Channel selection and Band Selection

The type of filtering speculated above is called“channel-selection filtering”to indicate that it“selects”the desired signal channel and“rejects”the interferers in the other channels.We make two key observations here：（1）all of the stages in the receiver chain that precede channel-selection filtering must be sufficiently linear to avoid compression or excessive intermodulation，and（2）since channel-selection filtering is extremely difficult at the input carrier frequency，it must be deferred to some other point along the chain where the center frequency of the desired channel is substantially lower and hence the required filter Q’s are more reasonable.（The Q of a band-pass filter may be roughly defined as the center frequency divided by the -3-dB bandwidth.）

Nonetheless，most receiver front ends do incorporate a“band-select”filter，which selects the entire receive band and rejects“out-of-band”interferers（Fig.4.3），thereby suppressing components that may be generated by users that do not belong to the standard of interest.We thus distinguish between out-of-band interferers and“in-band interferers，”which are typically removed near the end of the receiver chain.

The front-end band-select（passive）filter suffers from a trade-off between its selectivity and its in-band loss because the edges of the band-pass frequency response can be sharped only by increasing the order of the filter，i.e.，the number of cascaded sections within the filter.Now we note from Chapter 2 that the front-end loss directly raises the NF of the entire receiver，proving very undesirable.The filter is therefore designed to exhibit a small loss（0.5 to 1dB）and some frequency selectivity.

Figure 4.4 plots the frequency response of a typical duplexer（As mentioned in Chapter 3，a duplexer consists of two band-pass filters，one of the TX band and another for the RX band.），exhibiting an in-band loss of about 2dB and an out-of-band rejection of 30dB at 20-MHz“offset”with respect to the receive band.That is，an interferer appearing at f₁（20MHz away from the RX band）is attenuated by only 30dB，a critical issue in the design of both the receive path and the frequency synthesizer（Chapter 10）.

The in-band loss of the above duplexer in the transmit band also proves problematic as it“wastes”some of the power amplifier output.For example，with 2-dB of loss and a 1-WPA，as much as 370mW is dissipated within the duplexer—more than the typical power consumed by the entire receive path！

Our observations also indicate the importance of controlled spectral regrowth through proper choice of the modulation scheme and the power amplifier（Chapter 3）.The out-of-channel energy producted by the PA cannot be suppressed by the front-end BPF and must be acceptably small by design.