SECTION 8
GENERAL
THEORY OF
OPERATION
(See Figure 5)
Figure 6 is a block diagram of the ANC-4 showing the elements that are used to cancel
locally generated noise from the signal received by the station main antenna system.
The station antenna picks up noise, both locally and remotely generated, as well as desired
signals and passes everything directly to the receiver. If the noise level is strong enough,
it will take over the receiver AGC system and reduce the receiver gain for weaker desired
signals, masking them. The detected audio output then contains mostly noise information
unless the desired signal is of sufficient strength to overcome the noise signal amplitude.
This reduces the number of signals actually heard by the operator to those stronger than
the received noise level. When the noise level from local sources, such as power lines,
computers, etc., is strong enough to give high “S” meter readings on the receiver, the
number of intelligible signals received by the operator is greatly reduced when compared
to what could be heard if the local noise level could be overcome. Now DSP noise
reduction devices can allow the operator to improve the intelligibility of signals close to
or in the noise level, but cannot “dig out” those signals which have been masked by the
local noise. The ANC-4 provides a means to allow the operator to cancel the local noise
and let him hear those signals previously masked by that noise.
The signal from the MAIN antenna is combined in a special network with a signal from
the NOISE antenna. The MAIN antenna signal contains desired information as well as
noise. The NOISE antenna signal, which contains mostly the local noise, is amplified and
passed through a phase shift network so that its phase can be adjusted to 180 degrees
compared to the MAIN antenna signal. Front panel controls, NOISE PHASE and NOISE
GAIN, let the operator adjust the NOISE antenna phase and gain to just equal the noise
signal magnitude picked up by the MAIN antenna, but shifted 180 degrees in phase. Two
phase shift networks (PHASE) are incorporated to give maximum flexibility and cover
most local noise situations. A switch (FREQ RANGE) is also provided to tailor the noise
cancellation network for operation at frequencies above 20 MHz. The HI range on this
switch is also usable at lo frequencies and provides an additional phase shift network
which may be required if a satisfactory phase null cannot be obtained with either PHASE
switch position. The combined signals, with the local noise cancelled, are passed on to
the receiver via the RADIO connector on the rear panel. The networks in the ANC-4 allow
cancellation of local noise up to an operating frequency of about 80 MHz or so.
Desired Atmospheric Noise + Local Noise - Local Noise =
Desired Atmospheric Noise
When properly adjusted, 50 dB reduction of the locally generated noise is possible,
leaving only the normal atmospheric noise level existing on the band. (Observing the “S”
meter readings before and after local noise cancellation will make this apparent.) Signals
that were masked by the local noise can now be heard. Now that the local noise has been
removed from the received signal, a DSP noise reducer is much more effective when
operating at the audio end of things. Just by eliminating or reducing noise, listening
fatigue can be greatly reduced.
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