U3741BM
UHF ASK/FSK Receiver
Description
The U3741BM is a multi-chip PLL receiver device
supplied in an SO20 package. It has been specially
developed for the demands of RF low-cost data
transmission systems with low data rates from 1 kBaud to
10 kBaud (1 kBaud to 3.2 kBaud for FSK) in Manchester
or Bi-phase code. The receiver is well suited to operate
with the TEMIC PLL RF transmitter U2741B. Its main
applications are in the areas of telemetering, security
technology and keyless-entry systems. It can be used in
the frequency receiving range of f
0
= 300 MHz to
450 MHz for ASK or FSK data transmission. All the
statements made below refer to 433.92-MHz and
315-MHz applications.
Features
D
Minimal external circuitry requirements, no RF
components on the PC board except adaptation to the
receiver antenna
D
ESD protection according to MIL-STD. 883
(4KV HBM) except Pin POUT (2KV HBM)
D
D
D
D
High sensitivity, especially at low data rates
Sensitivity reduction possible even while receiving
Fully integrated VCO
Low power consumption due to configurable self
polling with a programmable timeframe check
operating temperature range –40°C to 105°C
D
High image frequency suppression due to 1 MHz IF
in conjunction with a SAW front-end filter. Up to
40 dB is thereby achievable with newer SAWs.
D
Programmable output port for sensitivity selection or
for controlling external periphery
D
Communication to
m
C possible via a single,
bi-directional data line
D
Supply voltage 4.5 V to 5.5 V,
D
Single-ended RF input for easy adaptation to
l
/4 antenna or printed antenna on PCB
D
Power management (polling) is also possible by
means of a separate pin via the
m
C
(300 kHz and 600 kHz)
D
Low-cost solution due to high integration level
D
2 different IF bandwidth versions are available
System Block Diagram
UHF ASK/FSK
Remote control transmitter
UHF ASK/FSK
Remote control receiver
1 Li cell
U2741B
PLL
U3741BM
Demod.
Control
1...3
m
C
Encoder
Keys M44Cx9x
XTO
IF Amp
Antenna Antenna
VCO
PLL
XTO
Power
amp.
LNA
VCO
14917
Figure 1. System block diagram
Rev. A1, 15-Oct-98
1 (25)
Preliminary Information
U3741BM
Ordering Information
Extended Type Number
U3741BM-M2FL
U3741BM-M2FLG3
U3741BM-M3FL
U3741BM-M3FLG3
Package
SO20
SO20
SO20
SO20
Remarks
2: IF bandwidth of 300 kHz, tube
2: IF bandwidth of 300 kHz, taped and reeled
3: IF bandwidth of 600 kHz, tube
3: IF bandwidth of 600 kHz, taped and reeled
Pin Description
SENS
1
20
DATA
Pin
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
FSK/ASK
2
19
18
ENABLE
TEST
POUT
CDEM
3
AVCC
4
17
AGND 5
DGND 6
7
16
MODE
15
DVCC
MIXVCC
14
XTO
LFGND
LF
LNAGND
LNA_IN
8
9
13
12
n.c. 10
14844
11
LFVCC
Figure 2. Pinning SO20
17
18
19
20
Symbol
Function
SENS
Sensitivity-control resistor
FSK/ASK Selecting FSK/ASK
Low: FSK, High: ASK
CDEM Lower cut-off frequency data filter
AVCC
Analog power supply
AGND Analog ground
DGND Digital ground
MIXVCC Power supply mixer
LNAGND High-frequency ground LNA and
mixer
LNA_IN RF input
n.c.
