SWRU064
13
4.1.4
LNA/PA matching
The input/output antenna matching network is optimised for 869.3 MHz operation. The
component values were calculated in the SmartRF
®
Studio software program, and
consists of C51, 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
The Voltage Controlled Oscillator (VCO)
The VCO tank is optimised for the 868MHz band, covering approximately 840 –
904MHz. The tank contains the components C91, C92, C93, L91 and the variable
capacitance D2.
4.1.6
The crystal oscillator
The crystal oscillator frequency is 12.000 MHz set by X1. We have chosen a +/-10 ppm
crystal for this application. The crystal is specified as 10/10/10/12, which means that has
a +/-10ppm initial tolerance, and will drift less than +/-10ppm over a temperature range
of –10 to 70°C. SmartRF
®
Studio specifies a total crystal accuracy of 0/-25ppm for the
parameters used here. This means that we have 10+10=20ppm of worst case deviation
from the specified frequency over the entire temperature range, leaving 5ppm for ageing
and safety margin. A crystal of this type will drift with age less than +/-3ppm/year (often
specified for +/-5ppm the first year), if long product life is anticipated, a higher accuracy
crystal, or one with low ageing, 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
The antenna
This antenna is an example of a PCB integrated antenna. Several antenna configurations
were evaluated, and the
λ
/4 PCB stub was found to be the best solution for this
application. The stub is approximately one-quarter wavelength (
λ
/4 = 8.63 cm at
869MHz), and on therefor just need a 0 ohm at L8.
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
be avoided, a magnet loop antenna could be used. The loop is less sensitive to nearby
objects, but suffers of very low efficiency due to very low radiation resistance. The stub
antenna implemented at the Demonstrator PCB showed better efficiency than a loop of
similar size, even when operated hand-held. For maximum efficiency, the antenna
should be pointed straight up.