Noise Blankers; Features Of The Nb1 And Nb2 - Kenwood TS-990S In-Depth Manual

03 RECEPTION

Noise Blankers

Features of the NB1 and NB2

The TS‑990S has two noise blankers; NB1 for analog processing and NB2 for digital processing by DSP.
NB1 is effective for short‑cycle pulses, such as ignition noise. NB2 is effective for noise that the analog noise blanker
(NB1) cannot follow. The following block diagram shows the analog noise blanker (NB1).
NB1 has the circuit configuration inherited from that of conventional models, which is effective for weak noise. A pulse
signal after passing through a narrowband filter changes its noise waveform, increasing the pulse width. Thus, at the
roofing filter input stage, which is not subject to influence of pulse noise, signals are picked up and switch circuits
operate.
For example, if a pulse with a short cycle is entered as shown in diagram 1, the AGC in the noise blanker circuit does not
react to it, thus the switch functions and a muted signal will be present.
To the contrary, if a pulse with a long cycle is entered as shown in diagram 2, the AGC reacts to it and corrects the gain,
thus the switch does not function and the signal will not be muted.
To adjust the effect of NB1, rotate the [NB1] knob for the main band, or press the [NB1/SEL] key for the sub band. The
larger the value displayed, the more the effect to noise.
The DSP digital noise blanker (NB2) may insufficiently take effect if the desired signal is strong or the roofing filter
bandwidth is narrow. However, in CW mode, and in the case of a bandwidth of 500 Hz or lower, it may be unexpectedly
effective. This is because NB2 operates flexibly, conforming the blanking time to the pulse length.
NB2 produces an effect on acquisition of a weak desired signal that is buried in noise and with a long pulse width that
NB1 cannot eliminate. Chapter 6, DSP, describes the details of NB2.
● How to Use the NBs and NRs
There is a term: "NB cross‑modulation". This means the situation of modulation in which a noise blanker is running
falsely recognizes the desired signal or adjacent signal as a noise pulse. It does not relate to the front‑end performance.
Once a false recognition takes place, the noise to be eliminated as of non‑desired signals clearly appears (or noise
clearly appears to the keying in CW) or the desired signal is heard with distortion.
The former case is caused by the noise blanker being unable to produce an effect when the desired signal is relatively
strong, or when a strong signal appears in an adjacent frequency. This is because a strong signal activates the AGC of
the noise blanker so that the noise amplifier gain decreases. If the level of the signal and the level of the pulse noise are
equivalent, placing an attenuator or disabling the pre‑amplifier decreases the front‑end gain and may restore the effect
of the noise blanker.
The latter case may easily take place by increasing the noise blanker level. This is a trade‑off and is inevitable. If the
received signal appears to be distorted, turn off the noise blanker and check the received sound. Turn on and adjust the
noise blanker level to rectify the distortion if doing so takes effect.
16
Buffer NB AMP2 NB AMP3


AGC
Fig. 23 Noise Blanker Circuit (NB1) Block Diagram
NB AMP4 Buffer
Noise
Detection
Switching
NB AGC
AMP
Noise Blanker
Threshold Level
Mute Signal