|
What
do Those Specs Really Mean?
By
Bob Grove W8JHD
Everyone
knows that specifications are important, but not everyone knows why. Oh, sure,
we can generalize: "A sensitive shortwave receiver is better for DX."
Maybe. Let's take a look at some of the more important specifications for
shortwave receivers and try to make sense out of what they are telling us.
Frequency
Range
While
the shortwave spectrum is officially 1.8-30 MHz, we have to keep in mind that
all receivers currently manufactured include the medium wave broadcast band as
well (540-1700 kHz, the same as 0.54-1.7 MHz). But there's more.
Since
virtually all portables are made and marketed overseas, the foreign domestic
broadcast band (150- 300 kHz) is included as well. There are no voice
transmissions below this, only some Navy digital communications; most tabletop
receivers go down to 100 kHz.
Keypad
Frequency Entry
Often
called "Direct Entry," keypads are far more convenient for selecting
discrete frequencies than rocking a dial back and forth, fine-tuning the desired
frequency. Until digital synthesis of receiver oscillators, such exact control
was impossible.
Tuning
Steps
In
the days of analog tuning, precise tuning of a signal to within a few hertz was
easily obtainable, but with digital synthesis, such accuracy is expensive.
Realistically, it becomes more of an issue with the reception of digital modes
and single sideband than AM, where being off by hundreds of hertz is no problem.
Voice
single-sideband stations, to sound natural, must be tuned within better than 25
Hz or so, while music, because of its absolute pitch intervals, must be even
tighter.
Some
receivers employ "direct digital synthesis," enabling increments as
small as 1 Hz; in fact, 10 Hz is probably plenty good for virtually any hobby
application.
Modes
Amplitude
modulation (AM) is still the preferred mode for domestic and international
broadcasting even though it does waste spectrum. It is sometimes called
"full carrier double sideband," and the same audio information is
duplicated in both sidebands (upper and lower). Synchronous detection (AM-Synch)
is a receiving mode which locks onto the station's signal frequency without
drifting. By choosing the stronger of the two sidebands, the reception remains
stable during fades, and eliminates distortion produced by unequal sidebands.
Single
sideband (SSB) actually transmits one sideband, eliminating both the carrier and
the opposite sideband, making it inherently more spectrum-efficient, and immune
from selective fading distortion. Virtually all two-way voice communications
heard in the shortwave spectrum are in upper sideband (USB). Exceptions include
amateur radio voice comms in the 160, 75, and 40 meter bands which are lower
sideband (LSB).
Sensitivity
The
measurement of a receiver's ability to respond to weak signals is its
sensitivity. Since shortwave radio signals are detected as minute voltages, the
measurement is made in microvolts (millionths of a volt).
Years
ago, less sensitive vacuum-tube receivers required significantly larger antennas
to capture enough signal energy to overcome their own noisy circuitry, the
result of the hot filaments and cathodes producing electrical noise ("thermionic
emission"). Modern solid-state electronics makes high sensitivity
practical, with half-microvolt (0.5 uV) ratings, and smaller antennas
commonplace.
Dynamic
Range
But
high sensitivity is only half the story. The ability of a receiver to respond
faithfully and equally to weak and strong signals is a measure of its dynamic
range, expressed in decibels (dB). Overly-sensitive receivers often become
overloaded by strong signals, producing spurious, phantom signals which
interfere with reception. Most common is intermodulation ("intermod"),
but desensitization ("desense") which lowers the weak-signal
capability of a receiver in the presence of strong signals.
Preamplifiers
and Attenuators
During
weak signal conditions, it is often an advantage to boost signal levels before
they come into the receiver. Preamps are wide-bandwidth devices that amplify all
signals over the entire frequency range at one time (with the possible exception
of the medium-wave broadcast band to avoid strong local signal overload).
And
if signal levels are generally excessive, an attenuator may be invoked to reduce
all signal strengths to make them more manageable or the receiver's tuning and
detecting circuitry.
Selectivity
Single-signal
reception is the goal; we want it audible and without interference. There is
little we can do to separate two signals on the same frequency, but there is
plenty we can do to separate two adjacent-frequency signals.
Filters
are frequency-selective components used in receivers to decrease the amount of
spectrum being detected at any one time. While it may seem prudent to make
filters as narrow ("sharp") as
possible, in fact different modes require different bandwidths, as we noted
before.
Since
the human voice occupies approximately 3 kHz of audio spectrum, and AM signals
double the amount of bandwidth, a conventional AM signal is about 6 kHz wide. If
we narrow it down much below 4 kHz, we reduce its high frequency components
considerably and it sounds muffled.
SSB
is already narrower, so selectivity on the order of 2.1-2.4 kHz is common. Even
narrower are digital modes; Morse code (continuous wave or "CW") is
the narrowest of all, with bandwidths of less than 0.5 kHz adequate in most
cases.
Passband
Tuning and IF Shift
These
two techniques allow the operator to manipulate a receiver's filtering circuitry
to favor one of two close-spaced signals without simply narrowing the passband,
which would produce muffling of the audio. Instead, the unwanted signal is
rejected and the desired signal's bandwidth is preserved.
Notch
Filter
A
filter which can be invoked and adjusted to remove single tones
("heterodynes") from the desired signal is quite useful. Some advanced
receivers use digital signal processing (DSP) to
do this
automatically and instantly without the listener having to turn a knob until the
irritating pitch disappears.
Noise
Blankers
Years
ago, crackly electrical noise interference was reduced by an audio noise limiter
(ANL). This was basically a voltage "clipper" which allowed an
adjustable amount of normal audio to pass to the amplifier, but would clip off
any sharp bursts of noise. These characteristically caused some distortion to
the sound.
More
modern receivers employ noise blankers which sense the arrival of the noise
spike and momentarily shut off the circuitry for the duration of the
interference spike. While they do result in less distortion, they are effective
over a narrower range of interference than the old ANL.
Scannable
Memory
The
ability to store a favorite frequency and mode into a memory channel is
certainly a benefit; switch the radio on, push a button, and there it is! Most
shortwave sets now have memory, and often offer the ability to scan as well,
allowing an automated hunt for active stations among the memorized channels.
Audio
Output Power
In
a home stereo system reserve audio powers in the 100-200 watt range are common.
But we seldom crank the volume up that loud! In actual practice, as little as 3
watts into a decent-size speaker can provide room filling sound. Engineers often
provide this specification along with another parameter: 10% total harmonic
distortion (THD). This is the maximum audio power the receiver can deliver to a
matched speaker without audibly distorting the sound. These definitions are
admittedly simplified.
For
more elaborate explanations of some of the often ignored or misunderstood
specifications, see the additional articles by Ian Poole. However, the above
summary should provide a guide to understanding the various circuit design
characteristics which make up a receiver's specifications. After reading them
over, you'll have a better idea of which specs are more important for your
listening requirements!
This
article first appeared in Monitoring Times, August
2000
|
