R a d io S y s te m
M o d e l
T r a n s m itte r
R F C h a n n e l
R e c e iv e r
Figure 1.2.1
4
R e c e iv e r S ig n a l P r o c e s s in g
C o u p lin g c a p a c ito r s iz e d e te r m in e d b y m a x im u m s ig n a l p u ls e w id th R X D e te c to r L o w -P a s s F ilte r L o w -p a d e te rm s ig n a th r s s filte in d e d l p u ls e e s h o ld r b a n d b y m in w id th s e ttin g w id th im u m a n d D a ta S lic e r T h r e s h o ld D a ta O u t
G a in
M in P u ls e W id th E n c o d e d D a ta S ig n a l
M a x P u ls e W id th
Figure 1.2.2
1.2.2 Data rate and bandwidth Figure 1.2.2 is a generic block diagram of an RF receiver. This is where most of the action takes place in a radio communication system. There are two filters in this block diagram that you need to know about before you start writing code. The low-pass filter limits the rate that data can be sent through the radio system. And it also has a major impact on the range of the system. As you probably guessed, there is a trade-off here. For a fixed amount of transmitter power, you can transmit farther if you transmit at a lower data rate. The coupling capacitor in the block diagram creates a high-pass filter (in other words, your signal is AC coupled). You have to choose a data rate and use a data encoding scheme that lets your information flow successfullythrough these two filters. And if you get this right, these filters will greatly contribute to the overall performance of your system. It is best to think in terms of the most narrow pulse (or most narrow gap) in your encoded signal, which must match the bandwidth of the low-pass filter, and the widest pulse in your encoded signal (or the widest gap), which must correctly match the time constant formed by the coupling capacitor and its associated circuitry. It is the minimum and maximum pulse widths (and gaps) in the encoded data that must be "in tune" with the filters in the receiver – not the underlying data rate.

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