What is self excitation?

07/07/2021

Self excitation means that the signal amplified by the repeater enters the receiver again for secondary amplification, resulting in the power amplifier working in saturation state. Self excitation only appears in the radio repeater. Because the fiber repeater is directly coupled with the base station signal, the fiber repeater will not generate self excitation. If the optical fiber repeater is unable to make a call with signal or the call quality is poor, it is recommended to check the uplink and downlink attenuation and repeater hardware equipment.

The gain and isolation change of signal repeater caused by the temperature change or base station parameter change would make the input signal to signal repeater higher, which causes self excitation.

When debugging the repeater, it is necessary to keep room for not too much pursuit of amplification function of repeater and too large gain. For the repeater with fault record, it is difficult to detect the self excitation in reverse channel of repeater. Because the forward channel of the repeater always has a base station signal input, if the repeater is self-excited, the forward amplifier may be overloaded. After some models of repeaters detect that the amplifier is overloaded three times, they immediately shut down the repeater and give a clear fault record, which is easy to find.

The input signal of the reverse channel amplifier changes greatly, and the mobile phone transmitter is not always in the transmitting state and the distance is far some times and near some times. In some cases, the reverse channel amplifier will self excite, but the amplifier will return to normal due to the sudden disappearance of the input. Reverse channel amplifier self-excited time is short, only a few seconds, and irregular, sometimes several hours are not self-excited once, very difficult to remove the fault.

In the area where the repeater is installed, if the mobile phone communicates with the local telephone, the mobile phone could hear clearly, but the local telephone cant, and the sound quality is extremely poor, which may be caused by the self excitation of the reverse channel amplifier of the repeater.

When the repeater is not installed properly, the isolation of the receiving and transmitting antenna is not enough, and the gain of the device is too large, the output signal is fed back to the input after delay, resulting in serious distortion of the output signal of the repeater and self excitation. After self excitation, the quality of signal spectrum waveform becomes poor, which seriously affects the quality of the call and causes the phenomenon of dropping the call.

There are two ways to solve the self excitation phenomenon, one is to increase the isolation between the donor and the re-transmission antenna, the other is to reduce the gain of the repeater. When the coverage of repeater is small, the gain can be reduced. When the coverage of repeater is large, the isolation should be increased.

The following methods are mainly used in the project:

- increase the horizontal and vertical distance of the transmitting and receiving antenna

- increase the shelter, such as installing the shielding net, etc.

- increase the directionality of the donor antenna, such as using the parabolic antenna

- select the re-transmission antenna with stronger direction, such as the directional angle antenna

- adjust the angle and direction of the donor and the re-transmission antenna, so as to make them as backward as possible