Not connected
LFVCC Power supply VCO
LF
Loop filter
LFGND Ground VCO
XTO
Crystal oscillator
DVCC
Digital power supply
MODE Selecting 433.92 MHz /315 MHz
Low: 4.90625 MHz (USA)
High: 6.76438 (Europe)
POUT
Programmable output port
TEST
Test pin, during operation at GND
ENABLE Enables the polling mode
Low: polling mode off
(sleep mode)
H:
polling mode on
(active mode)
DATA
Data output / configuration input
2 (25)
Rev. A1, 15-Oct-98
Preliminary Information
U3741BM
Block Diagram
V
S
FSK/ASK
CDEM
AVCC
SENS
IF Amp
Sensitivity
reduction
Polling circuit
and
control logic
FSK/ASK–
Demodulator
and data filter
RSSI
DEMOD_OUT
50 k
W
DATA
Limiter out
ENABLE
TEST
POUT
MODE
FE
CLK
DVCC
AGND
DGND
4. Order
MIXVCC
LPF
3 MHz
Standby logic
LFGND
LNAGND
IF Amp
LFVCC
LPF
3 MHz
VCO
XTO
XTO
f
LNA_IN
LNA
64
15011
LF
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
U3741BM
V
S
DVCC
C
L
XTO
MODE
MODE
R1 = 820
W
C9 = 4.7 nF
C10 = 1 nF
f
+
0 (USA) f
+
314
LO
IF
f
+
1 (Europe) f
+
432.92
LO
IF
LFGND
LF
V
S
R1
C9
C10
LFVCC
The relation is designed to achieve the nominal IF
frequency of f
IF
= 1 MHz for most applications. For
applications where f
RF
= 315 MHz, MODE must be set to
‘0’. In the case of f
RF
= 433.92 MHz, MODE must be set
to ‘1’. For other RF frequencies, f
IF
is not equal to 1 MHz.
f
IF
is then dependent on the logical level at Pin MODE and
on f
RF
. Table 1 summarizes the different conditions.
The RF input either from an antenna or from a generator
must be transformed to the RF input Pin LNA_IN. The
input impedance of that pin is provided in the electrical
parameters. The parasitic board inductances and
capacitances also influence the input matching. The RF
receiver U3741BM exhibits its highest sensitivity at the
best signal-to-noise ratio in the LNA. Hence, noise
matching is the best choice for designing the
transformation network.
A good practice when designing the network is to start
with power matching. From that starting point, the values
of the components can be varied to some extent to achieve
the best sensitivity.
If a SAW is implemented into the input network a mirror
frequency suppression of
D
P
Ref
= 40 dB can be achieved.
There are SAWs available that exhibit a notch at
D
f = 2 MHz. These SAWs work best for an intermediate
frequency of IF = 1 MHz. The selectivity of the receiver
is also improved by using a SAW. In typical automotive
applications, a SAW is used.
Figure 5 shows a typical input matching network, for
f
RF
= 315 MHz and f
RF
= 433.92 MHz using a SAW.
Figure 6 illustrates an according input matching to 50
W
without a SAW. The input matching networks shown in
figure 6 are the reference networks for the parameters
given in the electrical characteristics.
Figure 4. PLL peripherals
The passive loop filter connected to Pin LF is designed for
a loop bandwidth of BLoop = 100 kHz. This value for
BLoop exhibits the best possible noise performance of the
LO. Figure 4 shows the appropriate loop filter
components to achieve the desired loop bandwidth. If the
filter components are changed for any reason please
notify that the maximum capacitive load at Pin LF is
limited. If the capacitive load is exceeded, a bitcheck may
no longer be possible since f
LO
cannot settle in time
before the bitcheck starts to evaluate the incoming data
stream. Self polling does therefore also not work in that
case.
