Using RF Amplifiers for Isolation Page
4
HYBRID REVERSE AMPLIFIER
and PASSIVE ISOLATION
This is the high-level approach to reverse path headend isolation.   Hybrid amplifiers like our QRBA 050 or QRBA 200 may be used with any type of return signals, analog or digital.   In Figure 9, the eight fiber nodes are combined using a conventional 8-way splitter, which has a typical low-frequency loss of 11 dB.  The parallel paths of course add their broadband noise, which acts to limit the number of nodes that can be processed together.  Bit-error rates become excessive when too many nodes are combined.  The typical level of many return lasers is +30 dBmV.  The input to the hybrid amplifier usually does not need an equalizer since the input to reverse laser transmitters is FLAT. 
Output isolation relies on brute force padding on a common passive splitter as shown in figure 2 earlier in the presentation.  In Figure 9, the use of 10 dB inline pads will provide 50 dB port-to-port isolation to the various return path processing devices.  This isolation to feed digital receiving equipment prevents local oscillator radiation from the input of one device from affecting any other receiving device fed from the return path.
Figure 9.  ( Right )  Click for larger printable graphic.

Figure 9. CLICK for Larger PRINTABLE Image
Figure 10. CLICK for Larger PRINTABLE Image
Figure 10.  Click for larger image.
MONOLITHIC REVERSE AMPLIFIER
WITH ACTIVE ISOLATION
In the active splitter arrangement of Figure 10, the impedance matching of the active components to the directional couplers provides the 50 dB port-to-port isolation in a unity gain package known as QISO/R75-32 that operates best at +30 dBmV input and output.  The output impedance of the monolithic amplifier is typically –20 dB return loss.  This type of amplifier is limited to digital signals only due to its lower output power capability compared to the hybrid amplifiers.
In both forward and return path headend and hub site signal distribution, the control of spurious signals and interference reduction can be accomplished when the combining paths are forced into true one-way settings or have sufficient loss between offending receive devices.
Author Notes:
And with that comes the end of the article.  Here, on the web site, I can add my own thoughts that would be edited from a magazine article.   This article is the result of the successes of many of our customers in the bid to add high-speed web access and other "targeted area" communications to the broadcast path in the modern CATV system. 
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