8
4899B–RKE–10/06
ATA3741
4.3
FSK/ASK Demodulator and Data Filter
The signal coming from the RSSI amplifier is converted into the raw data signal by the ASK/FSK
demodulator. The operating mode of the demodulator is set via pin ASK/FSK. Logic “L” sets the
demodulator to FSK, Logic “H” sets it into ASK mode.
In ASK mode an automatic threshold control circuit (ATC) is employed to set the detection refer-
ence voltage to a value where a good SNR is achieved. This circuit also implies the effective
suppression of any kind of in-band noise signals or competing transmitters. If the SNR exceeds
10 dB, the data signal can be detected properly.
The FSK demodulator is intended to be used for an FSK deviation of
∆
f
≥
20 kHz. Lower values
may be used, but the sensitivity of the receiver will be reduced. The minimum usable deviation is
dependent on the selected baud rate. In FSK mode, only BR_Range0 and BR_Range1 are
available. In FSK mode, the data signal can be detected if the SNR exceeds 2 dB.
The output signal of the demodulator is filtered by the data filter before it is fed into the digital
signal processing circuit. The data filter improves the SNR as its bandpass can be adopted to
the characteristics of the data signal. The data filter consists of a 1st-order high-pass filter and a
1st-order low-pass filter.
The high-pass filter cut-off frequency is defined by an external capacitor connected to pin
CDEM. The cut-off frequency of the high-pass filter is defined by the following formula:
In self-polling mode, the data filter must settle very rapidly to achieve a low current consumption.
Therefore, CDEM cannot be increased to very high values if self-polling is used. On the other
hand, CDEM must be large enough to meet the data filter requirements according to the data
signal. Recommended values for CDEM are given in the
“Electrical Characteristics” on page 23
.
The values are slightly different for ASK and FSK mode.
The cut-off frequency of the low-pass filter is defined by the selected baud rate range
(BR_Range). BR_Range is defined in the OPMODE register (Section
). BR_Range must be set in accordance to the used baud rate.
The ATA3741 is designed to operate with data coding where the DC level of the data signal is
50%. This is valid for Manchester and Bi-phase coding. If other modulation schemes are used,
the DC level should always remain within the range of VDC_min = 33% and VDC_max = 66%.
The sensitivity may be reduced by up to 1.5 dB in that condition.
Each BR_Range is also defined by a minimum and a maximum edge-to-edge time (t
ee_sig
).
These limits are defined in the
“Electrical Characteristics” on page 23
. They should not be
exceeded to maintain full sensitivity of the receiver.
4.4
Receiving Characteristics
The RF receiver ATA3741 can be operated with and without a SAW front-end filter. In a typical
automotive application, a SAW filter is used to achieve better selectivity. The selectivity with and
without a SAW front-end filter is illustrated in
. This example relates to ASK
mode and the 300-kHz bandwidth version of the ATA3741. FSK mode and the 600-kHz version
of the receiver exhibit similar behavior. Note that the mirror frequency is reduced by 40 dB. The
plots are printed relative to the maximum sensitivity. If a SAW filter is used, an insertion loss of
about 4 dB must be considered.
f
cu_DF
1
2
π
×
30 k
Ω
×
CDEM
×
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=
