BK4819

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1.2.3 RF Frequency Synthesizer

An RF synthesizer is implemented to generate local oscillator(LO) signals. It

includes a voltage controlled oscillator(VCO), a fractional-N divider(frac-N), a

phase-frequency detector(PFD), a charge pump(CP) and a loop filter(LF). The

RF synthesizer is shared for RX mode and TX mode. The block diagram of the

synthesizer is shown in Figure 2.

In RX mode, the RF frequency synthesizer generates unmodulated LO signal.

And the unmodulated LO signal is then divided by an integer Ndiv for down-

conversion mixer in the FM receiver. In TX mode, FM modulation is realized in

the RF frequency synthesizer.

In RX mode, the locked frequency of the synthesizer is equal to Ndiv x (fwanted-fIF).

While in TX mode, the locked frequency of the synthesizer is equal to Ndiv x fwanted.

Channel selection is also implemented by programming the value of fraction-N

through 3-wire SPI interface. On power up or channel reselection, the

synthesizer takes less than 0.3m sec to settle.

For BK4819, the default crystal is 26MHz. The frequency tolerance of the crystal

should be within

2.5ppm to keep a reliable communication.

1.2.4 Input/output Matching

Since the LNA input and the PA output are of single-ended, external balun is not

necessary. Both the input matching and the output matching can be implemented

using low-cost discrete inductors and capacitors. The schematic of input/output

matching is shown in Figure 3.

Figure 3. Schematic of Input/Output matching

As for input matching, capacitor C1 is used for DC-blocking. The recommended

value of C1 is 4.7pF. The DC voltage at PIN15 is about 0V in RX mode. Inductor

L1 is used for impedance transformation. The recommended value of L1 is NC.

As for output matching, capacitor C2 is used for DC-blocking, too.