Noise Blankers; Features Of The Nb1 And Nb2 - Kenwood TS-890S In-Depth Manual

02 RECEPTION CIRCUIT

Noise Blankers

Features of the NB1 and NB2

The TS-890S 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 diagram "Noise Blanker Circuit (NB1) Block Diagram" is the block diagram of the analog noise
blanker (NB1).
IF Input
1
2
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.
The adjustment of the NB1 effect can be adjusted by the value displayed on the setting screen by a long press of the
[NB1] key. The larger the value, the larger the effect on noise.
As described above, the noise blanker may insufficiently take effect if the roofing filter bandwidth is narrow. However,
the DSP digital noise blanker (NB2) may be unexpectedly effective in CW mode even in a bandwidth of 500 Hz or lower.
This is because NB2 operates flexibly, conforming the blanking time to the pulse length.
NB2 produces an effect on the acquisition of a weak desired signal that is buried in the noise 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 state in which a noise blanker is behaving with the desired signal
or adjacent signal falsely recognized as a noise pulse. It does not relate to the front-end performance.
There are 2 states: the noise of non-desired signals clearly appears (or noise clearly appears according 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. If doing so takes effect,
then turning on the noise blanker and adjusting the noise blanker level is recommended.
10
Buffer Amp Noise Amp
NB AGC AMP
Fig. 11 Noise Blanker Circuit (NB1) Block Diagram
Noise
Detection
Threshold
voltage
Mute
Signal