Rainbow Electronics U3745BM User Manual

Features

• Supply Voltage 4.5 V to 5.5 V

• Operating Temperature Range -40°C to +85°C

• Minimal External Circuitry Requirements, No RF Components on the PC Board Except

Matching to the Receiver Antenna

• 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 Time

Frame Check

• Single-ended RF Input for Easy Matching to l/4 Antenna or Printed Antenna on PCB

• Low-cost Solution Due to High Integration Level

• ESD Protection According to MIL-STD 883 (4 KV HBM) Except Pin POUT (2 KV HBM)

• 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

• Programmable Output Port for Sensitivity Selection or for Controlling External

Periphery

• Communication to the Microcontroller Possible via a Single, Bi-directional Data Line

• Power Management (Polling) is also Possible by Means of a Separate Pin via the

Microcontroller

Description

The U3745BM 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 in Manchester or Bi-phase code. The receiver is well suited to operate with Atmel’s PLL RF transmitter U2745B. It can be used in the frequency receiving range of f mission. All the statements made below refer to 433.92-MHz and 315-MHz applications.

The main applications of the U3745BM are in the areas of outside temperature metering, socket control, garage door opener, consumption metering, light/fan or aircondition control, jalousies, wireless keyboard and various other consumer market applications.

= 310 MHz to 440 MHz for ASK data trans-

UHF ASK Receiver IC

U3745BM

Rev. 4663A–RKE–06/03

System Block Diagram

1 Li cell

Encoder

M44Cx9x

Keys

Pin Configuration

UHF ASK/FSK

Remote control transmitter

U2745B

PLL

XTO

VCO

Power

amp.

Figure 1. Pinning SO20

Antenna Antenna

ASK

CDEM

UHF ASK

Remote control receiver

U3745BM

Demod.

IF Amp

LNA VCO

Data interface

PLL XTO

DATA

ENABLE

TEST

1...3

µC

AVCC

AGND

DGND

MIXVCC

LNAGND

LNA_IN

POUT

MODE

U3745BM

DVCC

XTO

LFGND

LFVCC

U3745BM

4663A–RKE–06/03

Pin Description

Pin Symbol Function

1 NC Not connected 2 ASK ASK high 3 CDEM Lower cut-off frequency data filter 4 AVCC Analog power supply 5 AGND Analog ground 6 DGND Digital ground 7 MIXVCC Power supply mixer 8 LNAGND High-frequency ground LNA and mixer

9LNA_INRF input 10 NC Not connected 11 LFVCC Power supply VCO 12 LF Loop filter 13 LFGND Ground VCO 14 XTO Crystal oscillator 15 DVCC Digital power supply 16 MODE Selecting 433.92 MHz/315 MHz. Low: 4.90625 MHz (USA), High: 6.76438 (Europe) 17 POUT Programmable output port 18 TEST Test pin, during operation at GND 19 ENABLE Enables the polling mode. Low: polling mode off (sleep mode). High: polling mode on (active mode) 20 DATA Data output/configuration input

U3745BM

4663A–RKE–06/03

Block Diagram

ASK

CDEM

AVCC

Demodulator

and data filter

Limiter outRSSI

DEMOD_OUT

50 kW

DATA

AGND

DGND

MIXVCC

LNAGND

LNA_IN

LNA

IF Amp

4th Order

LPF

3 MHz

IF Amp

LPF

3 MHz

Sensitivity reduction

Polling circuit

and

control logic

FE CLK

Standby logic

VCO XTO

¸ 64

ENABLE

TEST

POUT

MODE

DVCC

LFGND

LFVCC

XTO

U3745BM

4663A–RKE–06/03

RF Front End

U3745BM

The RF front end of the receiver is a heterodyne configuration that converts the input signal into a 1-MHz IF signal. According to the block diagram, 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 oscillator) generates the drive voltage frequency f the voltage at pin LF. f

by the phase frequency detector. The current output of the phase frequency

to f

XTO

is divided by a factor of 64. The divided frequency is compared

detector is connected to a passive loop filter and thereby generates the control voltage

for the VCO. By means of that configuration, VLF is controlled in a way that fLO/64 is

equal to f

XTO

. If fLO is determined, f

XTO

--------=

can be calculated using the following formula:

XTO

The XTO is a one-pin oscillator that operates at the series resonance of the quartz crystal. According to Figure 2, 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 hereby of f

. When designing the system in terms of receiving bandwidth, the accuracy

of the crystal and XTO must be considered.

. The VCO (voltage-controlled

XTO

for the mixer. fLO is dependent on

and

XTO

Figure 2. PLL Peripherals

DVCC

XTO

LFGND

R1 = 820 W

C9 = 4.7 nF

LFVCC

C10 = 1 nF

C10

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 2 shows the appropriate loop filter components to achieve the desired loop bandwidth. If the filter components are changed for any reason, please note that the maximum capacitive load at Pin LF is limited. If the capacitive load is exceeded, a bit check may no longer be possible since f

cannot settle in time before the bit check

starts to evaluate the incoming data stream. Therefore, self polling also does not work in that case.

4663A–RKE–06/03

is determined by the RF input frequency fRF and the IF frequency fIF using the follow-

ing formula:

LOfRFfIF

–=

To determine fLO, the construction of the IF filter must be considered at this point. The nominal IF frequency is f quencies, the filter is tuned by the crystal frequency f fixed relation between f

= 1 MHz. To achieve a good accuracy of the filter’s corner fre-

and fLO that depends on the logic level at pin mode. This is

. This means that there is a

XTO

described by the following formulas:

MODE 0 (USA) f

MODE 0 (Europe) f

----------==

314

------------- -----==

432.92

The relation is designed to achieve the nominal IF frequency of f applications. For applications where f case of f not equal to 1 MHz. f

= 433.92 MHz, the MODE must be set to ‘1’. For other RF frequencies, fIF is

is then dependent on the logical level at Pin MODE and on fRF.

