Testing Splitters, Taps & Directional Couplers


 
  
Every CATV or MATV distribution system contains splitters, taps, directional couplers and other passive components. These components may develop excessive signal attenuation and losses, or poor isolation between inputs and outputs. These components can be tested using a RF signal source, termination resistors, and the Frequency Selective Voltmeter.

NOTE: Be sure to consult the manufacturers data sheet to obtain the parameters for the specific device you are testing. The parameters listed here are typical values which may vary slightly between manufacturer.

 


 

SPLITTERS

Splitters are used in distribution systems to divide an input signal into two or more output signals. As shown in figure 1., splitters have two important characteristics which determine whether the device is good or bad. Through loss is the amount of attenuation the signal receives as it passes from input to output.

 

 
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ISOLATION
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TROUGH LOSS
Fig. 1 - Trough loss and isolation are two important parametars of a splitter.

 

A typical two-way splitter has a through loss of about 3.5 dB from the input to each output, and an isolation of 20 dB or more. Four-and eight-way splitters are also common, having typical through losses of 7 and 11 dB typically.

To test a splitter for through loss, first measure and record the level of the signal source. Next terminate all but one of the output terminals of the splitter with a 75 ohm resistor.

Measure the signal level at the unterminated output port with the Frequency Selective Voltmeter. The difference between the measured signal at the output and the applied signal is the through loss of the splitter. Confirm that it is within the specifications for the particular splitter.

 

  

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Apply signal 
of known level

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Reading 1

Terminate

Reading 2

Read level

Read level

Terminate

Fig. 2 - To measure trough loss of splitter terminate one output and read the signal level at the other output.

Continue this procedure of terminating all but one output and measuring the signal at the unterminated output until all outputs of the splitter have been tested. Figure 2. illustrates the procedure to follow when testing through loss on a two-way splitter.

The next test to be performed on a splitter is isolation. The procedure for measuring splitter isolation is illustrated in Figure 3. Very simply the input port of the splitter is terminated in 75 ohms. Then a signal of known level is applied into one of the outputs and the signal level at the other output is measured. The difference between the two signal levels is the isolation of the splitter.

 

 
Terminated
75 ohms
sp2f90.jpg (8700 bytes) Apply signal 
of known level
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Read level

 

Fig. 3 - Test setup for checking splitter isolation.

Four-and eight-way splitters are checked in the same manner. Each output should be tested for isolation from the other outputs. Be sure to terminate the outputs that are not being tested.

 


 

TAPS AND DIRECTIONAL COUPLERS

Directional couplers, or Taps as they are sometimes called, are used to extract a small portion of the signal from the distribution cable to feed subscribers taps, while maintaining the proper characteristic impedance of the ditribution cable. A directional coupler has three important parameters to check: insertion loss, isolation, and tap loss. These parameters are illustrated in Figure 4.

 

 
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Isolation    Insertion loss     Isolation
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Tap loss
Fig 4. - Three important parameteres of a directionl coupler are insertion loss, tap loss and isolation.

The most important parameter of a directional coupler is the tap loss. Tap loss is how much lower the signal level at the tap output is, compared to the signal level at the input. Common tap loss values range from 3 dB to 28 dB. Directional couplers are placed at various locations throughout a distribution system based upon the required tap loss and signal level needed. If, for example, the signal level on a line is 28 dBm V, a directional coupler having a tap loss of 28 dB would be used to provide the signal of 0dBmV needed for a subscriber tap.

 

 
Apply signal of 
known level to Input
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Left Output
Read level
Middle Output
Terminated
75 ohms
Right Output
Read level
Fig. 5. Test setup for checking tap loss in a directional coupler.

Tap loss can be measured using the Frequency Selective Voltmeter by the following procedure illustrated in Figure 5. Apply a signal of known level to the input of the directional coupler. Terminate the output terminal of the directional coupler with a 75 ohm terminator. Then measure the signal level at the tap output with the Frequency Selective Voltmeter. The difference in level between the input signal and the signal at the tap output is the tap loss of the directional coupler.

The procedure for measuring the insertion loss of a directional coupler is illustrated in Figure 53. Apply a signal of known level to the input port of the dircectional coupler, while the tap output is terminated in 75 ohms. Then measure the signal at the output of the directional coupler using the Frequency Selective Voltmeter. The difference in signal level between the input and output is the insertion loss. The insertion loss of a directional coupler should be quite small, typically about 1.5 dB.

Isolation in a directional coupler is measured with a known signal applied to the output terminal with the input terminated, as shown in Figure 54. Between the applied signal level and the signal level measured at the tap output is the isolation of the directional coupler. The isolation of a directional coupler becomes greater as the tap loss increases, with a typical isolation of 20 dB for a 3 dB directional coupler. 

 


 

CABLE LOSS

The largest single passive device in an RF distribution system is the coaxial transmission cable itself. The purpose of the tansmission cable is to carry the RF signal with a minimum amount of loss. At the RF frequencies involved in CATV and MATV distribution systems, however, characteristics of the cable and losses in the cable must be taken into careful consideration. The Frequency Selective Voltmeter provides a good way of checking coaxial cable to determine if it is performing as it should with minimum loss.

One of the losses associated with coaxial cable is signal leakage. Signal leakage occurs when the coaxial cable can not contain the whole RF signal, and allows some of it to leak out into free space. Leakage loss should be identified and corrected. This is discussed in an earlier portion of the Application section.

Two other types of cable loss, dielectric loss and resistance loss, are illustrated in Figure 55. All coaxial cables have a specific amount of dielectric and resistance loss. These losses are taken into account when the distribution system is designed and built. Any changes in these parameters after the system is operating, however, may severely affect the performance of the distribution system. For this reason, it is important to briefly review cable loss and its affect on a distribution system in order to better understand how the Frequency Selective Voltmeter may be used to check for cable loss.

Resistance loss is by far the largest contributor of llosses in coaxial cable. Losses caused by the resistance of the inner conductor vary with the cross sectional area of the conductor. Most of the loss, however, is frequency related, a condition called "skin effect." Skin effect describes the condition where, as the frequency of the signal increases, the signal is carried through the conductor further and further away from the center. Thus, the resistance loss in any given cable type varies in direct proportion to the frequency of the RF signal-the higher the frequency the greater the loss.

Table on Coax Cable page lists some commonly used RF distribution cables and the typical losses for each. Use this chart as a guide to determine the normal amount of attenuation to expect in a piece of cable. Any attenuation which differs substantially from this amount indicates a problem with the cable, such as a poor connector, physical damage such as a sharp crimp or bend, or moisture in the cable.

Using the chart for an example, we see that 100 feet of RG 59/U cable normally attenuates the signal at channel 2 by 2.6 dB. Measuring a 100 foot drop of RG 59/U cable with the Frequency Selective Voltmeter, however, indicates 6.2 dBmV at one end of the cable and 1.1 dBmV at the subscribers drop. This 5.1 dB loss indicates a problem somewhere within the length of drop cable.

Naval Electronics Inc., Marine TV Antanna Systems for Ship's at Sea