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rfRXD0420/0920
3.2.2.1
DYNAMIC REFERENCE VOLTAGE
Selection of component values for R1 and C2 is an
A dynamic reference voltage can be derived by averag-
ing the received signal with a low-pass filter. The exam-
ple ASK application circuit shown in Figure 3-9, the
low-pass filter is formed by R1 and C2. The output of
the low-pass filter is then fed to OPA-.
The setting of the R1-C2 time constant depends on the
ratio of logical ones versus zeros and a trade off in
stability versus receiver reaction time. If the received
signal has an even number of logical ones versus
zeros, the time constant can be set relatively short.
Thus the reference voltage can react quickly to
changes in the received signal amplitude and differ-
ences in transmitters. However, it may not be as stable
and can fluctuate with the ratio of logical ones and
iterative process. First start with a time constant
between 10 to 100 times the signal rate. Second, view
the reference voltage against the RSSI signal to
determine if the values are suitable. Figure 3-12 is an
oscilloscope screen capture of an incoming RF square
wave modulated signal (ASK on-off keying). The top
trace is the data output of OPA (Pin 18). The two
bottom traces are the RSSI signal (Pin 21, bottom
square wave) and generated reference voltage (Pin 19,
bottom trace centered in the RSSI square wave). The
goal is to select values for R1 and C2 such that the
reference voltage is in the middle of the RSSI signal.
This reference voltage level provides the optimum data
comparison of the incoming data signal.
zeros. If the time constant is set long, the reference
3.2.2.2
STATIC REFERENCE VOLTAGE
voltage will be more stable. However, the receiver
cannot react as quickly upon the reception of a
received signal.
A static reference voltage can be derived by a voltage
divider network.
FIGURE 3-12:
RSSI AND REFERENCE VOLTAGE COMPARISON
OPA
(Pin 18)
RSSI
OPA-
(Pin 19)
(Pin 21)
DS70090A-page 16
Preliminary
? 2003 Microchip Technology Inc.
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