Other frequencies than expected are selected during the initial prescan

[incognito]

Hi,

We have configured a multiband emission test, that is using 3 bands:

• 150 kHz - 1 MHz
• 1 MHz - 5 MHz
• 5 MHz - 30 MHz

When this test is performed, we however also see that other frequency ranges are selected on the analyser. And these ranges do not seem to be measured. We for example see these frequency ranges:

• 2.575 MHz - 3.4 MHz
• 15.5 Mhz - 19.5 MHz

What is going on? And is this a problem?

[joro]

The temporary selection of these frequency ranges is indeed happening, and there is a very logical explanation for it.

Using the band configuration that you described, the following actions are performed:

1. The first band is being measured on the analyser for the frequency range from 150 kHz to 1 MHz. The analyser is then using a span of 850 kHz.
2. Then the analyser is changed to the next band.
3. First the center frequency is adjusted to 3 MHz. This is the middle of the desired 1 MHz – 5 MHz band. With the previous span of 850 kHz this results in a temporary selected range of: 2.575 MHz – 3.425 MHz.
4. After the center frequency is adjusted, the span is widened to 4 MHz such that a desired band of 1 MHz – 5 MHz is selected.
5. The second band is now selected, and is being measured on the analyser
6. Then the analyser is changed to the next band.
7. First the center frequency is adjusted to 17.5 MHz. This is the middle of the desired 5 MHz – 30 MHz band. With the previous span of 4 MHz this results in a temporary selected range of: 15.5 MHz – 19.5 MHz.
8. After the center frequency is adjusted, the span is widened to 25 MHz such that a desired band of 5 MHz – 30 MHz is selected.
9. The third band is now selected, and is being measured on the analyser

Changing from a frequency band to another frequency band is always done in two steps, first setting a higher center frequency, or first reducing the span, to ensure that no invalid situation is being triggered. Please remark that in the above examples, if we would have enlarged the span first, it would have caused the selection of a negative start frequency! That is why the center frequency is first adjusted in these cases.

During these temporary frequency changes, indeed no sweeps are taken, because that is not needed at all.