TRF6903

SINGLECHIP MULTIBAND RF TRANSCEIVER

SWRS022B − MARCH 2004 − REVISED AUGUST 2004

21

POST OFFICE BOX 655303

phase detector and charge pumps

The phase detector is a phase-frequency design. The phase-frequency detector gain is given by:

three steps: 250

µA, 500 µA, and 1000 µA.

loop filter

The loop filter must be carefully chosen for proper operation of the TRF6903. The loop filter as shown in Figure 9

is typically a second- or third-order passive design and in FSK operation should have a bandwidth wide enough

to allow the PLL to relock quickly as the external crystal frequency is pulled off-center during modulation. The

loop filter should also be wider than the data modulation rate. These requirements should be balanced with

making the loop narrow enough in consideration of the reference frequency. In OOK the VCO frequency is not

changed during data modulation, so the filter bandwidth may be narrower than the modulation bandwidth. Filters

can be calculated using standard formulas in reference literature. Some third-order filter examples are shown

in Table 4.

F(s)

+

1

) sC2R2

s(C1

) C2 ) sC1C2R2)

1

C3R3

s

) 1

C3R3

C1

lcp

C2

R2

C3

Vtune

R3

External-Loop Filter

15

13

VCO_VCC

VCO_TUNE

CP_OUT

Figure 9. Third-Order Loop Filter and Transfer Function

Table 4. Loop Filter Component Values For Various Data Rates at a Reference Frequency of 409.6 kHz,

0.5-mA Charge Pump Current

Bit rate − kbps

Manchester coding

1.024

2.048

4.096

8.192

16.384

19.2

32.768

65.536

Bit rate − kbps

NRZ coding

2.048

4.096

8.192

16.384

32.768

38.4

65.536

131.072

Data rate − kHz

Fundamental freq of BB

1.024

2.048

4.096

8.192

16.384

19.2

32.768

65.536

C1, nF

47

12

2.7

0.68

0.18

0.12

0.043

0.01

Loop filter component

C2, nF

1800

430

100

27

6.8

5.1

1.8

0.47

Loop filter component

(selected to nearest

standard value)

C3, nF

27

6.8

1.5

0.39

0.1

0.075

0.027

0.0068

(selected to nearest

standard value)

R2, k

Ω

0.39

0.75

1.5

3

5.7

6.8

12

24

R3, k

Ω

0.75

1.5

3

5.7

12

15

22

47

−3−dB bandwidth, kHz (approximate)

1.28

2.56

5.12

10.24

20.48

24

40.96

81.92

VCO

The voltage-controlled oscillator (VCO) produces an RF output signal with a frequency that is dependent upon

the dc-tuning voltage at terminal 13. The tank circuit is passive and has integrated varactor diodes and

inductors. The open-loop VCO gain is approximately 100 MHz/V.

A <1:0> is used to set the output divider ratio for operation within the 315-MHz, 433-MHz, 868-MHz, or 915-MHz

bands.

When the STDBY terminal is high, the reference, PLL, VCO, and dividers are powered up. When STDBY is low,

these blocks are powered down.