Rainbow Electronics RF12 - Manual (Page 26)

RF12
allowable local oscillator frequency error.
Whenever a low frequency error is essential for the application, it is possible to “pull” the crystal to
the accurate frequency by changing the load capacitor value. The widest pulling range can be achieved if
the nominal required load capacitance of the crystal is in the “midrange”, for example 16 pF. The
“pull-ability” of the crystal is defined by its motional capacitance and C
0
.
RX-TX ALIGNMENT PROCEDURES
RX-TX frequency offset can be caused only by the differences in the actual reference frequency. To
minimize these errors it is suggested to use the same crystal type and the same PCB layout for the
crystal placement on the RX and TX PCBs.
To verify the possible RX-TX offset it is suggested to measure the CLK output of both chips with a
high level of accuracy. Do not measure the output at the XTL pin since the measurement process itself
will change the reference frequency. Since the carrier frequencies are derived from the reference
frequency, having identical reference frequencies and nominal frequency settings at the TX and RX side
there should be no offset if the CLK signals have identical frequencies.
It is possible to monitor the actual RX-TX offset using the AFC status report included in the status
byte of the receiver. By reading out the status byte from the receiver the actual measured offset
frequency will be reported. In order to get accurate values the AFC has to be disabled during the read by
clearing the "en" bit in the AFC Control Command (bit 0).
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