A filter carrier with narrow bandwidth can be built using electronic diagram below.
This filter carrier with narrow bandwidth is constructed from standard components and is still a very narrow bandwidth, making it ideal for CW applications. As seen in circuit diagram, cascade filter is simple, standard 1 MHz crystal.
6 dB bandwidth is 120 Hz, while the -60 dB bandwidth is only 400 Hz.
Loss bandwidth is no more than 4 dB. Because tolerance of 1 MHz crystal is small, the filter is excellent reproducibility.