AS3909 datasheet(61/77 Pages) AMSCO

61 / 77 page

ams Datasheet, Confidential: 2013-Oct [3-02]

AS3909/AS3910 – 61

Appl icatio n I n f o rm a t i o n

The figure below depicts setting of the mod bits for some often

used modulation indexes.

Figure 50:

Setting mod bits for Modulation Indexes

Execution of Direct Command Calibrate Modulation Depth

The modulation level is adjusted by increasing the RFO1 and

RFO2 driver output resistance. The RFO drivers are composed

of 8 binary weighted segments. Usually all these segments are

turned on to define the normal, non-modulated level, there is

also a possibility to increase the output resistance of the

non-modulated state by writing the Antenna driver

non-modulated level definition register (#13).

Before sending the direct command Calibrate Modulation Depth

the oscillator and regulators have to be turned on. When the

direct command Calibrate Modulation Depth is sent the

following procedure is executed:

The Transmitter is turned on, non-modulated level is

established

The amplitude of the non-modulated carrier level

established on the inputs RFI1 and RFI2 is measured by

the A/D converter and stored in the A/D Converter

Output Register

Based on the measurement of the non-modulated level

and the target modulated level defined by the bits mod5

to mod0 the target modulated level is calculated

The output driver control is taken over by the eight bit

Calibrate Register with initial level defined in the

Antenna Driver Non-Modulated Level Definition Register

(#13). Content of the Calibrate Register is incremented

by 1 to increase the driver resistance, the reduced

amplitude is measured by the A/D converter and the

result is compared to the target modulation level.

The procedure from previous point is repeated as long

as the measured level is greater than target level.

modulation index [%]

a/b [dec]

a/b [bin]

mod5………mod0

1,1739

1,001011

001011

1,2222

1,001110

001110

1,3256

1,010100

010100

1,5000

1,100000

100000

1,8571

1,110111

110111

1,9843

1,111111

111111

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