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AS3935

Datasheet - Detailed Descri p ti on

8.8 Noise Floor Generator and Evaluation

The output signal of the AFE is also used to generate the noise floor level. The noise floor is continuously compared to a reference voltage (noise

threshold). Whenever the noise floor level crosses the noise threshold, the AS3935 issues an interrupt (INT_NH) to inform the external unit (e.g.

MCU) that the AS3935 cannot operate properly due to the high input noise received by the antenna (e.g. blocker). It is possible to set the

threshold for the noise floor limit with the bits REG0x01[6:4], as defined in the table below.

INT_NH is displayed as long as the input noise level (blocker) is higher than the noise floor threshold. By default the setting REG0x01[6:4] =010

is used.

8.9 Lightning Algorithm

The lightning algorithm consists of hardwired logic. False events (man-made disturbers) which might trigger the AS3935 are rejected, while

lightning events initiate calculations to estimate the distance to the head of the storm.

The Lightning algorithm is broken up into three sub blocks:

1. Signal validation: Verification that the incoming signal can be classified as lightning.

2. Energy calculation: Calculation of the energy of the single event.

3. Statistical distance estimation: According to the number of stored events (lightning), a distance estimate is calculated.

If the signal validation fails (the incoming signal does not have the characteristics of lightning), the energy calculation and statistical distance

estimation do not happen and the event is classified as disturber.

8.9.1 Signal Validation

The watchdog enables the lightning algorithm block in the event any activities are detected at the antenna. As this happens the output signal of

the AFE is evaluated by the Signal Validation block, which checks the pattern of the received signal. The signal validation checks the shape of

the received signal. In particular, the AS3935 can reject the impulse signals, like spikes, picked up by the antenna. The AS3935 has the ability to

improve the spike rejection with the register REG0x02[3:0]. By default, register REG0x02[3:0] =0010. Larger values in REG0x02[3:0]

correspond to more robust disturber rejection, with a decrease of the detection efficiency, as shown in the Figure 21.

Table 16. Settings for the Noise Floor Threshold

Continuous Input Noise Level

[µVrms] (outdoor)

Continuous Input Noise Level

[µVrms] (indoor)

REG0x01[6]

REG0x01[5]

REG0x01[4]

390

28

0

0

0

630

45

0

0

1

860

62

010

1100

78

0

1

1

1140

95

1

0

0

1570

112

1

0

1

1800

130

1

1

0

2000

146

1

1

1