37
DSP 07
AF Filter
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SSB, CW, FSK and PSK Modes
The AF filter, which is placed after the in-band AGC, switches in tandem with the IF filter when the low-cut filter, high-cut
filter or passband width is switched. For this reason, the AF filter is equipped with a similar function as the IF filter for
changing the low-/high-cut frequency, WIDTH (passband width) and SHIFT (center frequency of the filter).
The AF filter plays the roles of assisting the IF filter and making the desired signal easier for hearing. Other filters for the
audio signal after demodulation include the audio peak filter and RX equalizer.
Emphasis was also placed on the AF filter design of the TS-890S to process the signal in the stage preceding
demodulation in the SSB, CW, FSK and PSK modes.
By combining demodulation using the high-performance PSN system, band limitation in the stage preceding
demodulation will become equivalent to band limitation in the AF stage. This offers the advantages of shaping the filter
(slope characteristics) and achieving a design with the group delay size of the steep high-pass filter (low-cut) for the
audio signal maintained at about the same size as that of a low-pass filter (high-cut), thereby contributing to a high
sound quality.
The IF filter is equipped with a function for switching the IF filter shape (slope characteristics), while the AF filter allows
the bandwidth to be widened or narrowed with respect to the low-cut, high-cut or WIDTH settings configured for the cut-
off frequency of the IF filter. There are three options on the RX Filter screen, “Narrow”, “Medium” and “Wide”. Medium”
sets the passband to one that is the same as that of the IF filter, “Narrow” sets the passband to one that is narrower
than that of the IF filter, and “Wide” sets the passband to one that is wider than that of the IF filter. The AF filter has the
effect of enhancing the sharpness of the sound and improving the ease of hearing. Selecting “Narrow” makes the sound
sharper, while selecting “Wide” delivers a more analog-like sound with emphasis on the IF filter and AGC characteristics.
Amplitude (dB)
Frequency (Hz)
Fig. 43 Comparison of AF Filter Passband Width in the SSB Mode (Narrow, Medium, Wide)
Amplitude (dB)
Frequency (Hz)
Fig. 44 Comparison of AF Passband Width in the CW Mode (Narrow, Medium, Wide)
