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    Theory of duplex filter (duplexer)

    The creation of repeaters based on an economy version with two spaced antennas without the usage of filters has always led to mediocre results. The sensitivity of the modern receivers is rather high, and in order to achieve the equal uplink-downlink balance, it is necessary to create the favorable condition for realization of this sensitivity by the repeaterís receiver. Often the beginner builders of repeaters restrict themselves to installation of filters in the antenna circuit of the receiver. At that, they forget about the fact that the transmitter also requires the additional filtering of the signal. In order to understand the necessity to apply filters in both circuits, you can just look at Figure 1. In it you can see two zones where the parameters intersect which decrease the sensitivity of the repeater.

    Zone A is located in the area of the carrier of the repeater's transmitter. There the high-power signal penetrates through the low selecting input LC-circuits of the receiver and blocks the input amplifier. Its transmission factor drops radically, which leads to the general loss of delicacy of the entire receiver. Generally, the standard level of receiversí side signals suppression is 70 dB. Hence, it is understandable that in case of guided signals with the level higher than (Ė117 dBm +70dB)= -47 dBm the receiverís sensitivity will drop.

    Zone B is located in the area of frequencies at which user stations emit and, consequently, the repeaterís receiver itself as well. This zone is characterized by the high noise level generated by the transmitter. The modern transmitters use synthesizers which tend to generate noise. With regard to the fact that output LC-circuits of transmitters also possess low selective ability, through them proprietary high-level noises of the transmitter pass. This level is enough to ďchokeĒ the weak signal of a portable station located at a large distance. Looking at Figure 1, we see that with the noise level with the carrier +47 dBm and the standard value of its suppression on the transmitterís output circuits at 70 dB, we will still have the noise residue with the level Ė23 dBm. This level also leads to the significant decrease of the repeaterís receiver delicacy.

    One conclusion results from this Ė the influence of the carrier and of the transmitterís noises should be additionally lessened by means of additional filters. At that the influence of the carrier is reduced by means of filters installed in the antenna circuit of the receiving device. And the noises can be reduced only by means of filters in the transmitter's circuit. The reduction factor of the carrier of the closely located transmitter can by found in Chart 2, and the reduction of noises - in Chart 1 (see Fig.2). These numbers are normally close, and the structurally close filters are also used. Our example can demonstrate that RX circuit needs to have a filter with attenuation of +47+47 =94 dB added.

    In TX circuit a filter with the same value of 94 dB (-117-(-23) is required.



    Video guide: How to tuning filter
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