AN1833 Freescale Semiconductor / Motorola, AN1833 Datasheet - Page 14

• Manufacturer Part Number: AN1833
• Description: A Low-Power Wireless Remote Control Transmitter and Receiver
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
Automatic Frequency Control
14
The MC13158 OOK receiver uses automatic frequency control (AFC) to
adjust the VCO with respect to the RF to down convert it to the desired
IF frequency. The VCO's output frequency is adjusted by the output
voltage from a quadrature detector buffer amplifier circuit and the loop
filter. The loop filter input is taken from the buffered output of the
frequency discriminator of the MC13158. The discriminator V
frequency response is set by the resonant frequency and bandwidth of
the external quadrature tank circuit; while the dc output component is
determined by the combination of the external load feedback resistor,
R17, and the resistor from inverting input to ground, R15.
The detector dc level, V17, is calculated in this equation:
where V
In the application circuit, a V17 of nominally 2.2 Vdc is chosen, yielding
standard resistor values for R17 of 82 k and R15 of 39 k .
The AFC loop gain and settling time are set by the buffer amplifier
feedback resistor and the capacitor, respectively. The values of these
loop characteristics are left to the designer's discretion, depending on
the system requirements. Stable frequency acquisition responses within
milliseconds have been achieved with the schematic values of this
simple single-pole circuit.
The closed loop frequency error is 12 kHz per 100 kHz change in RF
frequency. This provides:
The AFC output (varactor control voltage) decreases with increasing IF
frequency. This system was designed for high side mixer conversion
(the LO is above the RF). The ground reference of the varactor is taken
•
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Freescale Semiconductor, Inc.
For More Information On This Product,
A bias to the discriminator output
An offset to dc match the loop filter frequency versus voltage out
response to VCO control characteristic
BE
is a diode drop = 0.75 Vdc.
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V17 = { [ (R15/R17) + 1] / (R15/R17)} * V
BE
MOTOROLA
Out
versus
AN1833