Demonstration Board Hardware Description; Rf-Section; The Loop Filter; If Filter - Texas Instruments CC900DB User Manual

4 Demonstration Board hardware description

4.1

RF-section

The RF section consists of a CC900 chip with a few external components. The different
parts of the circuit are explained below.

4.1.1 The loop filter

The PLL loop filter contains the components C121-C123 and R121-R123. The
®
SmartRF Studio software program has been used to calculate the component values.
Using the calculated component values for the loop filter gives an optimum loop
bandwidth for the selected system parameters.
The transmitted frequency is FSK modulated, which means that the bits '0' and '1' is
coded by jumping between two different frequencies. The loop filter bandwidth is
optimised according to the frequency separation (difference between the two
frequencies) and the data rate (the speed we jump between these two frequencies).

4.1.2 IF filter

The Demonstration Board uses the internal 200kHz IF filter. The 200kHz IF filter has a
larger bandwidth than the 60 kHz IF filter, and therefor allows us to use a less accurate,
and hence less expensive crystal. The sensitivity is slightly less than for the 60 kHz IF,
but is considered adequate in this application.

4.1.3 The modulation input/output

The modulation input/output (DIO) is connected to a micro-controller I/O pin. The data
to be sent is Manchester encoded (also known as bi-phase-level coding) by the micro-
controller. The Manchester code ensures that the signal has no DC component, which is
necessary for the FSK demodulator to work in an optimal way. The Manchester code is
based on transitions; a "0" is encoded as a low-to-high transition, a "1" is encoded as a
high-to-low transition. This is illustrated in the figure below.
1 0 1 1 0 0 0 1 1 0 1
TX
data
SWRU064
Time
12