CATV Bench Sweep Setup         Page 3

If you have any doubts about the test cables being foam versus solid dielectric, the RF bridge can demonstrate some things to you.  Cut some short pieces of drop cable of 10, 15 and 20 feet in length.  Install connectors on each cable, then connect them one at a time to the test port of the RF bridge.   Connect a barrel and terminator to each cable and observe the return loss and any sine wave shape that may be present in the reflected energy through each piece of cable.   Now compare the return loss with each piece of cable between the RF bridge and the test amplifier.  And the last test, as each piece of cable is attached to the bridge test port, observe the reflected sweep signal with the free end of the cable first shorted then with no connection (open).  Did you see the phase inversion of the sine wave response?  Now imagine that the sine wave shape you have seen in all these tests are part of your test bench because they were "convenient lengths" of drop cable.   These errors would be aligned into the amplifier under test to produce a "flat" response.  When the amplifier gets installed in the real world, the "flat response" reveals itself as the opposite of the sine wave error produced by the poor cables on the test bench.  That is what this whole article is about!

It is time to address line extender testing.  Connect the test cables and power inserter as in figure one with a housing-to-housing connector between the LE housing input port and the power inserter.  This will insure that the power inserter is not used with the trunk station.  Use the usual 5/8" to F adapters where needed.  Be sure to repeat the test of the 3 dB pads at the input side of the power inserter and the output port of the line extender.  If you forgot to stop the AC power from going through the line extender, the smell of smoke will be in the air "shortly".  If you did not see a difference in the swept response, leave out the pads. As for the alternative to powering with an AC source, a DC power supply that can provide +24 volts DC at 2 amperes rating can be connected to the correct point on the DC power bus of the LE.  Power the unit that way and leave out the power inserter.  The full load on the DC power supply should be around 1ampere for a dual-hybrid power-doubled line extender with a return hybrid installed.

Jerry K. Thorne                                                  (Click here for Technical Articles Menu) Applications Engineer July 16, 1998