AS3911 datasheet(21/159 Pages) AMSCO

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[v2-11] 2016-Jun-13

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AS3911 − Application Information

Filtering and Gain Stages

The receiver chain has band pass filtering characteristics.

Filtering is optimized to pass sub-carrier frequencies while

rejecting carrier frequency and low frequency noise and DC

component. Filtering and gain is implemented in three stages

where the first and the last stage have the first order high pass

characteristics, while the mid stage has second order low pass

characteristic.

Gain and filtering characteristics can be optimized for current

application by writing the Receiver Configuration Register 1

(filtering), Receiver Configuration Register 3 (gain in first stage)

and Receiver Configuration Register 4 (gain in second and third

stage).

Gain of first stage is about 20dB and can be reduced in six 2.5

dB steps. There is also a special boost mode available, which

boosts the maximum gain for additional 5.5 dB. In case of VHBR

(fc/8 and fc/4) the gain is lower. The first stage gain can only be

modified by writing Receiver Configuration Register 3. The

default setting of this register is the minimum gain. Default first

stage zero is located at 60 kHz, it can also be lowered to 40kHz

or 12 kHz by writing option bits in the Receiver Configuration

with common control bits (see Figure 16).

Gain in the second and third stage is 23 dB and can be reduced

in six 3 dB steps. Gain of these two stages is included in AGC

and Squelch loops or can be manually set in Receiver

Configuration Register 4. Sending of direct command Reset Rx

Gain is necessary to reset the AGC, Squelch and RSSI block.

Sending this command clears the current Squelch setting and

loads the gain reduction configuration from Receiver

Configuration Register 4 into the internal shadow registers of

AGC/Squelch block. Second stage has a second order low pass

filtering characteristic, the pass band is adjusted according to

sub-carrier frequency using the bits lp2 to lp0 of the Receiver

Configuration Register 1. See Figure 15 for -1dB cut-off

frequency for different settings.

Figure 15:

Low Pass Control

rec1<5> lp2

rec1<4> lp1

rec1<3> lp0

-1 dB point

1200 kHz

001

600 kHz

010

300 kHz

100

2 MHz

101

7 MHz

Other

Not used

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