Reception; Demodulation - Kenwood TS-890S In-Depth Manual

07 DSP

Reception

Demodulation

● SSB, CW, FSK and PSK Modes
Demodulation processing in the SSB, CW, FSK and PSK modes is based on the well-established PSN (Phase Shift
Network) system.
The optimal design of the PSN system is determined by the characteristics of the IF filter that comes with the transceiver.
If a steep and large-attenuation filter similar to the IF filter of the TS-890S is used, the number of reverse bandwidths to
be canceled by the PSN system is very small. In the earlier section on IF filters, we have explained that the passband
width, which is the basis for determining the shape factor, and also the IF filter configuration vary depending on the RX
mode. By adopting a PSN design according to the IF filter to minimize the filter order, it is possible to improve the group
delay characteristics on the low passband, which is a drawback of the PSN system.
Doing so enables quality sound to be obtained up to the lower band when the passband width of the IF filter is widened.
In the SSB mode, "0 Hz" can be configured as the low-cut frequency for the IF filter. This means that the cut-off
frequency has been configured for the carrier point so as to extend the low passband to its maximum using the PSN
system described above.
● AM Mode
In the AM mode, absolute demodulation is performed to obtain an envelope of the IF signal as a demodulation signal.
● FM Mode
In the FM mode, after mixer processing and conversion to I/Q signal are performed to convert the center frequency of
the IF signal to 0 Hz, demodulation signal is obtained by performing Atan processing to extract and differentiate the π ∼
+π phase component.
● FSK Mode
In the FSK mode, audio signal that is detected with the PSN system is output to a speaker or external output terminal
regardless of the status of the RTTY decoder (On or Off). When the RTTY decoder is turned on, frequency is detected
through a path that is different from that for audio output, and the RTTY baseband signal is acquired and used for
decoding.
The RTTY decoder operates in the FSK mode (only if 170 Hz is configured for the shift width).
● PSK31/63 Mode
In the PSK mode, audio signal that is detected with the PSN system is output to a speaker or external output terminal
regardless of the status of the PSK31/63 decoder (On or Off). When the PSK31/63 decoder is turned on, delay detection
is performed through a path that is different from that for audio output, and the PSK31/63 baseband signal is acquired
and used for decoding.
Also, the AFC (Automatic Frequency Control) feature can be used with the PSK31/63 decoder. AFC processing is
performed on the path from IF to the decoder, but audio output signals will not be processed. This is to output the audio
signal to the PC while using the internal decoder, thereby enabling AFC and decoding to be carried out concurrently by
the decoder software, etc. that is installed on the PC.
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