Simple Scalar Spectrum Analyzer

Simple Scalar Spectrum Analyzer

Sam Wetterlin

10/4/06

The following scans were made on an oscilloscope using the Simple Scalar Spectrum Analyzer (“SSSA”). The schematic of that device and the method of use is shown at the end of this document. In short, the input comes from a sweep generator and the output is displayed on a scope. The SSSA displays either the transmission characteristics or return loss of the DUT.

FIGURE 1—Transmission of 7ft. coax. Scanned 1-150MHz.

Each large vertical division is 10db (200mv). The transmission declines about 4db

due to attenuation. (And in small part due to declining “intercept” of

the AD8310 detector.

FIGURE 2—Return loss of 7ft. terminated coax. Scanned 1-150MHz.

Each large vertical division is 10db (200mv). Return loss is the difference between

the baseline and the displayed value.The baseline values (100% reflection)

are approximately the same as shown in Figure 1, or about 20db above the scope

center line, which puts the leftmost peak at RL=30db. Depending on preference,

the trace could be inverted. The series of peaks is characteristic of cable return loss, and results from the fact that the cable is not a perfect 50 ohms.

FIGURE 3—Transmission of 35MHz LPF. Scanned 1-100MHz.

Each large vertical division is 10db (200mv). The baseline is 30db above the scope

center line. This demonstrates a 65db dynamic range.

FIGURE 4—Return Loss of 35MHz LPF. Scanned 1-100MHz.

Each large vertical division is 10db (200mv). The baseline is 30db above

the scope center line. Maximum return loss is on left at about 30db,

and is near 0 above the passband of the filter.

FIGURE 5—Transmission of 10.7MHz LPF. Scanned 7.7-13.7MHz.

(20db per large vertical division)

This is the LC noise filter from the AD8330/AD8367 AGC loop. I did

not establish a baseline. I also did not show return loss, because the actual

filter is isolated from the input by resistors, so the return loss is very flat and uninteresting.