Modern wireless communications testing requires understanding the new capabilities offered by the latest RF signal generators.
By Ron Rausch, Keithley Instruments, Inc.

There are many different kinds of RF signal generators from which to choose. No single generator fits all user requirements. This article is a guide to help you select the best RF signal generator for your needs, and it focuses on selecting signal generators commonly used for today's commercial wireless communications systems.
Signal Modulation
First, make a list of the signal modulations that are being used now, since you will want to choose a signal generator that is able to create all of the signals you will need. To maximize the utility of your signal generator investment, consider other signals that you are likely to need in the future.

Figure 1. Keithley's SignalMeister RF Communications Toolkit is an example of a modern signal creation and analysis software package with an object-orientated GUI that's optimized for creating MIMO and other complex signals of wireless standards.
To your list of modulations and standards, add others that exist today that might be adopted or incorporated into new products that you will be testing in the future. You can find these by looking at your road map for new products and technologies. Other places to look are departments or groups in your company with closely-related products, as well as those used by competitive alternatives to your products.

Look at broader trends for other signal types to add to your list. The demand for bandwidth and spectrum efficiency has led to the trend of using OFDM (Orthogonal Frequency Division Multiplexing) modulated signals. These are multi-carrier signals that generally have much wider bandwidth than single-carrier signals. Bandwidths for current standards are up to 40 MHz for 802.11n WLAN. Signal bandwidth is a key specification for today's wireless communications systems. Signal generators with signal bandwidths less than 5 MHz are suitable for most single-carrier communications devices and equipment. Generators for most commercial systems that use OFDM signals have maximum bandwidths of 20 MHz, 40 MHz or wider.

Another trend is the move from single-input, single-output signals (SISO) to multiple-input, multiple-output (MIMO) signals. Wireless standards that use up to 4 X 4 MIMO configurations include 802.11n WLAN, 802.16e-2005 mobile WiMAX and LTE (Long-Term Evolution). Modern signal generators designed for MIMO are capable of at least four RF output signals, with some up to eight outputs. Key specifications are the synchronization time and jitter between the waveform samplers of any two generators. Synchronization time of MIMO signal generators varies from about 20 nsec which is usually acceptable for production applications, to 1 nsec which is needed for many research and product development applications.

In the not too distant future, MIMO communication systems will be doing beam forming. Here, the amplitude and phase of the RF signals at each antenna is varied to change the antenna pattern to improve signal strength at the receiver. For these measurements, it is critical that the synchronization between the individual signal generators be highly precise and stable. A key specification requirement is RF carrier phase jitter of 2º or less between signal generator outputs.
Measurement Range
Because high frequency coverage is expensive, most people only pay for what they need now or in the near future. RF signal generators covering up to 6 GHz are specifically designed for wireless markets. Models are also available with less frequency range coverage, such as 2.4 GHz to 3 GHz models to cover the mobile phone market and devices using the unlicensed IMT (industrial, medial and scientific) frequency bands.

Figure 2. The Keithley Model 2920 is an example of a modern RF vector signal generator that's small yet powerful with high-speed tuning and waveform switching, up to 80 MHz signal bandwidth, and is MIMO-ready to sync up to eight MIMO outputs.
Some signal generators only cover frequency ranges of specific wireless services. For example, banded WiMAX testers may only cover the 2.5 GHz and 3.5 GHz frequency bands. Having a signal generator with continuous frequency coverage helps ensure that products operate as they should near the band edges and behave predictably in between the bands. This may also allow coverage of new wireless bands that may become available. For example, new wireless services are expected to be deployed in the lucrative 700 MHz band auctioned by the FCC in the US in January, 2008.

When modulating signals, the peak power is limited by the generator's output power amplifier power, so the maximum modulated power will be less than the maximum CW power. How much less depends on the crest factor of the signal waveform. For example, WLAN and WiMAX use OFDM signal modulations and have relatively high crest factors, so the maximum modulated power is about 10 to 13 dB less than the maximum CW power.

Modern signal generators, called vector signal generators (VSG), use an I-Q (in-phase and quadrature) modulator that can be used to generate virtually any signal modulation. This allows complex signals used in today's wireless systems, such as W-CDMA, WLAN and WiMAX, to be easily generated. Most VSGs have an internal signal generator's arbitrary waveform generator, or ARB. Users create signals using software that makes a file of I and Q data values that define a waveform which are then downloaded into the generator's ARB memory. The key specification of a VSG is signal bandwidth. For most new signals that use OFDM modulation, such as WLAN, WiMAX and LTE, bandwidths up to 40 MHz are required. You will want plenty of ARB memory to store multiple waveforms because most measurement applications require several signals.

Signal generator suppliers have PC-based software tools, such as Keithley's SignalMeister or Agilent's Signal Studio, that create signals for many wireless standards such as WLAN. General-purpose tools, like MatLAB or LabVIEW, can be used to create a wide variety of signal waveforms. These are most useful when defining non-standard signal types. Modern PC tools use a visual block-diagram orientated GUI that simplifies and speeds the creation of signals which is especially important for MIMO signals and those involving transmitter distortion and channel emulation.
Absolute amplitude accuracy is a principal signal generator specification. This is a measure of the signal power's accuracy, traceable to a national standard. High accuracy is especially important in production applications, because it directly impacts measurement uncertainty guard bands and product yield. The specification will often vary with frequency and power level. Modern signal generators have circuitry that is highly stable and repeatable, allowing use of extensive calibration techniques to improve accuracy to the 0.5 dB to 0.7 dB range. Also, be sure to look at the instrument's Standing Wave Ratio (SWR) specification which is not included in the amplitude accuracy specification.

Modulation accuracy is another principle signal generator characteristic. The key specification is rms EVM (error vector magnitude). The EVM is a measure of how accurately the I and Q vectors on a constellation display were generated. The EVM varies for different signals (W-CDMA, WLAN, WiMAX, etc.), so look at the EVM specification for the signals of interest to you. You will want the instrument EVM to be at least 5 to 10 dB better than the DUT.
Modern signal generators are capable of making high speed measurements without sacrificing accuracy. This gives test engineers increased flexibility over slower instruments to optimize a production line output by making measurements. Measurements can be made at the fastest speed possible, or more measurements can be made and averaged to improve accuracy, for example at low signal levels.

Key specifications are frequency and amplitude switching times. Modern signal generators can switch frequencies in the 1 msec to 5 msec range, roughly five to 10 times faster than traditional synthesizers. Electronic attenuators have switching time in the low millisecond range, over 10 times faster than their mechanical cousins. Other speed parameters to compare are waveform switching times. Since most testing requires multiple signals, the time to switch between different waveforms is important. Look for switching times in the low millisecond range and features like arbitrary waveform sequence that let you switch waveforms virtually instantly. Waveform download time can slow measurement speed. Having a large ARB memory size that can hold all of your test waveforms allows the waveforms to be downloaded once and lets you quickly switch between them.
When selecting an RF signal generator, focus on your application and look to requirements that you can identify in the near future. New signal characteristics to consider for wireless testing today and tomorrow are the ability to generate multiple complex signals, maximum BW, MIMO compatibility and synchronization stability, and ARB memory size and switching speed.

Ron Rausch is senior marketing manager, RF Products for Keithley Instruments, Inc., Cleveland, OH;