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RFbeam Microwave GmbH | Schuppisstrasse 7 | CH-9016 St. Gallen | www.rfbeam.ch | K-LD2 | data sheet

09/2018 - Revision B | Page 8/20

Start up time

During start up, the sensor calculates the mean over

the number of FFT frames specified with the para-

meter start up learn. The start up time of the sensor

depends on this parameter, the sampling frequency

and the FFT size.

Threshold generation

The calculated mean during start up represents

the  noise floor of the sensor and is stored as spec-

trum average. During operation the spectrum average

is adapted continuously. The speed of this adaption is

configurable using the parameter threshold noise ad-

aption speed. This mechanism automatically adapts

interferences that are present in both planes of the FFT.

This adaptive spectrum average is used together with

the parameter minimum threshold margin to genera-

te the minimum possible threshold level. This means

that the threshold level for each bin cannot be smaller

than the spectrum average + the minimum threshold

margin setting and this is independent of the sensitivity

setting. Adapted interferences are thus automatically

filtered out in the threshold level and do not generate

a detection.

The noise floor of different sensors can vary. The

sensitivity setting is referenced to the ground line in

order to get an as constant as possible movement de-

tection over different sensors.

The threshold level is defined as an addition of the

parameter minimum threshold offset and the set sen-

sitivity setting for each bin (Further information about

the adjustment of the sensitivity setting can be found

in chapter Adjust Hold Time and Sensitivity).

t

=

ƒ

Sample

N

ƒ

Sample

256 N

Value of S05

Figure 8: Minimum threshold level and interference adaption

If the addition of the minimum threshold off-

set and the set sensitivity setting is smaller

than the minimum threshold level (defined

over the spectrum average and the parame-

ter minimum threshold margin), the threshold

is set to its minimum level.