f
LO
is determined by the RF input frequency f
RF
and the
IF frequency f
IF
using the following formula:
f
LO
= f
RF
– f
IF
To determine f
LO
, the construction of the IF filter must be
considered at this point. The nominal IF frequency is
f
IF
= 1 MHz. To achieve a good accuracy of the filter’s
corner frequencies, the filter is tuned by the crystal
frequency f
XTO
. This means that there is a fixed relation
between f
IF
and f
LO
. This relation is dependent on the
logic level at pin mode. This is described by the following
formulas:
Table 1. Calculation of LO and IF frequency
ÁÁÁÁÁÁÁÁÁÁÁÁÁ
Á
ÁÁÁÁÁÁÁÁÁÁÁÁÁ
Á
ÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁ
Á
Á
ÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁ
Á
Á
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
Á
300 MHz < f
RF
< 365 MHz, MODE = 0
365 MHz < f
RF
< 450 MHz, MODE = 1
f
LO
Conditions
f
RF
= 315 MHz, MODE = 0
f
RF
= 433.92 MHz, MODE = 1
Local Oscillator Frequency
f
LO
= 314 MHz
f
LO
= 432.92 MHz
f
RF
f
LO
1
1
314
+ )
f
+
1
)
Intermediate Frequency
f
IF
= 1 MHz
f
IF
= 1 MHz
f
RF
f
LO
314
f
LO
RF
1
432.92
+
f
+
432.92
RF
4 (25)
Rev. A1, 15-Oct-98
Preliminary Information
U3741BM
8 LNAGND
8
LNAGND
U3741BM
C3
22p
L
25n
9
LNA_N
C3
47p
L
25n
9
U3741BM
LNA_N
C16
100p
C17
8.2p
TOKO LL2012
F27NJ
C16
100p
C17
22p
TOKO LL2012
F47NJ
f
RF
= 433.92 MHz
L2
TOKO LL2012
F33NJ
C2
8.2p
33n
L3
27n
f
RF
= 315 MHz
L2
TOKO LL2012
F82NJ
C2
10p
14105
L3
47n
RF
IN
1
IN
2
IN_GND
OUT
OUT_GND
CASE_GND
3,4 7,8
B3555
5
6
RF
IN
82n
1
IN
2
IN_GND
OUT
OUT_GND
CASE_GND
3,4 7,8
B3551
5
6
14106
Figure 5. Input matching network with SAW filter
f
RF
= 433.92 MHz
8 LNAGND
f
RF
= 315 MHz
8 LNAGND
U3741BM
9
15p
25n
LNA_N
9
33p
25n
U3741BM
LNA_N
RF
IN
3.3p
22n
100p
TOKO LL2012
F22NJ
14107
RF
IN
3.3p
39n
100p
TOKO LL2012
F39NJ
14108
Figure 6. Input matching network without SAW filter
Please notify that for all coupling conditions (see
figures 5 and 6), the bond wire inductivity of the LNA
ground is compensated. C3 forms a series resonance
circuit together with the bond wire. L = 25 nH is a feed
inductor to establish a DC path. Its value is not critical but
must be large enough not to detune the series resonance
circuit. For cost reduction this inductor can be easily
printed on the PCB. This configuration improves the
sensitivity of the receiver by about 1 dB to 2 dB.
f
RF
= 315 MHz or f
RF
= 433.92 MHz is used. For other
RF input frequencies refer to table 1 to determine the
center frequency.
The U3741BM is available with 2 different IF
bandwidths. U3741BM-M2, the version with
B
IF
= 300 kHz, is well suited for ASK systems where the
TEMIC PLL transmitter U2741B is used. The receiver
U3741BM - M3 employs an IF bandwidth of
f
IF
= 600 kHz. This version can be used together with the
U2741B in FSK and ASK mode. If used in ASK
applications, it allows higher tolerances for the receiver
and PLL transmitter crystals. SAW transmitters exhibit
much higher transmit frequency tolerances compared to
PLL transmitters. Generally, it is necessary to use
B
IF
= 600 kHz together with such transmitters.
Analog Signal Processing
IF Amplifier
The signals coming from the RF front end are filtered by
the fully integrated 4th-order IF filter. The IF center
frequency is f
IF
= 1 MHz for applications where
Rev. A1, 15-Oct-98
5 (25)
Preliminary Information
