12704_SCTE_Broadband_Nov2016_COMPLETE_lowres

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The capacitor C, is used to compensate for the inductive reactance of the transformers at high frequencies. Other impedance correction capacitors may be added across the input and output ports. Non-Symmetrical Splitters This is the type of splitter with typical side losses of 6 to 15dB and through losses of the order of 2 to 0.5dB respectively. The non-symmetrical splitter is normally designed using the principle of directional coupling. The easiest form of coupler to understand simply uses the electromagnetic coupling between two cables as shown in Fig.15.

Signal entering from the output cable again induces a secondary current in T 2 as shown by the dotted arrow. The current induced in T 1 is also shown in the diagram. It will be noted that the current in T 1 is in the same direction as before. However, the direction of current in T 2 has reversed and cancellation occurs at the output tap. The amount of power available at the tap is determined by the ratios of T 1 and T 2 . These transformers are made such that voltages across the two secondaries are as near identical as possible to ensure good cancellation of the reverse signal and therefore good directional properties. The 75 ohm resistor is used to provide back matching for the unit. If a signal enters from the tap the majority of this is absorbed across the 75 ohm load, the remainder being transferred to the input terminal. The loss from tap to input is the same as that from input to tap. This makes the unit suitable as a signal combiner. A signal entering at either output of the device and flowing out of the input has the same loss as a signal entering the input and passing out via that output. Both variations on the basic circuit are to be found but all operate on the same principles. Better symmetry and impedance matching can be obtained from the circuit shown in Fig. 18. Practical considerations, however, make this circuit difficult to construct at UHF.

The signal flowing in the main cable induces a signal into the adjacent cable and the signal flows in the same direction as the main signal. A 75 ohm load is placed at the end of the secondary cable and this acts as a load. The load also absorbs any signal attempting to pass down the coupled cable in the reverse direction; this gives a simple directional tap-off unit. Unfortunately, the amount of coupling is dependent upon the signal frequency and the device cannot be designed for a flat frequency response over a wide bandwidth. Some ‘spur end’ tap-off units have been constructed using this principle and the lowest side loss is designed to be at the highest system frequency. The earlier types employed coils of special cable but later designs used printed circuit lines. The use of ferrite transformer techniques to form directional couplers enables uniform coupling to be achieved over the full television bands. A typical hybrid directional coupler is shown in Fig. 16.

The operation of this type of coupler is as follows: When a signal enters the input it passes into T 1

Summary The way in which a directional splitter is defined is as follows: (a) Through loss - this is the loss on the main line between the input and output or output and input of the splitter. (b) Side loss - this is the loss between the input terminal and the tap-off or vice versa. (c) Reverse loss or directivity - this is the loss between the tap or spur output and the output terminal.

, and in the

secondary of T 1

, a signal is induced in antiphase. The signals are

represented by the broad arrows (in Fig.16). The signal entering from the input also induces a signal in the secondary of T 1 , in antiphase. These two signals add together at the tap to form the output signal. If a signal attempts to enter from the output of the directional coupler the action is shown in Fig. 17.

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Vol. 38 No. 4 - November 2016 Issue