FM Antenna Diversity
A quadruple FM aerial diversity system is fitted as standard on the E90.
FM antenna diversity comprises:
- FM1, FM2 and FM3 aerials
- FM4 aerial, formed by combining the signals from the FM1 and FM2 aerials
- FM antenna amplifier with antenna diversity module
For reasons of space, the rear window of the E90 can accommodate only three FM aerials. The FM4 aerial is not a physical component. To improve FM reception, the FM4 aerial is created by overlaying the HF signals from the FM1 and FM2 aerials.
The signal quality of the currently selected FM antenna (FM1 to FM4) is evaluated and assessed in the diversity module. The next FM antenna is selected if the signal quality of the received radio station deteriorates in connection with the active antenna. The changeover takes place such that no interruption can be heard.
The high-frequency signal from the active FM aerial at any particular time is fed by the aerial amplifier and Diversity module via a co-axial cable to the tuner in the radio or navigation system. The signal is demodulated in the tuner and output in the form of an audio signal through the speakers.
The radio or navigation system detects that a diversity module is installed and generates the changeover voltage Us and the intermediate frequency signal (ZF) necessary for diversity operation. The ZF signal is analyzed by the electronic circuitry in the diversity module and is a copy of the currently selected radio station transmission on a fixed frequency of 10.7 MHz.
The changeover between AM reception, FM diversity mode and diagnosis mode is effected by means of the DC voltage signal Us. This voltage is generated by the radio and used in the diversity module for evaluation purposes.
Diversity mode is active at Us = 2.5 V. AM mode is active at Us = 0 V, i.e. the FM1 antenna is selected. Diagnosis mode is active at Us = 5 V.
In total, up to three signals are therefore applied on the coaxial cable simultaneously.
- HF signal (e.g. 87.5-108 MHz) from diversity module to radio
- Control DC voltage Us from the radio to the diversity module
- Intermediate frequency (fIF = 10.7 MHz) from the radio to the diversity module as the basis for evaluating the quality of the RF signal.