U3741BM Datasheet, PDF (6/25 Pages) ATMEL Corporation – UHF ASK RECEIVER IC

U3741BM
RSSI Amplifier
The subsequent RSSI amplifier enhances the output
signal of the IF amplifier before it is fed into the demod-
ulator. The dynamic range of this amplifier is
DR_RSSI = 60 dB. If the RSSI amplifier is operated within
its linear range, the best S/N ratio is maintained in ASK
mode. If the dynamic range is exceeded by the transmitter
signal, the S/N ratio is defined by the ratio of the
maximum RSSI output voltage and the RSSI output
voltage due to a disturber. The dynamic range of the RSSI
amplifier is exceeded if the RF input signal is about 60 dB
higher compared to the RF input signal at full sensitivity.
In FSK mode the S/N ratio is not affected by the dynamic
range of the RSSI amplifier.
The output voltage of the RSSI amplifier is internally
compared to a threshold voltage V_{Th_red}. V_{Th_red} is
determined by the value of the external resistor R_Sense.
R_Sense is connected between Pin Sense and GND or V_S.
The output of the comparator is fed into the digital control
logic. By this means it is possible to operate the receiver
at a lower sensitivity.
If R_Sense is connected to V_S, the receiver operates at a
lower sensitivity. The reduced sensitivity is defined by the
value of R_Sense, the maximum sensitivity by the signal-to-
noise ratio of the LNA input. The reduced sensitivity is
dependent on the signal strength at the output of the RSSI
amplifier.
Since different RF input networks may exhibit slightly
different values for the LNA gain the sensitivity values
given in the electrical characteristics refer to a specific
input matching. This matching is illustrated in figure 6
and exhibits the best possible sensitivity.
R_Sense can be connected to V_S or GND via a µC or by the
digital output port POUT of the U3741BM receiver IC.
The 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 disap-
pear when the input signal is lower than defined by the
reduced sensitivity. Instead of the data stream, the pattern
according to figure 7 is issued at Pin DATA to indicate
that the receiver is still active.
DATA
tmin2
t _{DATA_L_max}
Figure 7. Steady L state limited DATA output pattern
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,
applying ‘H’ sets it into 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 inband noise signals or competing transmitters.
If the S/N ratio exceeds 10 dB, the data signal can be
detected properly.
The FSK demodulator is intended to be used for an FSK
deviation of Df ≥ 20 kHz. Lower values may be used but
the sensitivity of the receiver is reduced in that condition.
The minimum usable deviation is dependent on the
selected baudrate. In ASK mode, only BR_Range0 and
BR_Range1 are available. In FSK mode, the data signal
can be detected if the S/N Ratio 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 S/N ratio as its
passband can be adopted to the characteristics of the data
signal. The data filter consists of a 1st-order highpass and
a 1st-order lowpass filter.
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:
+ fcu_DF
2
p
1
30 kW
CDEM
In self-polling mode, the data filter must settle very rap-
idly to achieve a low current consumption. Therefore,
CDEM cannot be increased to very high values if selfpol-
ling 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 values are
slightly different for ASK and FSK mode.
The cut-off frequency of the lowpass filter is defined by
the selected baudrate range (BR_Range). BR_Range is
defined in the OPMODE register (refer to chapter
‘Configuration of the Receiver’). BR_Range must be set
in accordance to the used baudrate.
6 (25)
Rev. A1, 15-Oct-98
Preliminary Information