What does sequences like 1-3, 1-10, 1-3-10 etc in a spectrum analyzer mean?
Notation like "1-3", "1-10", "1-3-10", etc. describes how the range for a particular parameter is varied in steps.

Instruments often cover wide dynamic ranges, over several decades. For example, a voltmeter might cover anywhere from 200 mV to 2000V full-scale, and will have range selections of 200 mV, 2 V, 20 V, 200 V and 2000 V. Each one is 10&times; the previous one; on a datasheet, this would be described as "1-10" sequence.

With oscilloscopes and spectrum analyzers, such steps are often too coarse, so they sub-divide each decade using a roughly logarithmic scale. If they add one additional step per decade, they use a "1-3" or "1-3-10" sequence. For RBW, it might go 100 Hz, 300 Hz, 1 kHz, 3 kHz, 10 kHz, etc. The ratios between successive steps alternate between 3&times; and 3.33&times;. These two ratios are close to each other, creating a rough approximation to a logarithmic scale.

In some cases, you want more choices than that &mdash; three steps per decade &mdash; so you'll see a "1-2-5" sequence. For example, the horizontal timebase on a scope might go 100 ns, 200 ns, 500 ns, 1 &micro;s, 2 &micro;s, 5 &micro;s, 10 &micro;s, etc. Now the ratios between successive steps follow a pattern of 2&times;, 2.5&times; and 2&times;. Again, these ratios are all similar to each other.