The Vector Signal Generator and Wireless Technology

By: BY JOHN HANSEN, AGILENT TECHNOLOGIES INC.
( 1 May 2007 )

It’s no secret that wireless communications has evolved greatly over the past 10 years. Mobile phones and wireless LANs with increasingly complex physical interfaces pose formidable challenges for both design and manufacturing engineers. Currently, a wireless device might include not only a cellular phone capable of operating on multiple bands with various modulation technologies, but it could also contain other receivers or transceivers such as Bluetooth, WiFi, GPS and FM radio. 

The test equipment needed to develop and manufacture these devices has had to keep pace with this ever advancing technology and the huge volumes demanded by the market. In fact, test equipment must be available far in advance of a wireless device or standard coming to market. To have the needed equipment ready for the development of devices supporting a new communication standard, test equipment vendors must participate in the creation of the standard. This helps to ensure that the timeliness, accuracy and performance of these measuring tools meet the requirements of the user.

The vector signal generator has become the instrument of choice to provide real-world test stimuli for modern wireless transceivers and their components. At a high level, a vector signal generator is composed of several fundamental blocks as shown in Figure 1: frequency reference, synthesizer, baseband generator, I-Q modulator, and an output section with an automatic level control (ALC) circuit. Elements unique to the vector signal generator that differentiate it from a classical analog signal generator are the baseband generator (BBG) and the I-Q modulator. 

The BBG in the signal generator takes the binary data created from signal generation software, maps it to I and Q, converts the digital I-Q signal to analog and sends it to the I-Q modulator. 

The pattern RAM is where the signals reside in the BBG. The signal can be created and sent here in many different ways. The signal files are also referred to as arbitrary waveform files and can be created internally in the vector signal generator or can be created from external applications and stored here. Signals can also be created in real-time and streamed through the BBG. 

When the data leaves the pattern RAM, it undergoes symbol mapping. This is the process in which the digital data from the file are mapped to the proper vector constellation points for the modulation technology that is being used. These digital signals are then digitally filtered. After the baseband filters, the I and Q signals are converted to analog signals and then filtered. The analog reconstruction filters are there to filter off the images of the signal produced by the DACs. The data is then sent off to the I-Q modulator.  

In the vector signal generator, the I and Q signals that come from the BBG are used to create a combined IF signal. In the I-Q modulator, I and Q signals are mixed with the same local oscillator (LO) that comes from the synthesizer section of the source. A 90° phase shifter is placed in one of the LO paths. Signals that are separated by 90° are also known as being orthogonal to each other or in quadrature. Signals that are in quadrature do not interfere with each other and they are two independent components of the signal. When recombined, they are summed to form a composite output signal.

This implementation of an I-Q modulator is used almost exclusively today. It is simple to implement and they interoperate well with the digital circuitry of the generator (e.g. DACs, DSP processors). 

Agilent Technologies offers a wide range of products for mobile and short distance radio applications for use in both engineering development and manufacturing. Signal generation and signal analysis make up the core of the wireless device test set and the performance of these instruments is critical to the success of the product and the assurance of profitability. 

The Agilent MXG vector signal generator is well suited to the manufacturing environment. In two rack units, the Agilent MXG offers scalable performance that can be tailored to the test requirements of components and devices used in cellular communications and wireless networking. It has a frequency range up to 6GHz, an internal baseband generator up to 125 megasample/second and enhanced dynamic range with an ACLR for 3GPP W-CDMA at -76dBc (max.) for one carrier and -70dBc (max.) for four carriers.  The Agilent MXG can switch frequency, amplitude and waveforms in less than 1.2ms and has an innovative design that improves reliability and lowers cost of ownership.

Appropriate for the R&D lab, the Agilent ESG vector signal generator enables capabilities for use at all phases of product development from device modeling to baseband firmware to RF verification. The ESG can be integrated with Agilent's EDA Software (ADS) to become part of a connected solution that enables new design and verification capabilities. The term connected solution refers to the combination of simulation and measurement, and allows the sharing of signals, measurements, algorithms, and data in both directions between the two domains. The ESG allows the input and output of baseband waveforms in analog or digital I/Q format representing the waveform to be modulated onto the RF carrier. In addition, this platform allows impairments to be added to the signal in the form of multipath fading or AWGN to accurately recreate the operational signal environment. 

Agilent’s set of Signal Studio waveform creation software products enable the generation of a wide range of application specific test signals using Agilent’s vector signal generators. Signals are easily created to evaluate the performance of radio designs, and the components that comprise them, under various parametric and functional test conditions at baseband and RF frequencies. Supported standards and applications include but are not limited to WCDMA with HSDPA/HSUPA, WiMAX, cdma2000, 1xEV-DO, GSM/EDGE, wireless LAN (WLAN), and TD-SCDMA. 

About the Author

John Hansen is currently a senior product manager for Agilent Technologies' Electronic Measurements Group. At Agilent, he is responsible for the launch of new high frequency microwave signal generator products.