Spectrum analyzer self-calibration

Hewlett-Packard Journal, June, 1991 by Timothy L. Hillstrom, Joseph F. Tarantino

The 310.1875-MHz first IF signal is filtered by a four-section coupled helical-resonator bandpass filter having a bandwidth of approximately 3 MHz. Performance and ease of manufacture were important in the development of this filter. The helix assemblies within the filter are wound on supporting cores molded of a very low-loss fluoroplastic to minimize the reduction in resonator Q caused by dissipation loss. The resonators are aperture-coupled and the input and output connections are tapped directly onto the helix assemblies. Tuning screws with self-locking pellets make tuning of the filter easier than if lock nuts were used. The helical-resonator filter months directly into a cutout at the edge of a printed circuit board and the whole assembly slides into a card slot as a unit.

The second conversion translates the signal from 310.1875 MHz. The local oscillator for this conversion (the second LO), is a phase-locked 300-MHz signal also used as an offset in the main (swept) LO. A seven-section LC filter and several gain stages make up the second IF section. To achieve the required instrument amplitude accuracy, the behavior of all of the IF amplifiers over temperature needed careful attention. Since operating frequencies and amplitudes are different for each IF section, various approaches were involved, but each amplifier stage received individual attention.

The third conversion is accomplished with an active mixer, providing gain as well as frequency translation. The third LO is a 10-MHz signal phase-locked to the instrument's common reference. This conversion translates the signal to 187.5 kHz. Following the conversion, the signal is further amplified and filtered, yielding a final IF bandwidth of approximately 40 kHz. An attenuator (settable in 1-dB steps under main processor control) is set so the signal is applied to the analog-to-digital converter at the optimum level. Conflicting performance goals made the design of the third IF challenging. To prevent degradation of swept measurement data, the transient response of the filter must be well-behaved. However, for good narrowband zoom performance, the frequency response of the filter should be reasonably flat over the entire 40-kHz IF bandwidth, and a filter designed for flat amplitude response often has poor transient response. The filter circuit was simulated and refined until good transient response at high sweep rates was achieved, and the flatness variation was kept to a level that can easily be calibrated out.

The analog-to-digital converter section consists of a track-and-hold circuit operating at 250 kHz followed by a two-pass converter. This entire section was leveraged from a previous product. [2] The final resolution bandwidth filtering, detection, and video filtering are all accomplished digitally in the HP 3588A by two custom gate arrays.

Digital IF Section

One of the goals for the HP 3588A project was to reuse existing modules wherever practical to minimize development costs. Hewlett-Packard dynamic signal analyzers use a chip set of gate arrays that perform many signal processing steps: triggering, optional quadrature detection from an arbitrary center frequency, programmable information bandwidth reduction by decimation filtering, and accumulation of complete sample records for later block processing. The HP 3588A digital IF section needed to be compatible with the same analog-to-digital converter as this chip set and at similar sample rates. It also needed to provide programmable information bandwith reduction and be fairly inexpensive to build. Modification of the dynamic signal analyzer signal processing chip set seemed to be a good approach for the HP 3588A digital IF section, since a rapid development cycle was desired.