A good crystal band-pass filter for a single-sideband phone would be?

A good crystal band-pass filter for a single-sideband (SSB) phone communication is typically chosen to accommodate the necessary bandwidth for voice transmission while minimizing interference from adjacent channels. A bandwidth of 2.1 kHz is widely accepted for SSB operation. This bandwidth effectively balances the need for clarity in voice communication and efficient use of the radio spectrum.

In SSB operations, having just the right amount of bandwidth ensures that the signal remains intelligible while not occupying excessive space on the frequency spectrum, which is crucial in crowded bands. The 2.1 kHz filter allows for adequate fidelity without overly broadening the signal, which would increase susceptibility to noise and interference.

Selecting a 1.5 kHz filter could restrict the audio quality too much, limiting the clarity of received signals. Likewise, options of 2.5 kHz and 3.0 kHz would allow wider bandwidths than typically necessary for standard SSB communication, which may invite interference from neighboring signals. Thus, 2.1 kHz represents an optimal choice for SSB phone applications, ensuring effective and clear communication while adhering to bandwidth efficiency standards.