Rainbow Electronics RF02 - Manual (Page 17)

RF02
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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).
CRYSTAL SELECTION GUIDELINES
The crystal oscillator of the RF02 requires a 10 MHz parallel mode crystal. The circuit contains an
integrated load capacitor in order to minimize the external component count. The internal load
capacitance value is programmable from 8.5 pF to 16 pF in 0.5 pF steps. With appropriate PCB layout,
the total load capacitance value can be 10 pF to 20 pF so a variety of crystal types can be used.
When the total load capacitance is not more than 20 pF and a worst case 7 pF shunt capacitance
(C0) value is expected for the crystal, the oscillator is able to start up with any crystal having less than
100 ohms ESR (equivalent series loss resistance). However, lower C0 and ESR values guarantee faster
oscillator startup. It is recommended to keep the PCB parasitic capacitances on the XTL pin as low as
possible.
The crystal frequency is used as the reference of the PLL, which generates the RF carrier frequency
(fc). Therefore fc is directly proportional to the crystal frequency. The accuracy requirements for
production tolerance, temperature drift and aging can thus be determined from the maximum allowable
carrier frequency error.