U3742bm – Rainbow Electronics U3742BM User Manual

Page 5

U3742BM

4735A–RKE–11/03

The passive loop filter connected to pin LF is designed for a loop bandwidth of
B

Loop

= 100 kHz. This value for B

Loop

exhibits the best possible noise performance of the

LO. Figure 4 on page 4 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. Self-polling does therefore
also not work in that case.

is determined by the RF input frequency f

and the IF frequency f

using the follow-

ing formula:

= f

- f

To determine f

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

nominal IF frequency is f

= 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

and f

, that depends on the logic level at pin MODE. This is

described by the following formulas:

The relation is designed to achieve the nominal IF frequency of f

= 1 MHz for most

applications. For applications where f

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

case of f

= 433.92 MHz, MODE must be set to '1'. For other RF frequencies, f

is not

equal to 1 MHz. f

is then dependent on the logical level at pin MODE and on f

. Table

1 on page 6 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 U3742BM 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

Ref

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

f = 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.

Figure 5 on page 6 shows a typical input matching network for f

= 315 MHz and

= 433.92 MHz using a SAW. Figure 6 on page 6 illustrates input matching to 50

without a SAW. The input matching networks shown in Figure 6 on page 6 are the refer-
ence networks for the parameters given in the electrical characteristics.

MODE

0 (USA) f

314

----------

MODE

1 (Europe) f

432.92

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