A 10-Hz-to-150-MHz spectrum analyzer with a digital IF section

Hewlett-Packard Journal, June, 1991 by Kirsten C. Carlson, James H. Cauthorn, Timothy L. Hillstrom, Roy L. Mason, Joseph F. Tarantino, Jay M. Wardle, Eric J. Wicklund

THE HP 3588A SPECTRUM ANALYZER (Fig. 1) is a heterodyned, synthesized instrument with a tracking source. It is designed for spectrum and scalar network measurement from 10 Hz to 150 MHz. The key measurement contribution of the HP 3588A is fast, high-resolution, narrowband measurements. The HP 3588A is a pioneer in a new era in which spectrum analyzers will increasingly use digital signal processing technology to improve performance and features. By combining digital filtering and FFT (fast Fourier transform) analysis with traditional swept spectrum analysis techniques, the HP 3588A typically provides four times faster swept measurements and up to hundreds of times faster narrowband measurements than were previously available in a swept analyzer. The HP 3588A accomplishes this while achieving superior amplitude accuracy.

The HP 3588A brings digital signal processing technologies to RF spectrum analysis in two ways. For improved swept measurements, the HP 3588A is the first RF spectrum analyzer with an all-digital final IF (intermediate frequency) section. The selectable bandwidth of the IF section's digital filters determines the resolution bandwidth of the measurement. The filters have precise, predictable characteristics and performance that would be impractical to achieve using analog technologies. Signal detection for swept measurements is also done digitally. FFT signal processing is used for a narrowband zoom mode. For measurement spans less than 40 kHz, narrowband zoom mode provides all the speed and resolution to 150 MHz that previously were only available to users of FFT-based analyzers.

For swept measurements, the digital resolution bandwidth filters provide shape factors of less than 4:1 with the same inherent accuracy obtained using more traditional IF filters with typical shape factors of 11:1. Since these filters are implemented digitally, their response is completely predictable and repeatable, and they can be swept faster than the traditional rate of one half the square of the resolution bandwidth. Any effects of the filters on the response are known and are calibrated out of the measurement. The result is a swept measurement that is typically four times faster and has better than two times narrower shape factor than analyzers with analog resolution bandwidth filters. It is important to note that these speed improvements are achieved together with the improved shape factors. If speed comparisons are made for equivalent 60-dB bandwidths, even more dramatic improvements are seen. Speed improvements under these conditions can be in excess of 16 times for swept measurements and 1000 times for narrowband zoom measurements.

FFT analysis for fast narrowband zoom measurements is available at any center frequency from 10 Hz to 150 MHz. Signals from the last analog IF section are digitized and subsequently filtered. Frequency spans from 40 kHz down to 1.2 Hz are available in this mode, and signals can be resolved with 4.5-mHz resolution at frequencies to 150 MHz. An ovenized reference oscillator is available to provide the frequency stability necessary to make these narrowband measurements.

All instrument functions are available over the HP-IB (IEEE 488.2, IEC 625), and the instrument can act as an HP-IB system controller as well. Optional HP Instrument BASIC is available for programming and automated testing. A 3.5-inch disk drive is an option for storage of instrument states, data, and Instrument BASIC programs.

Principal applications for the HP 3588A include general spectrum and scalar network analysis to characterize circuits and systems in laboratory and automated production environments. Examples of applications that benefit from some of the new features of this instrument include:

* Analysis of nonstationary or short-duration signals such as vibration-induced sidebands on oscillators

* Radio surveillance and measurements of moving sidebands on modulated carriers

* Circuit adjustments where fast feedback of results saves time

* Automated testing using the instrument's internal Instrument BASIC programming to control repetitive tasks (system controller capability over the HP-IB allows system integration without the need of a separate controller in ATE applications)

* Phase noise and broadband noise measurements (because of the detection scheme used, a true rms level is given without correction factors).

Development Challenges

Throughout the development of this product, many technical obstacles were overcome in integrating the digital IF and FFT technologies into the instrument:

* Custom gate arrays were developed for the digital filters.

* Because the instrument measures to 10 Hz, a special mixer design and a local oscillator (LO) feed-through cancellation circuit were developed to reduce local oscillator signal levels in the IF path at low frequencies.

* To achieve good amplitude accuracy in FFT-based measurements, a special technique was developed to calibrate out analog IF frequency response effects.

* Residual spurious signals within the IF passband that would never have been resolved in a swept-only analyzer were initially seen using an FFT, requiring special attention in development to the reduction of spurious and residual signals.