ATA5760 ATMEL Corporation, ATA5760 Datasheet - Page 7

• Manufacturer Part Number: ATA5760
• Description: Ata5760 Uhf Ask/fsk Receiver
• Manufacturer: ATMEL Corporation

4.3
4896C–RKE–04/06
FSK/ASK Demodulator and Data Filter
Figure 4-1.
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 the bit ASK/_FSK in the
OPMODE register. Logic L sets the demodulator to FSK, applying H to 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 in-band noise signals or competing transmitters. If the
S/N (ratio to suppress in-band noise signals) exceeds about 10 dB the data signal can be
detected properly, but better values are found for many modulation schemes of the competing
transmitter.
The FSK demodulator is intended to be used for an FSK deviation of 10 kHz
FSK mode the data signal can be detected if the S/N (ratio to suppress in-band noise signals)
exceeds about 2 dB. This value is valid for all modulation schemes of a disturber signal.
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 S/N ratio as its passband can be adopted
to the characteristics of the data signal. The data filter consists of a 1
2
The highpass filter cut-off frequency is defined by an external capacitor connected to pin CDEM.
The cut-off frequency of the highpass 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 sig-
nal. Recommended values for CDEM are given in the electrical characteristics.
The cut-off frequency of the lowpass filter is defined by the selected baud-rate range
(BR_Range). The BR_Range is defined in the OPMODE register (refer to section
of the Receiver” on page
The ATA5760/ATA5761 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
V
Each BR_Range is also defined by a minimum and a maximum edge-to-edge time (t
These limits are defined in the electrical characteristics. They should not be exceeded to main-
tain full sensitivity of the receiver.
fcu_DF
nd
DC_max
-order lowpass filter.
= 66%. The sensitivity may be reduced by up to 2 dB in that condition.
=
------------------------------------------------------------ -
2
Steady L State Limited DATA Output Pattern
30 k
1
DATA
23). The BR_Range must be set in accordance to the used baud-rate.
CDEM
t
DATA_min
t
DATA_L_max
ATA5760/ATA5761
st-
order high pass and a
DC_m in
“Configuration
f 100 kHz. In
= 33% and
ee_sig
).
7