ATA3742_07 ATMEL [ATMEL Corporation], ATA3742_07 Datasheet - Page 9

• Manufacturer Part Number: ATA3742_07
• Description: UHF ASK/FSK Receiver
• Manufacturer: ATMEL [ATMEL Corporation]

4.4
4900B–RKE–11/07
FSK/ASK Demodulator and Data Filter
R
full sensitivity to reduced sensitivity or vice versa at any time. In polling mode, the receiver will
not wake up if the RF input signal does not exceed the selected sensitivity. If the receiver is
already active, the data stream at pin DATA will disappear when the input signal is lower than
defined by the reduced sensitivity. Instead of the data stream, the pattern shown in
issued at pin DATA to indicate that the receiver is still active.
Figure 4-3.
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 mode; 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 signal-to-noise ratio is achieved. This circuit also implies
the effective suppression of any kind of inband 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
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 pass band can be adopted to
the characteristics of the data signal. The data filter consists of a 1st-order high-pass 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
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 (refer to Section
the Receiver” on page
The ATA3742 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 V
sensitivity may be reduced by up to 1.5 dB in that condition.
f
cu_DF
Sense
=
can be connected to V
------------------------------------------------------------ -
2
Steady L State Limited DATA Output Pattern
30 k
1
20). BR_Range must be set in accordance to the used baud rate.
DATA
CDEM
S
or GND via a microcontroller. The receiver can be switched from
t
min2
t
DATA_L_max
“Electrical Characteristics” on page
DC_min
= 33% and V
20 kHz. Lower values
DC_max
ATA3742
“Configuration of
Figure 4-3
= 66%. The
26. The
is
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