U3741BM Datasheet, PDF (3/25 Pages) ATMEL Corporation – UHF ASK RECEIVER IC

Block Diagram
FSK/ASK
CDEM
AVCC
SENS
AGND
DGND
MIXVCC
LNAGND
U3741BM
FSK/ASK–
Demodulator
and data filter
DEMOD_OUT
RSSI
Limiter out
V_S
50 kW
DATA
IF Amp
Sensitivity
reduction
4. Order
Polling circuit
and
control logic
FE
CLK
ENABLE
TEST
POUT
MODE
DVCC
LPF
3 MHz
Standby logic
LFGND
IF Amp
LFVCC
LPF
3 MHz
VCO
XTO
XTO
LNA_IN
LNA
f
LF
64
15011
Figure 3. Block diagram
RF Front End
The RF front end of the receiver is a heterodyne
configuration that converts the input signal into a 1-MHz
IF signal. According to figure 3, the front end consists of
an LNA (low noise amplifier) LO (local oscillator), a
mixer and RF amplifier.
The LO generates the carrier frequency for the mixer via
a PLL synthesizer. The XTO (crystal oscillator) generates
the reference frequency f_XTO. The VCO (voltage-
controlled oscillator) generates the drive voltage
frequency f_LO for the mixer. f_LO is dependent on the
voltage at Pin LF. f_LO is divided by factor 64. The divided
frequency is compared to f_XTO by the phase frequency
detector. The current output of the phase frequency
detector is connected to a passive loop filter and thereby
generates the control voltage V_LF for the VCO. By means
of that configuration V_LF is controlled in a way that
f_LO/64 is equal to f_XTO. If f_LO is determined, f_XTO can be
calculated using the following formula:
f_XTO = f_LO/64
The XTO is a one-pin oscillator that operates at the series
resonance of the quartz crystal. According to figure 4, the
crystal should be connected to GND via a capacitor CL.
The value of that capacitor is recommended by the crystal
supplier. The value of CL should be optimized for the
individual board layout to achieve the exact value of f_XTO
and hereby of f_LO. When designing the system in terms
of receiving bandwidth, the accuracy of the crystal and
the XTO must be considered.
Rev. A1, 15-Oct-98
3 (25)
Preliminary Information