AN1833 Freescale Semiconductor / Motorola, AN1833 Datasheet - Page 11

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

External Varactor Controlled Local Oscillator
AN1833
MOTOROLA
Figure 3
For easy reference, the components are labelled in the following
discussion.
The LC oscillator uses the on-board transistor. The MMBV809L is a low-
voltage varactor suitable for UHF applications; it is a single varactor in
an SOT-23 package. This oscillator is tunable over a range of
approximately 1.5 MHz (393.8 to 395.3 MHz) with the control voltage
V
Figure 4
VCO frequency decreases with increasing control voltage since the
varactor diode cathode is taken to V
to the anode.
This circuit is breadboarded using a custom PC board in which the RF
ground is V
mount components were used except where otherwise specified. The
absolute values of the components used will vary with layout placement
and component parasitics.
In the VCO circuit, R1 is the emitter bias resistor. R2 forms the base bias
circuit. C1 and C2 make up the Colpitts capacitive divider network. CS
and CB are capacitive trimmers for the LC oscillator. The shunt capacitor
CS across the varactor diode, MMBV809L [6], and series capacitor CB
are used to adjust the tuning range by varying the capacitance of the LC
oscillator circuit.
This equation shows the relationship:
where C is the equivalent capacitance of CB, CS, and CV.
The relationship for C is:
VCO
1. f = (0.159) / [ { [ [(C1)(C2)] / [(C1 + C2)] + C] * [L] }1/2]
2. C = (CS + CV )(CB) / (CB + CS + CV)
Freescale Semiconductor, Inc.
For More Information On This Product,
from the AFC.
is a schematic of the receiver external local oscillator circuit.
is a curve of VCO frequency versus AFC V
CC
Go to: www.freescale.com
and RF path lengths are minimized. High quality surface
External Varactor Controlled Local Oscillator
CC
and the control voltage is applied
VCO
. Note that the
Application Note
11