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AS3930
Datasheet - D e t a i l e d D e s c r i p t i o n
8.3 Channel Amplifier and Frequency Detector
The channel amplifier consists of a variable gain amplifier (VGA), an automatic gain control, and a frequency detector. The latter detects the
presence of a carrier. As soon as the carrier is detected the AGC is enabled, the gain of the VGA is reduced and set to the right value and the
RSSI can be displayed.
8.3.1
Frequency Detector / AGC
The frequency detection uses the RTC as time base. In case the internal RC oscillator is used as RTC, it must be calibrated, but the calibration
is guaranteed for a 32.768 kHz crystal oscillator only. The frequency detection criteria can be tighter or more relaxed according to the setup
described in R2<1:0> (see Table 14) .
Table 14. Tolerance Settings for Wake-up
R2<1>
0
0
1
1
R2<0>
0
1
0
1
Tolerance
relaxed
tighter (medium)
stringent
reserved
The AGC can operate in two modes:
?? AGC down only ( R1<5> =0)
?? AGC up and down ( R1<5> =1)
As soon as the AGC starts to operate, the gain in the VGA is set to maximum. If the AGC down only mode is selected, the AGC can only
decrease the gain. Since the RSSI is directly derived from the VGA gain, the system holds the RSSI peak.
When the AGC up and down mode is selected, the RSSI can follow the input signal strength variation in both directions.
Regardless which AGC operation mode is used, the AGC needs maximum 35 carrier periods to settle.
The RSSI is stored in the register R10<4:0> .
Both AGC modes (only down or down and up) can also operate with time limitation. This option allows AGC operation only in time slot of 256μs
following the internal wake-up. Then the AGC (RSSI) is frozen till the wake-up or RSSI reset occurs.
The RSSI is reset either with the direct command 'clear_wakeup' or 'reset_RSSI'. The 'reset_RSSI' command resets only the AGC setting but
does not terminate wake-up condition. This means that if the signal is still present the new AGC setting (RSSI) will appear not later than 300μs
(35 LF carrier periods) after the command was received. The AGC setting is reset if for duration of 3 Manchester half symbols no carrier is
detected. If the wake-up IRQ is cleared the chip will go back to listening mode.
In case the maximum amplification at the beginning is a drawback (e.g. in noisy environment) it is possible to set a smaller starting gain on the
amplifier (see Table 15) . In this way it is possible to reduce the false frequency detection.
Table 15. Bit Setting of Gain Reduction
R4<3>
0
0
0
0
0
1
1
1
1
R4<2>
0
0
0
1
1
0
0
1
1
R4<1>
0
0
1
0
1
0
1
0
1
R4<0>
0
1
0 or 1
0 or 1
0 or 1
0 or 1
0 or 1
0 or 1
0 or 1
Gain Reduction
No gain reduction
n.a.
n.a.
-4dB
-8dB
-12dB
-16dB
-20dB
-24dB
www.ams.com/LF-Receiver/AS3930
Revision 1.5
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