T5761

T5760 /

Preliminary Information

Rev. A2, 19-Oct-00

1 (32)

UHF ASK/FSK Receiver

Description

The T5760/T5761 is a multi-chip PLL receiver device
supplied in an SO20 package. It has been especially de-
veloped for the demands of RF low-cost data transmission
systems with data rates from 1 kBaud to 10 kBaud in
Manchester or Bi-phase code. The receiver is well suited
to operate with the Atmel Wireless & Microcontrollers'
PLL RF transmitter T5750. Its main applications are in

the areas of telemetering, security technology and key-
less-entry systems. It can be used in the frequency
receiving range of f

= 868 to 870 MHz or f

= 902 to

928 MHz for ASK or FSK data transmission. All the
statements made below refer to 868.3 MHz and
915.0 MHz applications.

Features

D Fully integrated LC-VCO and PLL loop filter
D Very high sensitivity with power matched LNA
D 30 dB image rejection
D High system IIP3 (16 dBm), system 1-dB compres-

sion point (25 dBm)

D High large-signal capability at GSM band (blocking

30 dBm @ + 20 MHz, IIP3 = 12 dBm @ + 20 MHz)

D 5 V to 20 V automotive compatible data interface
D Data clock available for Manchester- and Bi-phase-

coded signals

D Programmable digital noise suppresion
D Receiving bandwidth B

= 600 kHz for low cost

90-ppm crystals

D Low power consumption due to configurable polling
D Temperature range 40

C to 105

D ESD protection 2 kV HBM, 200 V MM
D Communication to mC possible via a single

bi-directional data line

D Low-cost solution due to high integration level with

minimum external circuitry requirements

System Block Diagram

Demod.

IF Amp

LNA

VCO

PLL

XTO

Control

T5760/
T5761

1...5

Power

amp.

XTO

VCO

PLL

T5750

Antenna

UHF ASK/FSK

Remote control transmitter

UHF ASK/FSK

Remote control receiver

Figure 1. System block diagram

Ordering Information

Extended Type Number

Package

Remarks

T5760-TG

SO20

Tube, for 868 MHz ISM band

T5760-TGQ

SO20

Taped and reeled, for 868 MHz ISM band

T5761-TG

SO20

Tube, for 915 MHz ISM band

T5761-TGQ

SO20

Taped and reeled, for 915 MHz ISM band

T5760 T5761

Rev. A2, 19-Oct-00

Preliminary Information

2 (32)

Pin Description

Pin

Symbol

Function

SENS

Sensitivity-control resistor

IC_

ACTIVE

IC condition indicator
Low = sleep mode
High = active mode

CDEM

Lower cut-off frequency data fil-
ter

AVCC

Analog power supply

TEST 1

Test pin, during operation at GND

AGND

Analog ground

n.c.

Not connected, connect to GND

LNAREF

High-frequency reference node
LNA and mixer

LNA_IN

RF input

LNAGND DC ground LNA and mixer

TEST 2

Do not connect during operating

TEST 3

Test pin, during operation at GND

n.c.

Not connected, connect to GND

XTAL

Crystal oscillator XTAL connec-
tion

DVCC

Digital power supply

TEST 4

Test pin, during operation at
DVCC

DATA_

CLK

Bit clock of data stream

DGND

Digital ground

POLL-

ING/_ON

Selects polling or rceiving mode
Low: receiving mode
High: polling mode

DATA

Data output / configuration input

AVCC

TEST 1

AGND

n.c.

LNAREF

LNA_IN

IC_ACTIVE

CDEM

DATA_CLK

TEST 4

XTAL

n.c.

TEST 3

POLLING
/_ON

DGND

LNAGND

TEST 2

DATA

DVCC

SENS

T5760/
T5761

Figure 2. Pinning SO20

T5761

T5760 /

Preliminary Information

Rev. A2, 19-Oct-00

3 (32)

Block Diagram

SENS

CDEM

AVCC

AGND

DGND

LNAGND

LNA_IN

DATA

POLLING/_ON

DATA_CLK

DVCC

XTAL

Polling circuit

and

control logic

Rssi

Limiter out

LCVCO

:256

XTO

Standby logic

CLK

FSK/ASK

demodulator

and data filter

RSSI IF

Amp.

PolyLPF

fg=7MHz

LNA

4. Order

f0=950 kHz/

Dem_out

Sensitivity

reduction

LPF

fg=2.2MHz

IF
Amp.

IC_ACTIVE

Data

interface

LNAREF

Loop

filter

1 MHz

Figure 3. Block diagram

RF Front End

The RF front end of the receiver is a low-IF heterodyne
configuration that converts the input signal into a
950-kHz/ 1-MHz IF signal with an image rejection of typ-
ical 30dB. According to figure 3 the front end consists of
an LNA (low noise amplifier), LO (local oscillator), I/Q
mixer, polyphase lowpass filter and an IF amplifier.

