desired multitone output. While this has the advantage that a single source can be used to produce multiple tones, it has many disadvantages. First, the modulation can only create sidebands over a narrow frequency range around the carrier. Thus, a multitone VSG that can create two tones that are 80 MHz apart can only create as many as 16 tones that are 5 MHz apart. Additionally, if a VSG has +10-dBm output power, its maximum output power per tone in an eight-tone test will be less than +2 dBm. With a VSG, the capability to control individual phases and amplitudes is limited.
With the availability of cost-effective programmable broadband synthesizers such as the PHS-5000 from Pronghorn Solutions (Fig. 2), it is now possible to create multiple test tones with multiple independent synthesizers phase locked to a common reference source, and capable of frequency, phase, and amplitude controls over the entire synthesizer frequency range. At a fraction of the cost of a VSG—with each synthesizer measuring only 6 x 3 x 0.5 in., weighing less than 1.5 lbs, and consuming less than 5 W power, and with IVI and other standard programmability functions—the multiple synthesizer multitone method overcomes the performance and size limitations of the VSG approach.
2. By adding modular synthesized sources, multitone test signals of any required complexity can be created by adding more sources.
To specify a multitone test system, it is necessary to specify the number of tones required, the frequency range of each tone or signal, the power range and level accuracy of each tone, the frequency accuracy and spectral purity, any phase adjustment requirements, and any modulation requirements. Flexibility is often yet another requirement: the capability to use a test solution for different purposes in a laboratory or production facility. Small size, light weight, and low power consumption are often very important, especially for on-site applications where battery-powered operation is often desirable.
This “new generation” of low-cost signal generators can be combined for multitone testing, and including compact, battery-powered units such as the PHS-4000 signal generator. This handheld signal source (Fig. 3) offers a fundamental frequency range of 150 MHz to 9 GHz that is available in extended-frequency units for a range of 50 MHz to 18 GHz. Multiple PHS-4000 generators can be combined for multitone testing, and each unit runs for about four hours on a rechargeable battery for ease of on-site testing.
3. Battery-powered signal sources such as this can provide multitone outputs to 9 GHz for on-site testing.
For higher frequencies, the same company’s model PHS-5000 modular source operates from 150 MHz to 18 GHz with 10-kHz tuning resolution, weighs less than 2 lbs, and consumes less than 6 W power, so that several modules can be combined in a mainframe to create a low-power multitone test-signal source (which the firm specifies as a model PHS-6000 frequency synthesizer). Each module can be programmed separately in terms of amplitude, frequency, and phase, for full control of multitone output signals in terms of their differential signal characteristics. The source modules operate from a common 10-MHz reference oscillator to maintain accuracy among the modules.
Ganesh R. Basawapatna, Chief Technical Officer
Anand Basawapatna, Director, Operations and Marketing