New Digital Demodulation Option Enhances Aeroflex’s High Performance Spectrum Analyzers
By Bryan Harber, Product Manager, Microwave and Spectrum Analyzers, Aeroflex Inc.

The Aeroflex 3280 Series of spectrum analyzers now offers a new digital demodulation option. The digital demodulation capability allows engineers to analyze the transmitter characteristics of a wireless device and demodulate digital modulation formats as used by Wireless LAN systems and cellular telephone systems. Aimed at wireless design, R&D, and production test engineers, the Aeroflex 3280 Series’ digital demodulation is ideal for the analysis of 802.11a, b and g wireless networks.

The demodulation of analog AM and FM signals and the demodulation of digital modulation formats both require a zero span condition before selecting the appropriate demodulator, then additional display and processing options are offered.

Each type of digital modulation format requires a slightly different software suite. This requirement is catered for in the 3280 Series under the “mode” key that allows the user to select the required modulation format.

The new digital demodulation option hardware provides the user with two unique choices of operation:

1) Full-frequency mode allows operation over the full frequency range (up to 26.5 GHz) when the digitizer can be connected directly to the output of the first IF that has a BW of 50/120 MHz, dependent upon the selected operating-center frequency.

2) Dual-channel, or direct-input mode, links directly from the instrument’s front panel or optionally via a rear panel connector to create a unique dual-channel instrument. The frequency range for the direct input of the demodulator is 300 MHz to 3 GHz. This frequency range allows coverage of IEEE 802.11b and 802.11g, while the 3280 Series measures 802.11a systems by utilizing the spectrum analyzer front end as a fixed tuned downconverter (13.2 GHz or 26.5 GHz units) with the first IF (421.4 MHz) feeding the digitizer input. This is achieved by setting the analyzer into zero span mode. The same technique can be used for any signals above the digitizer direct input of 3 GHz and below 330 MHz.

Although the unit’s hardware does not directly output analog I/Q signals, a further hardware option is offered for users who require streaming I/Q data. The low voltage differential signal (LVDS) cable option provides a streaming digital I/Q output from the digitizer option that the user may connect to an appropriate card installed in a PC to capture the digital bit-stream for further processing.

Software options provide demodulation of several digital modulation systems with their normally required performance parameters. The WLAN software option covers IEEE 802.11a, 802.11b and 802.11g and provides displays of constellation diagrams, error vector magnitude (EVM) versus frequency, EVM versus chips (symbol rate), adjacent channel power (ACP, 802.11b), and complementary cumulative distribution function (CCDF).

The constellation and EVM functions provide a useful insight into failures in the system and can be used to troubleshoot some of the common problems. By selecting the constellation display and comparing the diagram with the ideal, asymmetry can identify IQ imbalance or a tilted display can point to quadrature error. Non-square corners can point to compression in amplifiers. Switching to EVM, the power versus time plot can show up amplitude non-linearity by observing the error peaks and group delay issues can be identified with a bath tub type curve in place of a flat response.

When selection of the digital demodulator is required, the user is presented with soft keys to select either “basic,” the base mode of the digitizer, or one of the digital demodulation modes, e.g. WLAN.

As for AM or FM modes, the digital modulation modes require that the analyzer is operating in “zero span” mode and the operating software ensures this is the case when any of these modulation modes are selected.

Digital Demodulation - Basic Mode
The “Basic” mode of the digitizer option provides simple FFT spectrum analysis of a digitally modulated signal and user screens allow selection of power versus frequency, power versus time and frequency error versus time. The FFT analysis package is automated so that the user-selected span determines the number of samples for the analysis. This mode is provided to allow the user to check that the wanted signal is correctly tuned to the center of the display.

Digital Demodulation - WLAN Mode
Selection of WLAN provides the user with a selection choice of three IEEE 802.11 standards (a, b & g). Analysis tools include: spectral masks with pass/fail limits, adjacent channel power (ACP), error vector magnitude (EVM), IQ polar vector and IQ wave versus time. A tabular measurement screen is also selectable for display of all measured parameters. For example, when IEEE 802.11b is selected, the following parameters are displayed: system type, modulation type, data rate, frequency error, number of bits, number of symbols, carrier leakage, RMS EVM percentage, peak EVM percentage and chip clock error.

The high performance range of spectrum analyzers from Aeroflex covers frequency ranges from 3 Hz to 3 GHz, 13.2 GHz and 26.5 GHz. The analyzers incorporate an all digital IF system to provide regular resolution bandwidths (RBW) from 1 Hz to 5 MHz and have a low displayed average noise level (DANL) of < -150 dBc in a 1 Hz bandwidth. An optional tracking generator covering 9 kHz to 3 GHz is available for all three models.

The units feature a 10.4-inch LCD display, one of the largest available on the market, to display the spectrum traces, the soft key menus and can provide additional tabular data for many of the built-in measurement functions.

A low noise local oscillator system provides the ability for the analyzers to incorporate a phase noise measurement mode that allows the user to apply a signal and plot its noise profile. Frequency offsets from 10 Hz to 1 GHz are user selectable and are displayed in a log frequency format as a ratio to the applied input signal level in dBc/Hz. The spectrum analyzer local oscillator remains phase locked at a fixed frequency determined by the input signal frequency, as would be the case for zero span operation.

AM and FM demodulators are user selectable and supplied as standard.

Built in as standard are common measurement functions such as occupied bandwidth (OCBW), adjacent channel power (ACP), channel power, harmonic distortion and third order intermodulation (TOI).

Future software upgrades will incorporate IEEE 802.11n and offer demodulation of WiMax/WiBro systems, along with a variety of other digital modulation systems.

Aeroflex Inc.
www.aeroflex-inmet.com
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