Capturing time-varying signals is a challenge for most measurement tools. Oscilloscopes, of course, are known for their ability to trigger on a high-speed event and show time-domain information about the captured waveform. But engineers seeking transient signal analysis in the frequency domain have had few options until now. With the introduction of the RSA2200A and RSA3300A Series of real-time spectrum analyzers from Tektronix, Inc. (Beaverton, OR), RF designers can now capture time-varying and transient signals at frequencies to 8 GHz, even when such signals carry wideband modulation.

The RSA Series real-time spectrum analyzers (see figure) allow an engineer to capture an entire frequency span at one time by triggering a selected bandwidth around a center frequency. In contrast, a conventional spectrum analyzer uses superheterodyne receiver techniques to sweep a local oscillator (LO) across a frequency span of interest, with a resolution-bandwidth filter also employed to suppress unwanted spurious and harmonic signals in the band of interest. The conventional method provides an excellent way to study stationary signals or signals with known narrow modulation bandwidths, although it is less than ideal for examining transient signals or signals with wide modulation bandwidths. The real-time spectrum analyzer captures a full time record of signals across an entire frequency span of interest. Similar to a still-photography camera, the real-time analyzer captures every signal occurring in the bandwidth of interest during the instant in which that "frame" was captured. The real-time spectrum analyzer can continue to capture and store these frames until stopped, storing a series of time-sequenced frequency spans that can reveal even the most difficult-to-capture transient signals.

In addition to this wideband signal capture capability; the real-time spectrum analyzers share some of the triggering capabilities of modern digital storage oscilloscopes (DSOs). The analyzer can be programmed to trigger on specific frequency and amplitude conditions, for example, triggering on signals that fall within or outside of a defined amplitude/frequency mask.

The RSA2200A Series analyzers include RSA2203A, which operates from 10 MHz to 3 GHz (with optional extension to DC) and the RS2008A, which operates from 10 MHz to 8 GHz (with optional extension to DC). Both instruments feature 2-MB signal capture memory and maximum signal capture bandwidth of 10 MHz.

The RSA3300A Series analyzers include the RSA3303A, which operates from DC to 3 GHz, and the model RSA3308A, which operates from DC to 8 GHz. Both instruments feature 15-MHz signal-capture bandwidths, and 64-MB standard signal-capture memory (with option for 256-MB signal-capture memory). Both series of instruments are equipped with demodulators to process a wide range of modulation formats, including amplitude modulation (AM), frequency modulation (FM), amplitude-shift-keying (ASK) modulation, frequency-shift-keying (FSK) modulation, and phase modulation. The real-time spectrum analyzers provide the measurement power to capture in-phase (I) and quadrature (Q) modulation signal components like a vector signal analyzer, but with the flexibility of frequency-domain triggering.

Although both series of analyzers offer unique measurement capabilities, they can also be used in conventional spectrum-analyzer mode to "sweep" across a frequency range using resolution-bandwidth filters from 1 Hz to 10 MHz in the RSA2200A analyzers and to 15 MHz in the RSA3300A analyzers. The analyzers also offer a time-domain mode based on the use of Fast Fourier Transform (FFT) capability to display signal information as a function of time using a variety of display window types (such as Hamming, Blackman, Parzen, Welch, and Blackman-Harris windows).

Noise sidebands in the RSA2200A instruments are below −133 dBc/Hz measured 7 MHz from a 1-GHz center frequency, and below −132 measured 7 MHz from a 2-GHz center frequency. Noise sidebands in the RSA3300A instruments are below −135 dBc/Hz measured 7 MHz from either a 1- or 2-GHz center frequency. Reference levels can be set from −51 to +30 dBm with a wide range of attenuation capabilities; amplitude marker readout resolution is a precise 0.01 dB. Frequency resolution can be set from 1 MHz to 1 MHz; the carrier-frequency-measurement accuracy is ±4.01 kHz at 2 GHz.

These real-time spectrum analyzers bring new measurement capability to a wide range of applications. The analyzers are well equipped with digital-signal-processing (DSP) capabilities for detailed analysis. P&A: $22,990 and up; six weeks. Tektronix, Inc., 14200 SW Karl Braun Dr., P.O. Box 500, M/S 55-513, Beaverton, OR 97077; (800) 426-2200, FAX: (503) 627-3678, Internet: www.tektronix.com.