4 DSP
Figure 4-10 shows how BC cancels beat signals, as monitored by an FFT analyzer. Notice how multiple beats are clearly removed by BC.
There are two methods available for beat cancellation: BC1 and BC2. BC1 is tuned to be effective against weak or continuous beat interference, while BC2 cancels intermittent beats such as a CW signal. Note that since BC is designed to remove beats, it does not function in CW mode.
BC is a signal process method at the AF stage. Therefore, if there is a beat signal in proximity that is stronger than the target signal, BC effectively removes the beat interference from the audio output, but in the event the AGC is activated by the beat signal, the target signal is suppressed when received.
In such an occasion, the auto notch or manual notch filter that works at the IF stage is more effective.
4.6.2 Noise Blanker NB2 (IF Processing)
We explained in the section of RX circuity that
A noise blanker is designed to remove pulse noise at the IF stage to reveal the target signal suppressed by the AGC that was activated by the pulse noise. In addition to the analog noise blanker (NB1), the
NB2 employs a newly developed envelope tracking method, making it effective against noise that defies the tracking of the analog noise blanker (NB1).
Unlike the analog noise blanker, the procedure of NB2 is not a simple blanking of pulse noises from the target signal. NB2 removes pulse noises by tracking the RX signal level to automatically detect pulses and comparing the level of the pulses and of the target signal excluding the pulses to attenuate the pulse parts appropriately. Hence, even a long pulse can be processed without seriously degrading the target signal.
Figure 4-11 shows the time waveform of a signal containing pulse noises and a CW signal while NB2 is inactive, and Figure 4-12 shows the time waveform of the same signal while NB2 is active.
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