Chipcon CC400DB, Руководство пользователя

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SWRU068
13
4.1.4 LNA/PA matching
The input/output antenna matching network is optimised for 433.92MHz operation. The
component values were calculated in the SmartRF
®
Studio software program, and
consist of C51, C61, L51 and L61. To compensate for layout parasitics the component
values were tuned to optimise for 50 Ohm input impedance in receive mode, and
maximum output power in transmit mode.
4.1.5 Voltage Controlled Oscillator (VCO)
The VCO tank is optimised for the 433MHz band, covering approximately 410 –
450MHz. The tank contains the components C91, C92, C93, L91 and the variable
capacitance D2.
4.1.6 Crystal oscillator
The crystal oscillator frequency is 12.000 MHz set by X1. We have chosen a low cost
crystal for this application. The crystal is specified as 50/50/10/12, which means that it
has a +/- 50ppm initial tolerance, and will drift less than +/-50ppm over a temperature
range of –10 to 60°C. SmartRF
®
Studio specifies a total crystal accuracy of +/-50ppm
for the parameters used here. This means that the total system is only specified for room
temperature, as all the accuracy is “used up” for the initial tolerance. If a wider
temperature range is required, a higher accuracy crystal should be used.
The loading capacitors, C151 and C161, together with the input capacitance and
paracitics makes up the total 12pF load capacitance the crystal is designed for.
4.1.7 Antenna
This antenna is an example of a PCB integrated antenna. Several antenna configurations
were evaluated, and the short stub was found to be the best solution for this application.
As the stub is shorter than one-quarter wavelength (
λ
/4 = 16.4 cm at 433.92MHz), it has
to be prolonged and matched using lumped elements. The short stub has a capacitive
reactance and is thus prolonged using an inductor (L8) in series. Another inductor in
shunt (L7) matches the antenna to 50 Ohm.
The antenna is fairly omni-directional, but have slightly more emission sideways. This
follows from the fact that the antenna elements and the ground plane have the largest
physical dimensions normal to this direction. Placing the board in a vertical direction, or
placing two units parallel sideways will give the best range.
In general, small electrical antennas are not very efficient. Electrical stub antennas
should be designed as long as possible, up to one-quarter wavelength. They should be
kept away from the ground plane and other surrounding metallic bodies. For hand-held
remote controls, the antenna should not be in close contact with the hand. If this can not