Operating instructions
Choosing a Ham Radio 19
to multiple strong signals. A difference of 6 dB or more represents a noticeable change in
performance. While commercial transceivers have adequate strong-signal performance,
the more you expect to operate on HF, the more important these figures will become. The
ARRL’s Product Reviews are a valuable source of information about receiver performance
and are free to ARRL members on the ARRL Web site and in QST magazine.
Four other features help a receiver reject interference and other unwanted signals. Hav-
ing these controls on the front panel is very useful when operating on a crowded band.
Receive Incremental Tuning (RIT), sometimes called “Clarifier”, changes the receive
frequency without moving the transmit signal. Passband tuning or passband shift, controls
where the receiver’s filters are tuned relative to the main receive frequency. This allows
you to reduce off-frequency interference without changing the receiver’s main tuning
frequency.
Notch filters can reduce interference appearing as continuous tones, not uncommon
on the HF bands, created by unwanted signals or commercial broadcast stations. A notch
filter removes a very narrow slice of the audio range that can be adjusted to match the tone
of the interference. This either eliminates the tone or reduces it to a tolerable level.
A noise blanker suppresses impulse noise created by motors and vehicle ignition sys-
tems.
Digital Signal Processing
Many new radios employ Digital Signal Processing (DSP) to perform many functions
in software that were previously performed by electronic circuits. Special microproces-
sors do the job inside the radio. The latest generation of radios using DSP has very good
performance. DSP filtering was discussed in the preceding section.
DSP is also employed to get rid of unwanted noise. Noise reduction (NR) is used to
reduce the hissing and crackling of static and other random noise present in the audio of
received SSB and CW signals. This function is usually available with several levels of
Figure9
Filter
Frequency
Strength
Desired
Signal
Undesiredsignals
Whatthe
Radio
Receives
What
You
Hear
Figure9—Selectivityiscreatedbylterswithdifferentbandwidthssuitedtothesignal’s
mode.