Texas Instruments CC400DK, User Manual, Page: 7 / 22

CC400DK Development kit (rev. 1.5) 2002-09-17 Page 7 of 22
The Voltage Controlled Oscillator (VCO)
The VCO tank on the evaluation board is optimised for the 433MHz band, covering
approximately 410 – 450MHz. The tank contains the components C91, C92, C93, L91 and
the variable capacitance D2.
To operate in other frequency bands, C93 and L91 can be altered. To increase the operation
frequency C93 or L91 (or both) should be decreased (use 8.2nH for L91, and 1.5pF or 1.8pF
for C93). To decrease the operation frequency C93 or L91 (or both) should be increased (use
12nH for L91, and 2.7pF or 3.3pF for C93).
To find the tuning range for the new VCO tank, set the RF frequency to 300 MHz and 500
MHz in the software, update the device and measure the output frequency. In this way the
VCO tank will be tuned to its minimum and maximum operation frequency respectively.
For further details, please contact Chipcon .
The crystal oscillator
Crystal frequency is set to 12.000 MHz, X1. The crystal oscillator circuit has a trimmer
capacitor, CT152, which reduces the initial tolerance of the crystal to zero by careful
adjustment using a precision frequency counter. The crystal used at this board has ±10 ppm
initial tolerance and ±10 ppm drift over the –10 to +70 °C temperature range. The crystal
oscillator has an AC coupled (C153) test pin for external clock injection, TP1. Be sure to
remove the crystal when an external clock is used. The external clock should have amplitude
of 1-3V
pp
. If using other crystals they should be designed for 12pF load capacitance.
The preselector filter options
There are three preselector filter options: LC-filter, SAW filter, or no filter used. Each of the
three filter alternatives is equipped with a female SMA antenna connector. To choose
between the three filters there is a zero ohm resistor that can be moved (R61-R63).