Harmonics and spurious emissions
| Harmonics and spurious emissions | |
|---|---|
| Harmonic | A signal at a whole-number multiple of the operating frequency |
| Spurious emission | An emission outside the necessary bandwidth that can be reduced without affecting the information sent |
| Includes | Harmonics, parasitic oscillations, mixing and intermodulation products |
| US rule | §97.307 of Part 97 |
| Spurious emission limits | |
| Transmitting below 30 MHz | At least 43 dB below the fundamental (transmitters installed after January 1, 2003) |
| Transmitting 30–225 MHz | At least 60 dB below the fundamental, with allowances for transmitters of 25 W or less |
| Main cure | Low-pass or band-pass filtering |
| Test instrument | Spectrum analyzer, with suitable attenuation |
A harmonic is a signal at a whole-number multiple of a transmitter's intended frequency, which is called the fundamental. A signal on 7.1 MHz has its second harmonic on 14.2 MHz and its third on 21.3 MHz. Spurious emissions is the wider term for unwanted signals outside the frequencies a transmission needs, and harmonics are the most familiar kind. Every transmitter produces some; the design goal, and the legal requirement, is to keep them weak enough that they interfere with no one.
What counts as spurious
FCC definitions separate two kinds of unwanted signal. Out-of-band emissions lie just outside the necessary bandwidth and result from the modulation itself, such as splatter from an overdriven SSB signal or the key clicks of a CW signal keyed too abruptly. Spurious emissions generally lie farther away and can be reduced without affecting the information being sent. Hams often call them spurs. They include:
- Harmonics, produced whenever a signal passes through a nonlinear stage. Power amplifiers, especially efficient ones, generate strong harmonics.
- Parasitic oscillations, unwanted oscillation in an amplifier at an unrelated frequency, often in the VHF range in an HF tube amplifier.
- Mixing and intermodulation products from the frequency-conversion stages of a transmitter, and spurs from the synthesizer that generates its frequency.
Several HF amateur bands are harmonically related, so a harmonic often lands in another ham band; the second harmonic of the 40-meter band falls in 20 meters, for instance. A harmonic can just as easily land on an aviation, public-safety or broadcast frequency. On VHF, the third harmonic of a 2-meter signal falls in the 70-centimeter band.
FCC limits
Section 97.307 sets the rules for emission standards. In general terms:
- A transmission may occupy no more bandwidth than the information and mode require, in accordance with good amateur practice, and emissions outside the necessary bandwidth must not cause splatter or key-click interference to stations on adjacent frequencies.
- All spurious emissions must be reduced to the greatest extent practicable. If any spurious emission, including radiation from the equipment's chassis or power line, causes harmful interference, the licensee must take steps to eliminate it.
- Numerical limits are stated in decibels below the mean power of the fundamental. When transmitting below 30 MHz, spurious emissions from transmitters installed after January 1, 2003, must be at least 43 dB down; older equipment falls under somewhat looser limits. When transmitting between 30 and 225 MHz, they must be at least 60 dB down, with allowances for transmitters of 25 watts or less.
A spur 43 dB below a 100-watt signal is about 5 milliwatts. That is legal, but 5 milliwatts on an open HF band can be heard a long way, which is why the "greatest extent practicable" rule matters as much as the numbers. US rules do not require amateur transceivers to be certified by the FCC before sale, so responsibility for a clean signal rests with the station's licensee.
Causes and cures
- Filtering. Every modern HF transceiver has a bank of low-pass filters in its output, one for each band or group of bands, that pass the fundamental and attenuate its harmonics. An external low-pass filter adds protection and is good practice after an amplifier. An HF low-pass filter also blocks the 6-meter band, so a station that uses 6 meters needs a filter designed for it.
- Wrong band setting. A solid-state amplifier set to a higher band than the one in use selects a filter whose cutoff is too high, and the harmonics pass with little attenuation. A failed band-switching relay inside a radio has the same effect.
- Overdrive. Driving any stage beyond its linear range raises harmonics and splatter. Set the drive correctly and keep an eye on the ALC meter.
- Overdriven audio on digital modes. Too much sound-card audio creates harmonics of the audio tone. A 1,000 Hz tone has a second harmonic at 2,000 Hz, which passes through the radio's transmit filter and appears on the air as a weaker copy 1 kHz above the real signal.
- Poorly designed equipment. Laboratory tests have found some low-cost radios producing harmonics and spurs well above the permitted levels.
Testing
The standard instrument is a spectrum analyzer, which displays signal strength against frequency, so harmonics and spurs show as separate peaks above the noise. A transmitter must never be connected straight to an analyzer's input. Instead it feeds a dummy load, and the analyzer receives a small sample through a sampling tap or a set of power attenuators rated for the job. A software-defined radio with plenty of attenuation can do a similar job at lower cost, and a general-coverage receiver tuned to each harmonic frequency while the transmitter feeds a dummy load gives a rough check. Product reviews in ARRL's QST magazine include laboratory measurements of transmitter spectral purity.