Using RF Amplifiers to Optimize
Headend and Hub Site Isolation

By Jerry K. Thorne
For those of you who read my earlier article in Communications Technology of January 1997, this will sound familiar.  The portion of that article on headend isolation has spawned a few products that have found popular usage in modern headends.
        Figure 1.
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Figure 1.  Click above or here for larger printable graphic.
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The forward path diagrams begin with Figure 1 depicting a headend with traditional directional couplers and splitters.  This passive signal combining network feeds the various zones with many nodes in each franchise area.  The diagram includes up to 16 channels of franchise specific channels of programming, which may consist of local community interest programs or targeted local commercial insertion.
A co-channel feedback path exists across the passive splitters and directional couplers due to the interconnection of 16 dB return loss passives to each other.  With a proper termination of 26 to 30 dB, a typical splitter or directional coupler can provide up to 30 dB port-to-port isolation.  With degraded values of terminating impedance, that isolation is reduced.
A 30 dB gain amplifier is needed to raise signal levels to drive a large number of lasers.  The interstage slope control should be used to overcome the cable and splitter tilt to insure the RF signals arrive at the laser inputs FLAT!  The output RF slope of the amplifier will also improve the CTB performance of the amplifier.  This CTB improvement is approximately 0.7 dB for every 1 dB of slope adjustment reducing the low-frequency output levels.  For headends that do not need signal levels above +40 dBmV, power-doubled or quadra-powered output amplifiers will be sufficient.   Feedforward amplifiers are best for output signal levels in the range of +40 to +45 dBmV.  The four-way splitter may be installed internally in some types of rack-mounted amplifiers.  If an external splitter is used, it should be located as close as possible to the unit to minimize signal reflections.
        Figure 2.
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Figure 2.  Click above or here for a larger printable graphic.
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The isolation of splitters or directional couplers will vary with the source and load impedance values.  A "good" type-F terminator will have a return loss of 26 to 30 dB across the desired bandwidth.  The return loss for typical passive devices and trunk amplifiers is 16 dB. Figure 2 shows the methods of measuring insertion loss and isolation on the through path and tap leg of a directional coupler or splitter.  Using a sweep generator provides a full bandwidth isolation view of this type of device.
The splitter in this diagram also depicts a "poor-man’s RF bridge."  Remember that the internal circuit of a two-output splitter consists of a balanced ferrite winding fed from the center tap (input) and has a 150 ohm resistor across the outer windings (center conductor connection of the output ports).  The two 75-ohm loads on the splitter complete a balanced bridge circuit having a balance of 30 dB typically with accurate load impedance values.   Degrading the return loss of those loads reduces the isolation of the splitter.   The same basic rules apply to directional couplers.  < NEXT PAGE >