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componentes Descripción

Fabricante

T5760

UHF ASK/FSK Receiver

Atmel Corporation 

T5760 / T5761

The value of CL should be optimized for the individual

board layout to achieve the exact value of fXTO (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 fLO. 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.

VS

DVCC

CL

XTAL

n.c.

TEST 3

TEST 2

Figure 4. XTO peripherals

The nominal frequency fLO is determined by the RF input

frequency fRF and the IF frequency fIF using the following

formula (low side injection):

fLO = fRF – fIF

To determine fLO , the construction of the IF filter must

be considered at this point. The nominal IF frequency is

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

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

quency fXTO. This means that there is a fixed relation

between fIF and fLO.

fIF = fLO / 915

The relation is designed to achieve the nominal IF fre-

quency of fIF = 950 kHz for the 868.3 MHz version. For

the 915 MHz version an IF frequency of fIF = 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 fRF

= 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 fIF = 950 kHz for applications where fRF =

868.3 MHz and fIF =1.0 MHz for fRF = 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

DRRSSI = 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 VTh_red. VTh_red is deter-

mined by the value of the external resistor RSens. RSens is

connected between Pin SENS and GND or VS. 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 RSens 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 RSens is connected to VS, the receiver operates at a

lower sensitivity. The reduced sensitivity is defined by the

value of RSens, 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

4 (32)

Rev. A2, 19-Oct-00

Preliminary Information

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