Spectrum analyzer self-calibration

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

The HP 3588A can free-run on its internal 80-MHz VCXO, lock to an optional 10-MHz oven reference, or lock to a user's reference at 10/N MHz, where N is an integer from 1 to 10. The 80-MHz VXCO is designed to have low temperature drift and good stability for users who do not require the high performance of the optional ovenized reference. The 80-MHz reference is used to provide quadrature 20-MHz clocks for the digital filters, using a Johnson counter on the same circuit board as the filters. 10-MHz references are derived from the 80-MHz VCXO and are used in various places throughout the instrument.

The 300-MHz reference is an LC oscillator that acts as the LO for the second conversion and provides an offset frequency in the LO step loop (see Fig. 3). With this architecture, the net phase noise caused by the 300-MHz reference is:

(300-MHz phase noise) x (1 - [H.sub.step.H.sub.sum]),

where [H.sub.step] and [H.sub.sum] are the closed-loop phase transfer functions of the step and sum loops, respectively (with the gain of the step loop normalized out0.

This effectively high-pass filters the 300-MHz phase noise (in the offset frequency domain) and provides attenuation at low offset frequencies. Thus, the requirement for spectral purity of the 300-MHz reference at low offset frequencies is eased by this effect, allowing the use of an inexpensive LC oscillator rather than a more expensive low-noise oscillator employing a SAW (surface acoustic wave) resonator.

Low-frequency references are provided for the source (187.5 kHz), the ADC sample clock (250 kHz), and the interpolation loop reference (100 kHz). Synchronization is maintained between all these references and the Start[underscore]Meas signal so that data taking and the LO sweep are synchronized and repeatable.

Further synchronization capability is provided so that several HP 3588A's can be configured to sweep synchronously, within a few microseconds uncertainty, using an IBASIC application program. This allows synchronized offset or harmonic sweeps, facilitating mixer or generalized distortion testing. To achieve this, this synchronizing circuit allows the low-frequency references and Start[underscore]Meas signal of all instruments to be synchronized through a Trigger Out signal provided in the rear panel.

Speed Advantages of FFT-based Spectrum Analyzers

For several decades, Hewlett-Packard has offered spectrum analyzers using FFT technology in the baseband frequency range up to 100 kHz. The FFT is used essentially as a bank of parallel bandpass filters. Since all the filters settle and measure simultaneously, the measurements can be mush faster than with a swept filter of similar resolution. This is very useful for transient signals and time-varying signals, so this technique has been termed "dynamic signal analysis". Typical window functions designed for amplitude accuracy are three to four FFT frequency samples wide. A 512-point FFT thus provides over a hundred parallel filters at once and may provide measurements over 100 times faster than a single analog filter.