CABL Forum - Not Possible Testing At Tap "F" Port

Hi Fred,

First of all, it is IMPOSSIBLE to read the SAME noise and ingress level trace level from the tap "f" port(s) in the field. The design of the tap is not designated for this task unless it was spliced in backwards at the point you are testing from. There is isolation on the return (regardless of tap value) from the tap "f" port back towards the EOL (for example). This would be the same as trying to hook up a house splitter backward (input to one of the output legs) and attempting to read a forward output level from the other output leg. The excessive isolation would prevent you from obtaining accurate readings.

Depending on what type manufacturer taps and tools or devices you have available to you, your testing procedures may be limited. Lindsay did have many years ago a tap that allowed the internal directional-coupler to be physically reversed so the tap port could see the return from the "f" port temporarily BUT these taps are not common (unless you are working in Shreveport, LA).

You may have to utilize a probe on the "KS" ports of the tap that should be AC blocking for better results. Unfortunately short of physically disconnecting the cable from the plant and hooking to a BAFF connector (not recommended) to the cable to see what is there, you are limited by the testpoints in the amplifiers. This should be good enough depending on what you are trying to accomplish. If you narrow the problem down to a location "between actives", for example, you must look at drops at this point on the SDA.

Many years ago I designed a device(s) that could be used in the event of not having a man in the headend looking at the analyzer to measure progress. This would be for a "one man job" as I assume is your case. Take a system tap (value not important) and remove ALL internal wire mini-couplers and NOT the chokes and capacitors to the individual ports. Solder a fine wire from one of the "f" port lugs to the (removed) coupler TAP OUTPUT point. This will allow you to measure (with 0-dB attenuation at the tap port) what is present on the main line since the modified tap plate is REVERSED causing a temporary reverse testpoint in the network. DO NOT REMOVE the main choke or jumpers that allows voltage to pass through the tap as it could cause the following amplifier to be dead and act like a terminator past that point. It is actually very easy to do but time consuming. The soldering must be also done properly as not to short out any potential areas within the tap plate. USE a VOM to test prior to use.

Also reading prior replies to this post, just a comment about CPD. Yes, CPD can come from a customers drop or home. CPD can be present ANYWHERE where both forward and return signals are present at a point where 2 dissimilar metals are joined. This along with any type of corrosion at this junction will cause a "mixer diode effect" causing the forward signals to be super-imposed on the return causing a beat every 6-MHz in the return spectrum. There are many factors that determine the level of the distortion but none are exactly the same. If a defective (2-way) house amplifier that was sitting in a truck after being in the rain gets introduced to the house drop, this could cause 20dB or more of CPD depending on the conditions and even a high value tap value would not have enough isolation to consider it clean. (Just my 2-cents worth).

Good luck!

Greg Tobin
Interactive Broadband Technical Services, LLC
314-422-4423 cellular

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Not Possible Testing At Tap "F" Port