What do Those Specs Really Mean?
By Bob Grove W8JHD
Audio Output Power - Dynamic Range - Frequency Range - Keypad Frequency Entry - Modes - Noise Blankers - Notch Filter - Passband Tuning and IF Shift - Preamplifiers and Attenuators - Scannable Memory - Selectivity - Sensitivity - Tuning Steps
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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, return to the technical section to see additional online articles. 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