Spectrum analyzer self-calibration

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

The digital input to the signal processing chip set from the analog-to-digital converter is applied to two mixers simultaneously. The local oscillators for the two mixers are both at the same frequency, but are 90 degrees apart in phase. This provides two data streams, one representing the real part of the IF signal and the other representing the imaginary part. This quadrature detection is nearly identical to that done in the HP 3577A Network Analyzer. [3]

Swept spectrum analyzers detect the scalar magnitude of the input signals as they are swept through the analyzer's IF. The detector gate array converts the real and imaginary components of the signal into the magnitude of the signal. This magnitude is passed to the main processor for subsequent processing and display. The complex signal samples can also be passed into the main processor for use in a fast Fourier transform process whose output can then be converted into magnitude and phase.

The detector computes the magnitude by squaring the real and imaginary components and then adding them together. This results in the square magnitude of the filtered signal, which is proportional to the square of the input voltage or to the input power.

Since the resolution filter bandwidth may be much narrower than a bin (display point) on the instrument, a peak detection function is also required. This is a standard swept spectrum analyzer function. If the peak detector were not included and the response were just sampled, a response narrower in frequency than the equivalent frequency spacing of adjacent pixels on the display could be missed entirely. For this reason, all swept spectrum analyzers have a peak detector. The peak detector gets reset whenever its output is sent to the main processor and thus to the display. Each magnitude sample received by the peak detector is compared to the peak detector's current value. If the new magnitude is larger than the current value then the peak detector value is updated to the new value; otherwise, the current value is retained.

Swept analyzer users expect to see the screen updated as the instrument sweeps, so acquisition of a complete sample record is inappropriate. This means that no record needs to be acquired, and detected data needs to be immediately available to the main processor. The detected result is passed to the main processor by an on-chip DMA interface. This saves several ICs over the previous approach, which used a FIFO buffer to acquire a complete sample record.

Traditional swept spectrum analyzers must have a video filter. This filter is typically placed after a full-wave detector and removes the component at twice the intermediate frequency resulting from the full-wave detection. The detector in the HP 3588A can be called a power detector since it merely takes the sum of the squares of the real and imaginary components. This detection technique has no resultant component at twice the intermediate frequency, so no video filter is needed for most measurements. However, a video filter is provided for occasions where noise smoothing is desired.