Lauri's column

Interference - the ever-increasing problem?

The available number of radio frequencies is a limited natural resource, rigorously managed by responsible authorities according to the international agreements. Therefore it is becoming more and more difficult to get so called licensed frequencies except for the applications considered vital for public safety and other such reasons.

On the other hand, though, the unlicensed frequency bands have been made available for more general usage. The technical specifications are more relaxed than those required for the products intended for the licensed use. Besides, the frequency bands are narrow compared to the growing number of applications and users and are getting more and more crowded.

The relaxed performance requirements have also led to product designs that do not comply with the application requirement, in some cases causing economical or even safety risks. Unfortunately, in most cases the end user is not even aware of the risks. This is placing pressure and responsibility to the system integrator to take into account such issues.

More often than entering into the hazardous situations the issues described above are causing delays and less than expected performance of the complete system, in most of the cases hundreds of times more expensive than the WCS (Wireless Communications System).

The technical, economical, competition and knowledge issues are playing their roles behind this. In the forthcoming columns I will deal with different technical issues behind the radio receiver and transmitter susceptibility to interference.

Any well-engineered radio equipment is shielded and filtered to avoid picking up extraneous signals which could cause interference or radiating signals trough the power supply and signal lines - other but antenna port. Antenna bandwidth, the frequency band the antenna receives or transmits signals, is always wide compared to the channel bandwidth. Therefore the antenna is able to pick up all the signals that are on its bandwidth. The directional pattern and polarity of the antenna and the incoming signals determine the relative amplitude. The receiver is more susceptible to interference due to such signals; usually more so the closer the interfering signal frequency is to the receiver's nominal frequency.

The receiver properties known as:
Selectivity
Blocking
Intermodulation properties
Receiver topology
Type of modulation etc.

The larger the number of interfering signals and the higher the level at the antenna port the more probable the interference. The worst-case scenario is that very strong signals may be mixed at the receiver RF amplifier input and reradiated interfering other users.
The phenomenon above is even more probable during the transmission period. The transmitter Power amplifier usually has very good mixing properties and may cause severe intermodulation problems to other users over a large area.

Now one may think that the transmitter power levels used are low. Yes, but especially in the industrial environment it is not uncommon that the antennas of the different systems may get very close to each other and the field strengths may be substantial and thus also the signal levels to the antenna port. Besides, the cellular phones are increasing in numbers increasing the potential of system cut-offs.
As the first aid to such a problem is to place a band pass filter between the antenna and the radio, the narrower the better.

During my long career in radio technology I have come across with the interference problems like these - but not to the extent the integrators and users are facing nowadays. I do not believe the situation will get better all by itself. At least we at Ultra-Crea have decided to utilize our expertise in RF-filtering and introduce various types of filters that we believe will provide first aid help in overcoming some of the interference problems.

Look at the data sheets at RF Filter and combiners section on this site. If you have special needs, do not hesitate to contact us.

Lauri Kuokkanen