The PLL generates the carrier frequency for the mixer via
a full integrated synthesizer with integrated low noise
LC-VCO (voltage controlled oscillator ) and PLL-loop-
filter. The XTO ( crystal oscillator ) generates the
reference frequency f

XTO

. The integrated LC-VCO gen-

erates two times the mixer drive frequency f

VCO

. The I/Q

signals for the mixer are generated with a divide by two
circuit ( f

= f

VCO

/2 ). f

VCO

is divided by a factor of 256

and feed into a phase frequency detector and compared

with f

XTO

. The output of the phase frequency detector is

feed into an integrated loopfilter and thereby generates
the control voltage for the VCO. If f

is determined,

XTO

can be calculated using the following formula:

XTO

= f

/ 128

The XTO is a one-pin oscillator that operates at the series
resonance of the quartz crystal with high current but low
voltage signal, so that there is only a small voltage at the
crystal oscillator frequency at Pin XTAL. According to
figure 4, the crystal should be connected to GND with a
series capacitor C

. The value of that capacitor is recom-

mended by the crystal supplier. Due to a somewhat
inductive impedance at steady state oscillation and some
PCB parasitics a lower value of C

is normally necessary.

T5760 T5761

Rev. A2, 19-Oct-00

Preliminary Information

4 (32)

The value of C

should be optimized for the individual

board layout to achieve the exact value of f

XTO

(the best

way is to use a crystal with known load resonance fre-
quency to find the right value for this capacitor) and
hereby of f

. When designing the system in terms of re-

ceiving bandwidth and local oscillator accuracy, the
accuracy of the crystal and the XTO must be considered.

If a crystal with

$30 ppm adjustment tolerance at 25_C

$50ppm over Temperature 40_C to 105_C, $10 ppm

of total aging and a CM ( motional capacitance ) of 7 fF
is used, an additional XTO pulling of

$30 ppm has to be

added.

The resulting total LO tolerance of

$120ppm agrees with

the receiving bandwidth specification of the
T5760/T5761 if the T5750 has also a total LO tolerance
of

$120 ppm.

DVCC

XTAL

TEST 3

TEST 2

n.c.

Figure 4. XTO peripherals

The nominal frequency f

is determined by the RF input

frequency f

and the IF frequency f

using the following

formula (low side injection):

= f

To determine f

, the construction of the IF filter must

be considered at this point. The nominal IF frequency is
f

= 950 kHz. To achieve a good accuracy of the filter

corner frequencies, the filter is tuned by the crystal fre-
quency f

XTO

. This means that there is a fixed relation

between f

and f

= f

/ 915

The relation is designed to achieve the nominal IF fre-
quency of f

= 950 kHz for the 868.3 MHz version. For

the 915 MHz version an IF frequency of f

= 1.0 MHz

results.

The RF input either from an antenna or from a RF genera-
tor 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 capaci-
tances influence the input matching. The RF receiver
T5760/T5761 exhibits its highest sensitivity if the LNA
is power matched. This makes the matching to an SAW
filter as well as to 50

W or an antenna more easy.

Figure 33 shows a typical input matching network for f

= 868.3 MHz to 50

W. Figure 34 illustrates an according

input matching for 868.3 MHz to an SAW. The input
matching network shown in Figure 33 is the reference net-
work for the parameters given in the electrical
characteristics.

Analog Signal Processing

IF Filter

The signals coming from the RF front end are filtered by
the fully integrated 4th-order IF filter. The IF center fre-
quency is f

= 950 kHz for applications where f

868.3 MHz and f

=1.0 MHz for f

= 915 MHz. The

nominal bandwidth is 600 kHz.

Limiting 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 maxi-
mum RSSI output voltage and the RSSI output voltage
due to a disturber. The dynamic range of the RSSI ampli-
fier 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, because only the hard limited
signal from a high gain limiting amplifier is used by the
demodulator.

The output voltage of the RSSI amplifier is internally
compared to a threshold voltage V

Th_red

. V

Th_red

is deter-

mined by the value of the external resistor R

Sens

. R

Sens

connected between Pin SENS and GND or V

. 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

Sens

is connected to GND, the receiver switches to full

sensitivity. It is also possible to connect the Pin SENS di-
rectly to GND to get the maximum sensitivity.

If R

Sens

is connected to V

, the receiver operates at a

lower sensitivity. The reduced sensitivity is defined by the
value of R

Sens

, the maximum sensitivity by the signal-to-

noise ratio of the LNA input. The reduced sensitivity
depends 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 33

T5761

T5760 /

Preliminary Information

Rev. A2, 19-Oct-00

5 (32)

and exhibits the best possible sensitivity and at the same
time power matching at RF_IN.

Sens

can be connected to V

or GND via a

C. 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 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).

DATA

t DATA_L_max

DATA_min

Figure 5. 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 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

100 kHz. In FSK mode the

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

-order highpass and

a 2

-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 follow-
ing formula:

fcu_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 electrical characteristics.

The cut-off frequency of the lowpass filter is defined by
the selected baud-rate range (BR_Range). 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

DC_min

= 33% and V

DC_max

= 66%.

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

ee_sig

). These limits are

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

90 ppm is used, the total deviation is

120 ppm in that

case. Note that the receiving bandwidth and the IF-filter
bandwidth are equivalent in ASK mode but not in FSK
mode.

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