TRF6900
SINGLE-CHIP RF TRANSCEIVER
SLAS213C – SEPTEMBER 1999 – REVISED MAY 2000
low-pass filter amplifier/post-detection amplifier (continued)
C
1
External
Components
R
1
External
Components
R
1
C
C
2
1
32
31
30
32
31
30
From
Data
Switch
R
2
From
Data
Switch
R
2
–
+
To Data Slicer
–
To Data Slicer
V
ref
+
V
ref
st
nd
Figure 11. 1 -Order Low-Pass Filter
Example
Figure 12. 2 -Order Low-Pass Filter Example
st
nd
The amplifier can be configured as a 1 - or 2 -order low-pass filter with bandwidths that are determined by
nd
external components. The internal resistor R is set to 10 kΩ, hence the –3dB corner frequency for a 2 -order
2
low pass filter (as shown in Figure 12) can be derived from the following formula:
1
ƒ
g
2
10 k
R
C
C
2
1
1
data slicer
The data slicer is fundamentally a comparator. The data slicer provides binary logic level signals, derived from
the demodulated and low pass-filtered IF signal, that are able to drive external CMOS compatible inputs. The
noninverting input is directly connected to the internal reference voltage, V , and the inverting input is driven
ref
by the output of the low-pass filter amplifier/post-detection amplifier. The decision threshold of the data slicer
is determined by the internal reference voltage, V . The automatic frequency control (AFC) loop scheme for
ref
the TRF6900 is shown in Figure 13.
Low-Pass Filter Amplifier/
Post-Detection Amplifier
C1
External
Tank Circuit
R1
External
Components
C2
35
34
32
31
30
IF2_IN
FM/FSK
–
Demodulator
39
DATA_OUT
28
–
Limiter
+
V
ref
+
V
ref
–
+
Internal
Variable
Inductor
V
ref
Integrator
Figure 13. AFC Loop to Control the Data Slicer Decision Threshold
15
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