Subj : Todays New Gear Tip
To : All
From : Daryl Stout
Date : Sat Oct 21 2017 12:04 am
Filters & Receiver Performance
The HF bands are a challenging environment for receivers; signal strengths
vary from barely detectable to extremely strong. The bands are often
crowded with strong signals. The most important receiver characteristics
are selectivity (the ability to reject unwanted signals) and strong-signal
performance.
Selectivity is created by filters with different bandwidths suited to the
signal's mode. You'll encounter radios that use discrete electronic
crystal filters and software-based DSP (Digital Signal Processing)
filters. HF radios that use discrete filters will come with a
medium-bandwidth filter for SSB (about 2 kHz), an AM filter (6 kHz), and
an FM filter (15 kHz). A CW filter (500 Hz) or RTTY filter (250-300 Hz)
are needed if you expect to use those modes a lot. Filters can be added
after you purchase the radio.
DSP filters are created by software in the radio's controlling
microprocessors, reducing or eliminating the need for separate electronic
filters. The radio will come pre-programmed with several common filter
types. You can create new filter types and even adjust them while you are
using the radio.
The radio's ability to operate properly in the presence of strong signals
is measured primarily in two ways; blocking dynamic range (BDR) and
3rd-order intercept point (TOI). In both cases, higher figures are better.
BDR in decibels (dB) describes the receiver's ability to ignore unwanted
signals. TOI in dBm (a power level) describes the receiver's reaction 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. Having 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 receivers 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 systems.
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