About Filters & Multicouplers
Filters/multicouplers have manifold functions and benefits. Because of the importance of the topic the functions and benefits are described in the following in general and in detail:
In general
Highly selective filters and multicouplers:
- Protect and improve the operational quality of the customer’s own VHF/UHF radio installations
- Protect the radio system from externally generated interfering signals from all kinds of civil or military inband or out-of-band radio installations, including broadcasting, TV, microwave links or radar
- Protect other electromagnetically sensitive radio, navigation or radiomonitoring installations under the responsibility of the own or other services
- Provide frequency economy in an environment with high frequency congestion – filters/combiners make the use of already or newly assigned and critically spaced frequencies possible
- Provide space economy by installation of antennas on limited space – e.g. on the tower cabin roof, at the only available radio center, on board of a ship or in mobile or transportable systems (e.g. mobile towers)
RF filters improve the characteristics of transmitting and receiving systems – depending on the RF path they are integrated into – as follows:
Receive filter benefits - in detail
- Suppression of out-of-band intermodulation products by additionally attenuating all signals and their harmonics which are outside of the receiving range and - due to nonlinearities and mixing - could cause intermodulation products to fall within the useful VHF or UHF band and disturb reception
- Suppression of third-order crossmodulation products by attenuating strong interfering signals which could transfer their modulation to the wanted - but possibly weak - signal
- Increase of image-frequency rejection by additionally attenuating signals defined as image frequency relative to the received frequency
- Increase of intermediate frequency (IF) rejection by additionally attenuating interfering signals on one of the intermediate frequencies
- Prevention of desensitization (reciprocal mixing) by attenuating strong interfering signals which could otherwise transfer the noise sidebands of the receiver oscillator, mixed onto the interfering signal, to the useful band, thus reducing sensitivity
- Prevention of blocking by attenuating strong interfering signals which could reduce the amplitude of the useful IF signal by overdriving the mixer stage
Transmit filter benefits - in detail
- Attenuation of broadband transmitter noise caused by the exciter oscillator or power amplifiers
- Suppression of spurious emissions, mainly occurring in the small-signal stages during frequency conditioning
- Suppression of harmonics, mainly generated in the power stages
- Attenuation of transmitter (backward) intermodulation products of the 3rd or higher order: these are generated - within a simultaneously operating system with co-sited transmitters - by the radiation of transmitter signals via the antennas into the output of another transmitter. In this case the attenuation of the filter at the transmission end is effectively doubled and adds to the antenna-decoupling factors
Practical effects of improvement
Optimized radio installations using filters help to achieve vital operational benefits:
Benefits by filters
- No irritation of the operator
- No degradation of the receive sensitivity
- No irritation of the operator by spurious reception (phantom signals)
- No RF output power reduction of the transmitter
- No radiation of unwanted and interferingsignals
Background
- By unwanted response of the receiver carrier squelch
- Also with weak wanted receive signals
- Such interference may be generated by signals identical to the image frequencies or the intermediate frequency
- Interfering signals entering via the antenna socket influence the VSWR-dependent gain control loop
- Radiated TX intermodulation products can result in self-jamming of one’s own receive system or irritations of the aircraft pilot
