T5744 데이터 시트보기 (PDF) - Atmel Corporation

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T5744

UHF ASK/FSK Receiver

Atmel Corporation 

4 T5744

Figure 4. PLL Peripherals

VS

DVCC

CL

XTO

LFGND

LF

R1 = 820 W

C9 = 4.7 nF

C10 = 1 nF

VS

LFVCC

R1

C10

C9

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

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

ing formula:

fLO = fRF - fIF

To determine fLO, the construction of the IF filter must be considered at this point. The

nominal IF frequency is fIF = 1 MHz. To achieve a good accuracy of the filter's corner fre-

quencies, the filter is tuned by the crystal frequency fXTO. This means that there is a

fixed relation between fIF and fLO that depends on the logic level at pin mode. This is

described by the following formulas:

MODE = 0 USA fIF = fLO/314

MODE = 1 Europe fIF = fLO/432.92

The relation is designed to achieve the nominal IF frequency of fIF = 1 MHz for most

applications. For applications where fRF = 315 MHz, MODE must be set to '0'. In the

case of fRF = 433.92 MHz, MODE must be set to '1'. For other RF frequencies, fIF is

not equal to 1 MHz. fIF is then dependent on the logical level at Pin MODE and on fRF.

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 parame-

ters. The parasitic board inductances and capacitances also influence the input

matching. The RF receiver T5744 exhibits its highest sensitivity at the best signal-to-

noise ratio in the LNA. Hence, noise matching is the best choice for designing the trans-

formation 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

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

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 appli-

cations, a SAW is used.

4521B–RKE–01/03

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