Rf front end, U3741bm – Rainbow Electronics U3741BM User Manual

Page 4

U3741BM

4662B–RKE–10/04

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 the block diagram, the front end consists of
an LNA (low noise amplifier), LO (local oscillator), a mixer and 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

for the mixer. f

is dependent on

the voltage at pin LF. f

is divided by a factor of 64. The divided frequency is compared

to f

XTO

by the phase frequency detector. The current output of the phase frequency

detector is connected to a passive loop filter and thereby generates the control voltage
V

for the VCO. By means of that configuration, V

is controlled in a way that f

/64 is

equal to f

XTO

. If f

is determined, f

XTO

can be calculated using the following formula:

The XTO is a one-pin oscillator that operates at the series resonance of the quartz crys-
tal. According to Figure 2, 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

. When designing the system in terms of receiving bandwidth, the accuracy

of the crystal and XTO must be considered.

Figure 2. PLL Peripherals

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 2 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. Therefore, self polling also does not work in
that case.

is determined by the RF input frequency f

and the IF frequency f

using the follow-

ing formula:

XTO

--------

DVCC

XTO

LFVCC

LFGND

C10

R1 = 820

Ω

C9 = 4.7 nF
C10 = 1 nF

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