Spur is a powerful aid for designing frequency converters. It runs on Macintosh PowerPC computers but is not currently available for downloading. A shareware version is being considered.

Here I will describe some of the features that can be seen in the views that appear on the main page. A similar view is shown below for improved clarity. (Clarity for these Spur plots appears to be much better with Internet Explorer than with Netscape.)

Mixer responses are represented by lines (dark blue here) in a plot of LO (local oscillator) frequency vs. RF (the input) frequency, both normalized to the IF (output frequency). The normalization permits an essentially 3-dimensional figure to be viewed in 2 dimensions. Frequencies corresponding to the cursor location are displayed on the right for chosen values of one of the three variables. When the mouse cursor moves over one of the lines, the spur order (m x n) and relative strength are displayed at the top of the screen. This information can also be printed on the corresponding curve. (The cursor is not visible in these "screen shots.")

The frequency range of an input filter is represented by a horizontal line (red here). If a spur curve passes through that line, the spur will pass through the input filter. If the LO frequency varies (e.g., when a receiver is tuned), the line becomes a rectangle. Rectangles can be entered from the keyboard or by dragging the mouse or from a spreadsheet. Probably the most common way is by use of a small dialog box, which is visible in one scene. If we also represent a finite range for the IF, rectangles acquire a diagonal extent, becoming polygons. If a curve passes through a polygon, there will be a spurious response at some combination of RF, LO, and IF.

Perhaps the most common designs would be represented by one polygon but multiple polygons, as illustrated here, are also useful when the design is broken into several bands.

Information concerning the represented frequencies can be read in a bubble when the mouse button is pressed at some point on the polygon. This is possible because the entered definition of the polygon (passband) helps to resolve the 3-dimentional display. Frequencies at the intersection of the polygon and a spur curve can be similarly read.

The larger of the windows that appears over the main window is a spur level chart, showing the relative amplitudes of the various spurs for a particular mixer and particular signal levels. Test data is entered in this window and, in a second mode, the signal strengths can be changed from test values to design values and the program then estimates the resulting spur levels with the design values. These are the levels that appear in the main window.

A data window is also available. Among other information, it gives the shape factors for RF and IF filters to filter the various spurs and indicates the attenuations needed at those shape factors.

Since Spur represents the design in 3 dimensions, it is not necessary to step any of the frequencies.

If you came from the home page, it should still be there. Otherwise, you can open the home page.