Radio Receiver Design .page088

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Radio Receiver Design .page088 Powered By Docstoc
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                        Receiver Configurations                                                            73


                         amplified and converted to a first IF frequency just as in any other receiver. Any
                        tuning necessary is done by the preamplifier and local oscillator. In the case of
                         spread spectrum receivers, it is very important that the frequency of the first IF
                        be higher than the bandwidth of the preamplifier. Linearity is also very important.
                        The reason for both is to prevent unwanted vulnerability to interference, which we
                        will discuss in the following pages. This concern is especially important in spread
                        spectrum receivers that are employed to resist either deliberate interference or
                        inadvertent noise.
                               The despreading operation is usually performed at a second mixer, primarily
                        because it is more practical to design the local refernce at a fixed frequency than to
                        make it variable. The local reference itself consists of a code generator and a
                        modulator that generates a replica of the desired, received signal. (If the desired
                        signal is a direct sequence signal, then the local reference is also direct sequence.
                        If the desired signal is frequency hopped, then so is the local reference. The code
                        employed in both cases is the same as that used by the transmitter to spread the
                        signal.)
                               Following the mixer in which the local reference signal and the received
                        signal are multiplied together, the receiver is a TRF receiver, just like any other
                        superheterodyne receiver, consisting of the second IF and a demodulator. The
                        spread spectrum receiver includes a signal presence detector that is similar to the
                        squelch circuit in other receivers, clock tracking circuits similar to those used in
                        bit synchronizers, and automatic gain control (AGC) processing similar to that of
                        other receivers.
                               We will discuss the details of both spread spectrum and more conventional
                        receivers further in the succeeding chapters and in Appendix 11.


                        3.11 TRADEOFFS IN RECEIVER DESIGN

                        At every step in a receiver design, tradeoffs are necessary, and each choice affects
                        the overall performance of the receiver. The following list of tradeoffs and their
                        effects outline the overall problem. There are many more tradeoffs to be made in
                        superheterodyne receivers than in TRF or regenerative receivers and many more
                        possible effects. Nevertheless, the superheterodyne technique produces consis-
                        tently better overall performance than any of the alternatives. In the chapters that
                        follow, these tradeoffs and their effects will be covered in more detail.
                              Table 3.2 summarizes receiver design tradeoffs and their effects on a re-
                        ceiver’s characteristics, by receiver stage. Other receiver stages are auxiliaries to
                        those listed in Table 3.2. Although they contribute to overall receiver perfor-
                        mance, we have listed only those that directly process the desired signal in some
                        way. More information on the auxiliaries (AGC, squelch, etc.) will follow in the
                        succeeding chapters.




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