With the continually increasing complexity of digital modulation techniques employed in wireless and satellite communications systems, instruments that accurately generate and maintain a set level of bit energy-to-noise density ratio (E_b/N_o) have become valued tools in optimizing bit-error rate (BER) performance. These instruments have been steadily enhanced over the years with greater accuracy, higher frequency ranges, wider power range and control, and many other features that help make characterizing digital receiver performance more straightforward. The UFX-EbNo instrument is an E_bN_o generator that takes this performance a step further with comprehensive automation, output frequencies to 44 GHz and greater reliability.

The ratio of the actual E_bN_o to the required E_bN_o for a desired BER (system margin) is the broad determinant of how well a wireless communication system will perform under signal conditions that are affected by factors such as rain, hail or dust-induced attenuation. Every modulation scheme, such as BPSK, QPSK and QAM, has a different curve illustrating its theoretically-achievable BER for a given E_bN_o. The sharp E_bN_o curves of complex digital modulation schemes drive the need for extremely accurate instruments that generate a precise level of E_bN_o over a wide range of power levels and operating frequencies.

The UFX-EbNo generator is designed to satisfy these requirements. It is the successor to the UFX-Series, which was one of the industry's first solutions for E_bN_o generation, and was incorporated in test systems by manufacturers of subsystems and systems for wireless and satellite communications, as well as in various military test systems. The new instrument builds on the core capabilities of the company's precise noise generators, while focusing on the generation of accurate bit energy-to-noise density ratios rather than precise absolute noise power levels.

The basic application of the UFX-EbNo unit is to simplify and speed up the process of BER testing by automatically generating and setting carrier-to-noise ratio (C/N) and E_bN_o so that the performance of the system can be evaluated with confidence. It operates in five modes: C/N, E_bN_o, carrier-to-noise density (C/N_o), carrier-to-interferer (C/I) and as a true RMS power meter, while the focus is on E_bN_o and C/N ratios. It can also function as a precision generator of white Gaussian noise. All of the relevant input and output signal levels of the chosen operating mode can be displayed simultaneously, including carrier-to-noise ratios.

The various functions of the UFX-EbNo generator are highly automated, and the instrument compensates for bit rates, bandwidth and other factors. Most measurements can be performed by pressing a single button. For E_bN_o testing, the instrument automatically calculates noise density based on a user-specified bit rate, carrier output level and output E_bN_o ratio. To obtain a specific C/N_o ratio, the UFX-EbNo unit is simply set to E_bN_o mode and zero is entered for the bit rate. The instrument creates the required value of C/N_o.

The key to the accuracy and repeatability of this process is the instrument's ability to generate and maintain its specified characteristics over a wide range of power levels and operating frequencies. To achieve this, the UFX-EbNo generator is designed around precision components, including the calibrated noise sources, which have been the company's core product line for more than 18 years. In addition, the instrument has an integral RMS digital power meter, which is customized for operation within the frequency range of the instrument, and provides a C/N accuracy of ±0.2 dB received signal strength (RSS) within its range of -55 to +5 dBm. Leveraging on the company's experience and the power meter technology of Boonton Electronics, another Wireless Telecom Group member, the instrument can measure both the carrier signal and noise with Gaussian noise crest factors up to 18 dB. The power meter uses various averaging methods to ensure accurate measurements, and all measurements are made through couplers that allow the signal to pass through to the output connector without degradation by the power meter circuitry.

The UFX-EbNo generator utilizes the substitution calibration method to set all of the desired carrier-to-noise ratios. This technique eliminates the nonlinearity effects that can result from the instrument's power meter by maintaining both signal and noise at the same level at the power meter's input. The noise power can then be offset by the desired ratio. While the attenuator that varies the instrument's noise output power is the greatest potential source of inaccuracy, its impact on overall performance is negligible because the attenuators are well-characterized, precision devices.

Secondary effects such as thermal drift are also negligible because the measurement samples of both noise and power are very short. Active components can also be contributors to long term drift, but since they are common to the signal and noise paths, their variations do not affect the calibrated ratio. In addition, the signal path for both phase and amplitude is linear, which ensures that the desired signal passes through the instrument undistorted.

In every mode, data entry is a simple process, and front-panel indicators and a 4 x 20 line display provide clear indication of instrument settings. The UFX-EbNo instrument can be specified with remote control capabilities via IEEE-488-2, RS-232C, RS-422, RS-423, TCP/IP or Ethernet. A wide variety of options are available, such as a tracking feature that lets users with unstable input signals perform accurate long term testing at a given E_bN_o ratio by correcting input signal drift with 0.2 dB resolution to ensure a constant value of E_bN_o. Other options include 0.1 dB per step attenuation of output noise and C/N, dual channels, uninterrupted carrier during E_bN_o calibration, and 0.01 dB attenuation and E_bN_o resolution.

The number of standard frequency ranges available with the UFX-EbNo has been increased, and they now range (in bands) from 5 to 90 MHz, to 18 to 22 GHz, with other custom frequency ranges available between 5 MHz and 44 GHz. Each frequency range corresponds to a particular communications service, such as satellite communications frequencies, cable modems, 802.11a and 802.11b wireless LANs, and all current wireless communications bands. Table 1 lists the generator's performance specificiations.

Noise Com Inc., Parsippany, NJ (201) 261-8797, www.noisecom.com. Circle No. 304