Telex 38109-977 manual Receiver block diagram

occur. In some situations, it is possible for this to cause damage to either the transmitter, transmission line, and/or antenna.

Now let’s look at the receiver and it’s primary functional aspects (see Figure 7.11). The receiver in a wireless system is the exact compliment of the transmitter, but is usually much more sophisticated and complex in design. Its job is to receive the signal from the receive antenna and extract the source signal so that it matches the original exactly. In practice, there will always be some modification or distortion of the source signal in the course of transmission, but good quality wireless systems minimize this to a level that is indistinguishable.

Figure 7.11 Receiver block diagram.

	Front End	RF		Ist IF		2nd IF
				Filter		Filter
FROM	Filter	Amplifier	Mixer		Mixer

ANTENNA

1st Local	2nd Local
Oscillator	Oscillator		Demodulator
		Expander
	TO
	AUDIO PROCESSOR

As in the transmitter, the antenna will be covered in the next section. The receiver starts with the front-end filter. The front-end filter is extremely important to successful operation in high RF level environments. The front-end filter is the first line of defense. Its job is to limit the number of potential interfering frequencies that could affect the receiver. It is usually a passive, linear section and it must be impedance matched to the antenna for proper signal transfer. Linearity is the most important factor in a front end, even more so than how tight or narrow the section is. A high degree of linearity will ensure that no intermodulation products are generated in the front end before extraneous RF signals are filtered out. Having a front-end that is relatively tight and that is extremely linear is critical if the system is to work properly under worst-case RF scenarios.

The next section of the receiver is the first RF amplifier. The first RF amp’s job is to take the extremely low level RF signal coming through from the front end and bring it up to a usable level. The incoming RF signal at the first RF amp can vary dramatically from less than 0.5 ∝V to almost the value of the transmitter output. The key for the first RF amp is that it should be able to handle very small, as well as, relatively large incoming signals within it’s linear region of operation. See Figure 7.12. To maintain a good linear region, RF amps normally require a high current drain which can negatively impact battery life. A compromise between linearity and effective battery life must be managed carefully.

C h a p t e r 7 - D e s i g n o f W i r e l e s s I n t e r c o m S y s t e m s 103