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I have a question concerning BPSK and the bandpass filter that seems to be sometimes placed before the receiver circuit (the receiver is a mixer and lowpass filter).

Basically, from what I've noticed, there is sometimes a bandpass filter before the receiver of BPSK (and other types of receivers too I think). Is the only purpose of this bandpass filter to band limit the signal and basically just improve the carrier to noise ratio? It doesn't actually need to be there does it? It's optional right?

Thanks.

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The mixer/LPF filter that you describe as the receiver is typically one that is designed (possibly for optimal performance e.g. matched filter) using a mathematical model that assumes its input is the desired signal plus noise (often AWGN) and nothing else. If other signals are present, they are assumed to be orthogonal to the signal of interest (as in CDMA signaling) and thus not interfering with the receiver operations. Such assumptions are rarely valid in practice since the antenna picks up adjacent-channel signals etc. as pointed out in JasonR's answer, and such signals are not necessarily orthogonal to the desired signal. So the bandpass filter is needed to eliminate such signals. If out-of-band interference is guaranteed to be absent, then the bandpass filter might not be needed; but paranoid designers who don't trust such guarantees and suspect that the customer will come right back and complain that the receiver does not work as certified (usually because of out-of-band interference that the customer (who is always right) never told them about) will include the bandpass filter right from the start to avoid re-doing the work.

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The filter that you're referring to is called a preselection filter. Its purpose is to filter out everything but the desired signal of interest before mixing to baseband. Unwanted components could include other signals that are nearby in frequency, or just noise that lies outside the desired signal's bandwidth.

Preselection can serve multiple purposes:

  • It is common to have a low-noise analog amplifier in your receiver chain to reduce the receiver's overall noise figure. Bandlimiting the input of the amplifier using a preselector can result in improved amplifier performance in some cases, especially those where there are strong interfering signals in nearby bands.

  • In cases where bandpass sampling is used (i.e. the desired signal is sampled directly at its nonzero intermediate frequency), a bandpass filter will typically be required in order to remove other signal or noise components that would otherwise alias into the sampled signal bandwidth.

Whether the filter is optional or not would be application-dependent.

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