T5743 datasheet(4/41 Pages) ATMEL

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T5743

4569A–RKE–12/02

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 an 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. fLO is dependent on

the voltage at Pin LF. f

LO is divided by factor 64. The divided frequency is compared to

XTO by the phase frequency detector. The current output of the phase frequency detec-

tor 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 fLO/64 is equal to

XTO. If fLO is determined, fXTO can be calculated using the following formula: fXTO = fLO/64.

The XTO is a one-pin oscillator that operates at the series resonance of the quartz crys-

tal. 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.

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 bit

check may no longer be possible since f

LO cannot settle in time before the bit check

starts to evaluate the incoming data stream. Self polling does therefore also not work in

that case.

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

ing formula: f

LO = fRF - fIF

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 fre-

quencies, the filter is tuned by the crystal frequency f

XTO. This means that there is a

fixed relation between f

IF and fLO. This relation is dependent on the logic level at Pin

MODE.

DVCC

XTO

LFVCC

LFGND

C10

R1 = 820

C9 = 4.7 nF

C10 = 1 nF

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