TRF6901

SINGLECHIP RF TRANSCEIVER

SLWS110G − SEPTEMBER 2001 − REVISED JUNE 2004

11

POST OFFICE BOX 655303

detailed description (continued)

demodulator

The quadrature demodulator decodes digital frequency shift keying (FSK) modulation. An external ceramic

discriminator or an equivalent discrete circuit is required at terminal 35. The demodulator is optimized for use

with a ceramic discriminator. Thus, the use of a packaged ceramic discriminator is recommended for the best

performance. Internal resistors can be programmed with D<14:12> to tune the demodulator center frequency.

The recommended default setting for the demodulator tuning bits is D<14:12> = 110. The resonant frequency

of the discrete-component discriminator can be calculated from the inductor and capacitor values used in the

circuit. A parallel resistor may be added to reduce the quality factor (Q) of the tank circuit, depending on the

application.

ƒ

res

+

1

2

p LC

35

LR

C

External Tank

Figure 4. Optional External Discrete Demodulator Tank

post-detection amplifier/low-pass filter

The post-detection amplifier operates as a low-pass transimpedance amplifier. The external low-pass filter

circuit must be optimized for the data rate. The 3-dB corner frequency of the low-pass filter should be greater

than twice the data rate. Various low-pass filter designs use two to five components and may be first- or

second-order designs. Simple 2-element filter component values and 3-dB bandwidths are contained in Table 1.

Internal Low-Pass Amplifier

37

36

External

Low-Pass

Filter

R1

C1

C2

R2

Figure 5. Post-Detection Amplifier/Low-Pass Filter

Table 1. Various Post-Detection Amplifier/Low-Pass Filter

3-dB Bandwidth and Corresponding Component Values

f3dB (kHz)

10

20

30

60

R2 (k

Ω)

220

220

220

220

C2 (pF)

68

33

22

10