T5761 ATMEL [ATMEL Corporation], T5761 Datasheet - Page 5

• Manufacturer Part Number: T5761
• Description: UHF ASK/FSK Receiver
• Manufacturer: ATMEL [ATMEL Corporation]

and exhibits the best possible sensitivity and at the same
time power matching at RF_IN.
R
receiver can be switched from 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 accord-
ing to figure 5 is issued at Pin DATA to indicate that the
receiver is still active (see also figure 32).
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 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 reference 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) ex-
ceeds 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
data signal can be detected if the S/N (ratio to suppress
inband 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 pass-
band can be adopted to the characteristics of the data
signal. The data filter consists of a 1
a 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 follow-
ing formula:
Rev. A2, 19-Oct-00
Sens
DATA
nd
Figure 5. Steady L state limited DATA output pattern
-order lowpass filter
can be connected to V
t
DATA_min
Df
t DATA_L_max
100 kHz. In FSK mode the
S
or GND via a C. The
st
Preliminary Information
-order highpass and
fcu_DF +
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.
The cut-off frequency of the lowpass filter is defined by
the
BR_Range is defined in the OPMODE register (refer to
chapter ‘Configuration of the Receiver’). The BR_Range
must be set in accordance to the used baud-rate.
The T5760/T5761 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
The sensitivity may be reduced by up to 2 dB in that
condition.
Each BR_Range is also defined by a minimum and a
maximum edge-to-edge time (t
defined in the electrical characteristics. They should not
be exceeded to maintain full sensitivity of the receiver.
Receiving Characteristics
The RF receiver T5760/T5761 can be operated with and
without a SAW front-end filter. In a typical automotive
application, a SAW filter is used to achieve better selec-
tivity and large signal capability. The receiving frequency
response without a SAW front-end filter is illustrated in
figures 6 and 7. This example relates to ASK mode. FSK
mode exhibit similar behavior. The plots are printed rela-
tively to the maximum sensitivity. If a SAW filter is used,
an insertion loss of about 3 dB must be considered, but the
over all selectivity is much better.
When designing the system in terms of receiving band-
width, the LO deviation must be considered as it also
determines the IF center frequency. The total LO devi-
ation is calculated to be the sum of the deviation of the
crystal and the XTO deviation of the T5760/T5761. Low-
cost crystals are specified to be within 90 ppm over
tolerance, temperature and aging. The XTO deviation of
the T5760/T5761 is an additional deviation due to the
XTO circuit. This deviation is specified to be 30 ppm
worst case for a crystal with CM = 7 fF. If a crystal of
case. Note that the receiving bandwidth and the IF-filter
bandwidth are equivalent in ASK mode but not in FSK
mode.
90 ppm is used, the total deviation is 120 ppm in that
selected
2
p
baud-rate
T5760 /
DC_min
30 kW
1
= 33% and V
range
CDEM
ee_sig
(BR_Range).
). These limits are
T5761
DC_max
= 66%.
5 (32)
The