Microwave signal analysis once brought to mind a slow-sweeping spectrum analyzer with unsteady cathode- ray-tube (CRT) screen to display signal traces barely above the noise floor. By leveraging performance advancements in semiconductors, digital signal processing, and analog components, however, Agilent Technologies has given rise to its PXA series of high-performance signal analyzers that keep the noise floor low and signal levels high enough to read clearly, at frequencies through 26.5 GHz. The firm currently offers four PXA signal analyzer models covering 3 Hz to 3.6 GHz (model N9030A-503), 3 Hz to 8.4 GHz (model N9030A-508), 3 Hz to 13.6 GHz (model N9030A-513), and 3 Hz to 26.5 GHz (model N9030A-526).
The highest-performance member of the PXA family is the N9030A signal analyzer (Fig. 1), which offers a standard instantaneous measurement bandwidth of 10 MHz and real-time measurement bandwidths as wide as 140 MHz at frequencies to 26.5 GHz. The PXA instruments feature extremely low noise levels, with phase noise of -128 dBc/Hz offset 10 kHz from the carrier and only -145 dBc/Hz offset 1 MHz from the carrier. The displayed average noise level (DANL), which is a practical measure of the lower limit of the analyzer's dynamic range, is -160 dBm through 2 GHz and -155 dBm at 12 GHz using the analyzers' built-in noise-floor-extension (NFE) technology. In addition, a DANL of -155 dBm can be achieved at 12 GHz without NFE technology by switching to a low-noise path (LNP) signal routing. At the other end of the dynamic range, the third-order intercept (TOI) is better than +20 dBm at 2 GHz and better than +17 dBm at 12 GHz. The third-order dynamic range is 115 dB at 2 GHz and 112 dB at 12 GHz.
The PXA analyzers provide analysis bandwidths of 10, 25, 40, and 140 MHz with amplitude accuracy of at least 0.52 dB and as good as 0.19 dB with 95-percent confidence. Across the maximum available measurement bandwidth, the analyzers offer intermediatefrequency (IF) response of 0.63 dB to 3.6 GHz and 1.13 dB beyond 3.6 GHz. The wideband-code-division-multipleaccess (WCDMA) adjacent-channel power ratio (ACPR) is -81 dBc under normal operating conditions and with the NFE technology applied, and -83 dBc when using noise correction.
The new PXA analyzers incorporate a number of design innovations to achieve their high levels of performance, in particular with regards to extending the usable dynamic range. For example, noise generated by components within an instrument's signal path, such as amplifiers, can limit an analyzers' capability of detecting and accurately measuring low-level signals. Low-level signals are typically lost in the noise floor. And if higher-level signals are present and must be measured or processed, attenuation is typically added to reduce the amplitude of those higherlevel signals and prevent them from overloading the analyzer's front-end electronics and causing distortion. But the use of attenuation will also reduce the amplitude of the lower-level signals, often dropping them to the level of the noise floor and making it difficult to detect and measure those signals. In a spectrum analyzer, resolutionbandwidth (RBW) filters are used to improve the measurement selectivity when reading low-level signals, but the use of narrow RBW filters brings with it a tradeoff in measurement sweep speed. This tradeoff has long been an accepted part of using a signal or spectrum analyzer.
In order to overcome this tradeoff, and improve low-level measurement capability, the PXA analyzers feature a number of design improvements compared to traditional analog or digital signal analyzers. For example, the analyzers employ a noise-floor-extension (NFE) approach that can effectively improve the capability of extracting low-level signals from the noise floor, by as much as 3.5 dB for continuous-wave (CW) and pulsed signals and about 8 dB for noise-like signals, to as much as 12 dB for some applications. The NFE function provides a dramatic improvement in performance whether using an external preamplifier or the built-in optional preamplifier offered with the PXA analyzers (Fig. 2). The typical DANL for the PXA analyzers is -161 dBm without a preamplifier and as good as -172 dBm with a preamplifier.
Losses through the signal paths in a signal analyzer can also raise the noise floor and obscure low-level signals. To improve low-level-signal measurement capability in the PXA analyzers, an optional low-noise path (LNP) can be switched in. The path bypasses the lossy circuit elements and components normally found in the input signal-processing chain of a spectrum or signal analyzer to improve the sensitivity of low-level-signal measurements at microwave frequencies (above 3.6 GHz). The use of the LNP option can eliminate the need for a preamplifier, as well as the use of narrow RBW filters that slow the sweep speed. For example, Fig. 3 offers an example of the measurement improvement possible using the LNP option. Without the LNP option, a RBW filter of 62 kHz would have been needed for the same results, increasing the sweep time from 57.2 ms to 7.18 s.
At the other end of the dynamic range, the PXA analyzers are equipped with a new RF microcircuit-based front-end architecture designed to handle higher-level input signals without distortion. The microcircuit arms the PXA analyzers with a high TOI point of better than +20 dBm at 2 GHz. In addition, an attenuation offset mode adds 12 dB of transparent attenuation to any selected input attenuation to minimize distortion from overloads. With their excellent DANL performance, the PXA analyzers can afford the additional attenuation. The PXA analyzers are code compatible with the company's earlier spectrum analyzers, including the HP8566, HP8568, and PSA models (to 26.5 GHz) to simplify installation into automatic-test-equipment (ATE). As an option, MATLAB data analysis software from The MathWorks is available directly from Agilent on all of the PXA analyzers. Agilent Technologies, 5301 Stevens Creek Blvd., Santa Clara, CA 95051; (877) 424-4536, (408) 345-8886, FAX: (408) 345-8474, Internet: www.agilent.com.