Issue 146 September 2002
Killing the EMI Demon

THE CONTROVERSY

Perhaps it’s because the spectrum spreader solves EMI problems (it gives an advantage of 10 to 20 dB) that it’s not well accepted in some quarters. Those in the anti-spectrum spreader crowd argue that the use of spectrum spreaders is a form of cheating because the radio frequency energy is still present but just spread differently in the spectrum. It’s assumed that changing the spectrum from narrow band to broadband will simply spread the interference around or even make the total interference greater.

People holding competing points of view have performed experiments in an effort to demonstrate that spectrum spreaders either increase or decrease interference. But what both sides have failed to realize is that there are different types of spectrum spreaders that have different effects on different forms of communications signals. It’s important to realize that traditional radio communication is being gradually displaced by new techniques rooted in digital technology.

The newer digital techniques are generally broadband and transmit redundant information. For example, frequency hopping is a technique in which the transmitter and receiver hop from one frequency band to the next in a predetermined pattern. Interference on one or a few bands will not have a noticeable effect because the missing data will be retransmitted or recovered by the use of error correction techniques.

Another technique is Orthogonal Frequency Division Multiplexing (OFDM). This is used for digital radio and television, particularly in Europe. OFDM is also used for DSL Internet service over copper telephone wires. With this technique a wide band is divided into hundreds or thousands of narrow band channels, each of which carries a slow data signal. The bands are typically 1- to 10-kHz wide.

All of the data signals are combined and error corrected to create a fast datastream. Because as much as half of the transmitted information is redundant, this technique can suffer considerable interference and retain perfect transmission of the picture or sound. A big advantage is that it’s resistant to fading and multipath interference. OFDM is made possible by cheap digital signal processing that is now available via ASIC’s. The digital technology makes possible frequency analysis via techniques such as the Fast Fourier transform. In addition, the digital compression of sound and speech is achievable via techniques such as MPEG.

Most spectrum spreaders modulate the clock by sweeping the frequency or phase back and forth in a regular pattern. The repletion rate is typically in the range of 20 to 100 kHz. However, it’s also possible to modulate the frequency or phase by a random signal or a pseudo-random signal that does not repeat, or if it does, it has a long repetition period. If the modulation is periodic so that the clock pattern periodically repeats, then the spectrum will be split into separate spectral lines separated by the repetition frequency. If the modulation is random or has sufficient random noise in the circuit, then the spectrum will be smeared in a continuous fashion. This can make a difference.

In an OFDM TV system, for example, if a TV signal is transmitted in 8000 bands that are separated by 1 kHz and the spread spectrum clock splits the original clock into separate spectral lines that are separated by 100 kHz, then only one-one hundredth or perhaps one-fiftieth of the channels in the OFDM will be interfered with. This is a degree of interference that can be easily handled by the error correction facility. However, if the spectrum is continuously smeared, then it’s conceivable that every channel would experience interference and the TV picture would be lost.

The repetition frequency of the modulator can also have an effect on voice transmission. If the repetition frequency is 5 kHz and a harmonic of the clock falls in the traditional FM band, then a 5-kHz whistle will be heard in an FM receiver (this is something the ear is sensitive to). If the repetition frequency is 50 kHz, then the whistle may still be there, but it will be beyond the range of human hearing.