Si4312

10

Rev. 0.5

3.5. RATIO Selection Used for the Slicer Threshold Calculation Window

In OOK modulation, many different encoding schemes exist, which can result in variable ON and OFF times as

seen in the example data waveforms shown in Figure 4. In order to determine the proper slicer threshold used for

demodulating the OOK signal into digital data, the Si4312 must sample an "ON" and "OFF" event in a time window

called the threshold calculation window. The samples during the "ON" time will determine the ON voltage, and the

samples during the "OFF" time will determine the OFF voltage. These voltages are used to determine the slicer

threshold voltage as follows:

Defining the BT variable from the last section as the ON time and

then, the threshold calculation window is calculated as follows:

The threshold calculation window is 1.25 times longer than the ON plus OFF times for margin to allow sampling of

both ON and OFF times. Figure 5 shows a graph of the threshold calculation window.

Figure 5. Example of the Threshold Calculation

Ratio is a unit-less multiplier that relates the OFF time to the ON time. The Si4312 defines two constants for RATIO

as shown in Table 10 based on the logic level of pin 5.

Table 10. RATIO Constants Based on Logic Level of Pin 5

Pin 5

RATIO

05

110

SlicerThreshold

Max_ON_Voltage

Min_OFF_Voltage

+



2

-----------------------------------------------------------------------------------------------------------

=

RATIO

OFF

ON

------------

=

Threshold Calculation Window

1.25

ON

OFF

+





1.25

BT

BT

RATIO



+





1.25

BT RATIO

1

+





=

=

=

Threshold Calculation Window

Digital Data

PPM

Encoding

“1”

“0”

“1”

“1”

1000 us

100 us

OOK

Waveform

ON voltage

OFF voltage