= 315 MHz, the MODE must be set to ‘0’. In the

= 1 MHz for most

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 U3745BM exhibits its highest sensitivity at the best signal-tonoise 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

= 40 dB can be achieved. There are SAWs available that exhibit a notch at

Ref

Df = 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 3 shows a typical input matching network for f

= 433.92 MHz using a SAW. Figure 4 illustrates an input matching to 50 W without a

= 315 MHz and

SAW. The input matching networks shown in Figure 4 are the reference networks for the parameters given in the section “Electrical Characteristics”.

Table 1. Calculation of LO and IF Frequency

Conditions Local Oscillator Frequency Intermediate Frequency

fRF = 315 MHz, MODE = 0 fLO = 314 MHz fIF = 1 MHz f

= 433.92 MHz, MODE = 1 fLO = 432.92 MHz fIF = 1 MHz

300 MHz < f

365 MHz < f

< 365 MHz, MODE = 0

< 450 MHz, MODE = 1

U3745BM

------------ ------------- ---=

----------+

314

------------ ------+

432.92

------------ -------=

----------=

314

------------ ------=

432.92

4663A–RKE–06/03

Figure 3. Input Matching Network with SAW Filter

U3745BM

LNAGND

U3745BM

IN_GND

LNA_IN

C16

100p

27n

B3555

CASE_GND

3,4 7,8

C17

8.2p

TOKO LL2012

F27NJ

OUT

OUT_GND

5 6

22p

fRF = 433.92 MHz

8.2p

TOKO LL2012

F33NJ

33n

25n

1 2

Figure 4. Input Matching Network without SAW Filter

fRF = 433.92 MHz

15p

25n

LNAGND

U3745BM

LNA_IN

fRF = 315 MHz

10p

fRF = 315 MHz

33p

47p

TOKO LL2012

F82NJ

82n

25n

1 2

IN IN_GND

LNAGND

U3745BM

LNA_IN

C16

100p

47n

B3551

CASE_GND

3,4 7,8

LNAGND

U3745BM

LNA_IN

C17

22p

TOKO LL2012

F47NJ

OUT

OUT_GND

5 6

3.3p 22n

100p

TOKO LL2012

F22NJ

3.3p 39n

100p

TOKO LL2012

F39NJ

Please note that for all coupling conditions (see Figure 3 and Figure 4), 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.

4663A–RKE–06/03

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

= 433.92 MHz is used. For other RF input frequencies, refer to Table 1 to determine

the center frequency.

= 1 MHz for applications where fRF= 315 MHz or

The receiver U3745BM employs an IF bandwidth of B

= 600 kHz. This IC can be used

together with the U2745B. SAW transmitters exhibit much higher transmit frequency tolerances compared to PLL transmitters. Generally, it is necessary to use B

= 600 kHz

together with such transmitters.

RSSI Amplifier The subsequent RSSI amplifier enhances the output signal of the IF amplifier before it is

= 60 dB. If the

RSSI

Demodulator and Data Filter

fed into the demodulator. The dynamic range of this amplifier is DR 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.

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 4 and exhibits the best possible sensitivity.

The signal coming from the RSSI amplifier is converted into the raw data signal by the ASK demodulator.

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 exceeds 10 dB, the data signal can be detected properly.

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 bandpass can be adopted to the characteristics of the data signal. The data filter consists of a 1storder high-pass and a 1st-order low-pass filter.

The high-pass filter cut-off frequency is defined by an external capacitor connected to pin CDEM. The cut-off frequency of the high-pass filter is defined by the following formula:

cu_DF

------------ ------------- ------------- ---------- -----------=

p´ 30 kW´ CDEM´

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 section “Electrical Characteristics”.

The cut-off frequency of the low-pass filter is defined by the selected baud rate range (BR_Range). BR_Range is defined in the OPMODE register (refer to section “Configuration of the Receiver”). BR_Range must be set in accordance to the used baud rate.

U3745BM

4663A–RKE–06/03

U3745BM

The U3745BM 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 and V

= 66%. The sensitivity may be reduced by up to 1.5 dB in that condition.

DC_max

Each BR_Range is also defined by a minimum and a maximum edge-to-edge time

). These limits are defined in the section “Electrical Characteristics”. They should

ee_sig

not be exceeded to maintain full sensitivity of the receiver.

DC_min

=33%

Receiving Characteristics

The RF receiver U3745BM can be operated with and without a SAW front end filter. The selectivity with and without a SAW front-end filter is illustrated in Figure 5. This example relates to ASK mode of the U3745BM. Note that the mirror frequency is reduced by 40 dB. The plots are printed relatively to the maximum sensitivity. If a SAW filter is used, an insertion loss of about 4 dB must be considered.

When designing the system in terms of receiving bandwidth, the LO deviation must be considered as it also determines the IF center frequency. The total LO deviation is calculated to be the sum of the deviation of the crystal and the XTO deviation of the U3745BM. Low-cost crystals are specified to be within ±100 ppm. The XTO deviation of the U3745BM is an additional deviation due to the XTO circuit. This deviation is specified to be ±50 ppm. If a crystal of ±100 ppm is used, the total deviation is ±150 ppm in that case. Note that the receiving bandwidth and the IF-filter bandwidth are equivalent in ASK mode.

Figure 5. Receiving Frequency Response

0.0

-10.0

-20.0

-30.0

-40.0

-50.0

-60.0

dP (dB)

-70.0

-80.0

-90.0

-100.0

-6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0

df (MHz)

without SAW

with SAW

4663A–RKE–06/03

+ 20 hidden pages