Notch Filter

 
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überarbeitet am 26.9.2007

Some interferences are due to spurious signals within a sideband of a AM or a SSB transmission. By using a very narrow IF filter, You can cut off the interfering signal, but the sound will be very bassy and dull.

As an alternative, You can suppress a narrow band interference signal with the Notch filter.

In some receivers, the notch filter works on high frequency (intermediate frequency) - level, on the genuine high frequency signal. An eliminated strong interfering signal won't affect the gain control AGC any longer.
In other receivers, the notch filter works on the audio frequency -level, here the whistling noise is filtered out of the audio signal. This circuit has the advantage, that the success of the measure does not depend on the nature of the whistling noise. How the whistling noise is generated, is not of any importance to the circuitry. If the eliminated whistling lies in the language frequency range, the language can become incomprehensible or the music, with which some tones missing, may be unbearable.

After a station has been tuned in and the suitable mode of operation has been selected, passband tuning has been optimally adjusted, You activate the Notch Filter (NOTCH) as last step. Turn the control knob, until the whistling is suppressed as well as possible.
In a few sets, as the AOR AR-7030, You find a fully automatic Notch filter, that detects the pitch of the whistling automatically.