AS3911 datasheet(19/149 Pages) AMSCO

19 / 149 page

ams Datasheet, Confidential: 2013-Oct [2-03]

AS3911 – 19

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

Peak detector performs AM demodulation using peak follower.

Both, the positive and negative peaks are tracked to suppress

common mode signal. It is limited in speed; it can operate for

sub-carrier frequencies up to fc/8 (1700 kHz). It has

demodulation gain G = 0.7. Its input is taken from one

demodulator input only (usually RFI1).

AM demodulator mixer uses synchronous rectification of both

receiver inputs (RFI1 and RFI2). Its gain is G = 0.55. Mixer

demodulator is optimized for VHBR sub-carrier frequencies.

(fc/8 and higher). For sub-carrier frequency fc/8 (1700 kHz) both

peak follower and mixer can be used, while for fc/4 and fc/2 are

supported only by mixer.

By default the Peak detector is used, for data rates fc/8 and

higher use of mixer is automatically preset by sending direct

command Analog Preset.

PM demodulation is also done by a mixer. The PM demodulator

mixer has differential outputs with 60mV differential signal for

1% phase change (16.67 mV per degree). Its operation is

optimized for sub-carrier frequencies up to fc/8 (1700 kHz).

In case the demodulation is done externally of the AS3911 it is

possible to multiplex the LF signals applied to pins RFI1 and

RFI2 directly to the gain and filtering stage by selecting the

Receiver Configuration Register 2 bit lf_en.

Figure 12:

Receiver Block Diagram

digital

sub-carrier

Demodulator

Mixer

Peak Detector

AGC

Squelch

RSSI

rec4<7:4>

AGC

Squelch

RSSI

rec4<3:0>

rec3<7:5>

rec3<4:2>

rec3<2:0>

rec1<5:3>

digital

sub-carrier

AC coupling

+ 1st gain stage

Low-pass

+ 2nd gain stage

High-pass

+ 3rd gain stage

Digitizing

stage

Demodulation

stage

Demodulator

Mixer

rec1<7:6>

rec2<6:5>

RF_IN1

RF_IN2

RSSI_AM<3:0>

RSSI_PM<3:0>

RX_on

sg_on

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