4 DSP
4.3 Interference Elimination Within AGC Loop
TS-590S also incorporates rich and powerful interference elimination functions that work within the IF-AGC loop (Figure 4-3).
The previous model (TS-2000S) already featured a digital IF filter and the auto notch filter function, and in the TS-590S, a digital noise blanker (NB2) and a manual notch filter function*1 have been added.
These functions within the AGC loop eliminate interference to make a weak target signal emerge clearly.
*1 The auto notch filter and manual notch filter cannot be used at the same time.
4.3.1 Digital IF Filter
The digital IF filter of the TS-590S consists of slope tuning combining an IIR (infinite impulse response) LPF and an IIR HPF in SSB mode, of WIDTH/SHIFT using an IIR BPF in CW, FSK and SSB-DATA modes, and of an FIR (finite impulse response) BPF in AM mode. (In FM mode, since an FM detection IC is used, the signal at the IF stage is not processed by the DSP. Instead, the demodulated audio signal is processed by an AF filter.)
The attenuation of the filter used in SSB, CW, FSK and SSB-DATA modes is set to 110 dB and the filter slope is constantly sharp regardless of the setting of the slope tuning or of the WIDTH. Meanwhile, because the IF frequency has been raised, the filter’s own group delay characteristics are improved and the influence of the group delay is minimized even when a LO CUT frequency (HPF) that is close to the carrier point is selected in the SSB mode.
Figure 4-5 Results of Amplitude and Frequency Analysis of the Digital IF Filter (SSB Mode)
In CW, FSK and SSB-DATA modes, a BPF with bandwidth of as narrow as 50 Hz can be selected, but the group delay curve shows relatively large spectral excursions around the cut-off frequency. By inserting an additional group delay compensation filter, we have decreased the delay of the filter as a whole to obtain usable characteristics and to take advantage of the fast response of the IF-AGC.
