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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 6/20

Sampling and FFT calculation

The K-LD2 works with an internal I/Q doppler signal

sampling and a computation of a 256 point wide com-

plex FFT. I/Q doppler signals are phase shifted by +90°

or -90° depending on the direction of a movement in

the front of the sensor.

The signal processing unit samples the I/Q data

with a configurable sampling rate (see parameter S04)

Figure 5: I/Q doppler signals of an approaching movement (left) and a receding movement (right)

and  computes a complex FFT. The sampling rate is

an important parameter of the sensor because it direc-

tly estimates the speed resolution, the maximal speed,

and the response time of the system. The  response

time is doubled if the FFT average feature (described

below) is used.

Parameter

S04

Sample rate

[Hz]

Resolution

[Hz]

Max. frequency

[Hz]

Resolution

[km/h]

Max speed

[km/h]

Response time

[ms]

01

1280

5

640

0.11

14.3

200/400

02

2560

10

1280

0.22

28.6

100/200

03

3840

15

1920

0.34

43.0

67/134

04

5120

20

2560

0.45

57.3

50/100

05

6400

25

3200

0.56

71.6

40/80

06

7680

30

3840

0.67

85.9

33/66

07

8960

35

4480

0.78

100.2

29/58

08

10240

40

5120

0.89

114.5

25/50

09

11520

45

5760

1.01

128.9

22/44

0A

12800

50

6400

1.12

143.2

20/40

Table 2: Sampling rate vs. speed resolution vs. maximal speed vs. response time

The sampled I/Q doppler signals are transformed

with a complex FFT into the frequency domain with

256 bins. Those signals appear either in the real (right)

plane for an approaching movement or in the imagi-

nary (left) plane for a receding movement. The signal in

the centre is the DC offset caused by the amplifier and

the analogue to digital conversion.

To reduce random noise, the sensor features a FFT

average option (see parameter S0A) which is enabled

in the factory settings. It  is an average over two

FFT frames.