Directional coupler. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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The proposed directional coupler is intended for use in a device for separating television signals received by one antenna.

Very often in the practice of television reception a situation arises when one antenna receives several television signals with different levels. When connected to a TV antenna, the quality of the received signal usually satisfies the consumer. It is provided by the parameters of the television receiver (sensitivity, selectivity, dynamic range).

A different situation occurs if it is necessary to amplify the signal received by the antenna, for example, to feed it to the collective reception system, when the signal level from the antenna is insufficient to feed it to the channel amplifier. Using a broadband amplifier for several television channels always leads to disastrous results. Intermodulation distortion in the amplifier "clogs" a weaker channel, so that the presence of this channel can only be guessed at. The most correct solution in this situation is to install a separate antenna for each channel. However, in many cases, installing a separate antenna is not possible for various reasons.

To clarify further reasoning, let's take a specific example of the electromagnetic environment. At the output of the antenna of the 3rd TV band we have:

- level of the 7th channel - 77 dB μV;

- level of the 9th channel - 52 dB μV.

To feed signals to channel amplifiers, they must be separated, and the signal of the 9th channel should be additionally amplified, since for most channel amplifiers the nominal input level is about 70 ... 80 dBμV. Using broadband splitters with 3,5 dB forward attenuation, we obtain the signal distribution shown in Fig. 1.

As can be seen from Fig. 1, the division of signals is not entirely successful, since the signal level of the 9th channel additionally drops by 3,5 dB, and the signal-to-noise ratio deteriorates. In addition, such splitters have an output isolation of 22...26 dB, which is not always sufficient for stable operation of such a system.

For the optimal solution of this problem, a directional coupler was made according to the scheme from [1]. Characteristics of signal separation are shown in Fig.2.

The coupler has output isolation in the operating frequency range of more than 46 dB, which significantly weakens the signals from the output to the input of channel amplifiers, and thus provides a margin of stability with high signal amplification. The coupler circuit is shown in fig. 3.

Coils L1 and 12 are frameless, they contain 4 turns of wire PEV-2 00.7 mm each, the diameter of the coils is 5 mm. All capacitors are type KD-1 or K10-17. The input and outputs are designed to connect a load of 75 ohms. The response-coupler is tuned by coils to the maximum crosstalk attenuation between the outputs at the frequency of the 7th channel.

The general block diagram of signal generation for feeding to channel amplifiers is shown in Fig.4. As can be seen from fig. 4, the signals of the 7th and 9th channels, despite the difference in levels at the antenna output, are equalized by the reduced device and are reduced to a level of about 70 dBμV, which is quite enough to feed the channel amplifiers.

According to this scheme, more than a dozen devices were manufactured, which showed good repeatability and high quality of the received signal. Such a directional coupler (Fig. 3.) can be used in various devices (low-power transceivers, adders, mixers).

Literature

1. A.E. Aksenov. Directional coupler on lumped inductive and capacitive elements. - Radio engineering, 1976, N2.

Author: S. Novikov; Publication: cxem.net

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