003: Audio CW filter




When I heard EME signal for the first time, I had no CW filters but SSB filter of 2.7kc. Therefore the concentration on a weak signal less than 0 dB for a long time made me very tired. I came to want an AF filter, and the passive LC filter (88mH) was made soon in 1983. An active BPF (1985) and then smoothing filter of JM1MCF was manufactured (1989) after that. I have made a passive filter again using high Q coil of 43mH recently.

These four filters were compared and estimated at the same time this time.The output of a noise generator was mixed with weak signal of SG, it was put in a receiver, less than 0 dB of situation was made artificially and it was compared by audibility. JM1MCF noise smoother (analog correlation-type of averaging) met that it was best. Background noise becomes quietest clearly.

The next was a passive filter using a 43mH high Q inductor, and the center frequency of around 350Hz seemed to be excellent. A digital filter is in the mainstream already, I think it's still worth an analog LC filter in particular enough, as it can be made easily.

The actual response of the LC filter was measured, so the data as well asthe circuit diagram are raised. (added on Jan.16th of 2020)





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