Thursday, 20 August 2009

Noises in a CATV Reverse Path

The Signal Level Meter is probably the single most essential piece of instrumentation required for a CATV network monitoring, be it Analogue or Digital. With the introduction on CAS and HITS in the Indian market the demand for Analog-cum-QAM cable meters is growing fast. Such a meter is used for both installation of new head-ends in a network as well as for fault finding and routine maintenance of the already laid wire web. A signal level meter is used to measure and ensure that signal levels are delivered as required and as cost effectively. The distortion performance of an Amplifier is closely related to the output signal level. The amplifier distortion increases roughly by 2 dB for every 1 dB increase in the output level. Hence it becomes very crucial to adjust amplifier levels appropriately down the cable line up to the last mile. This can be achieved using a signal level meter that is accurate and has been calibrated at regular intervals. As a bare minimum requirement, an SLM would measure Video level, Audio level & C/N for an analogue relay: BER and MER for a digital relay and Tilt for gain-slope adjustment. On the higher end, however, an advanced SLM with features such as spectrum and constellation can make the task of identifying types of noise and its cause much simpler and quicker. 
The advancement in science and instrumentation has made communication much more reliable and appealing. One of those advancements had given birth to Interactive cable TV technology where a customer would respond to the services offered to him via his cable hardware and TV set seated at his home. The services vary from booking air or rail tickets to playing KBC or other game-plays. The TV set is no longer an Idiot box, it now interacts. Although, this technology is not really unthinkable and not very new but the upstream path or reverse path is now being increasingly used in CATV networks for relaying live programs and for exchanging channels between various Head-ends. The development in the field of computer connectivity has further pushed up streaming in CATV networks. The market is already seeing some meters with upstream range detection of 5 - 30 MHz; it would even be better to have an equipment which could detect somewhere between 5 - 80 MHz for future compatibility because upstream traffic would certainly increase over time.
For a two way system, Ingress and noise are far greater problems in the reverse path than in the forward path. Reasons are as many as there are types of noises associated with a CATV network. Accumulation of different type of noises, random noise, thermal noise, diode effect, second order and third order beats are the most affecting. Troubleshooting noise in the reverse path is much more difficult and time consuming than it is in the forward path and requires lot of attention and right tool for analysis. 
Noise has a compounding effect. In forward path, noise generated at one point in a network affects everyone downstream from the noise source. In the reverse path network, however, an interference or noise affects everyone in the same branch all the way back to the head-end receiver. To get an idea of how badly this affects the noise floor, just imagine how many points are there in the network through which a noise can be injected for a given customer. For forward path signals, the only problematic area could be the direct path between customer’s STB and head-end but for reverse path signals, a problem can arise from anywhere in the area served by one head-end receiver. This would mean that for the reverse path problems, there are much more points to test, greater possibility of making mistakes and lot of money to invest. And for that purpose a professional tool is a must.
The other most common type of noise is Ingress and there can be numerous sources for such type of noise. As we see there are an increasing number of RF energy transmitters in the band utilized by CATV reverse path such as mobile transmitters, short-wave radios, paging, etc., there can be multiple Ingress sources which adversely affect the network and may render the service inoperable at certain instances. Even worse, many transmitters may be placed just next to the CATV network hardware. Such problems could be reduced by using hard-lined and better quality instruments at the upstream connections and relatively low cost ones downstream. Measurement of upstream signals helps identifying strong signal Ingress at certain frequencies which would, as a result not permit the use of that particular reverse path channel.
Hum is an undesirable modulation of the TV video carrier by power line frequencies and harmonics (such as 50Hz AC in India) or other low frequency disturbances that may be introduced in the network through power-pass amplifiers. Since poor electricity problem prevails in most remote parts of India, humming noise becomes very prominent. The best practice would be to use a trunk line amplifier that has good power supply specifications and is adequately isolated from high tension cable on the electricity poles.
CPD (common path distortion) is a form of intermo-dulation distortion that occurs as the RF spectrum is mixed as it passes through non-linear junctions. This phenomenon results is production of beats that affects communication. CPD can be caused due to many reasons: corrosion at dissimilar metal joints, poor solder joints, oxidation of aluminum parts, cracked housings, exposed electronics, missing port caps and sealing gaskets, etc. Dissimilar metal contacts often lead to an effect known as diode effect which causes variations in electron densities around the contact leading to formation of depletion regions and virtual capacitances that adversely affects the signal quality. Seals have to be air-tight, otherwise, normal heating and cooling cycles bring in moisture through open and cracked joints in the CATV hardware that are exposed to the environment. One of the symptoms associated with CPD is the hum in the signal which is caused due to the semiconductor properties of the oxidation that forms on the soldered parts.
Noise can be generated by non-transmitting devices too. Switching relays, electric motors, light rods and other heavy electric machinery pollute the mains supply thereby affecting the reverse path band in CATV amplifiers that are tied to the same network. 
To deal with such type of noises and problems, their sources needs to be correctly identified, analyzed, isolated and replaced to avoid further loss of infrastructure and man-hour. Tracking down one source of Ingress is relatively easy but tracking many noise sources together at one particular frequency is very tedious. This is where a professional SLM with features as spectrum analyzer, constellation analyzer and upstream measurements come in handy.
We at Horizon Global Electronics understand the fresh needs of the existing cable market and are committed to its growth. The introduction of Digital Cable Meter range for both head-ends as well as last-mile technician had been our first step in that direction and more products are in the pipeline backed by our regional presence in the Indian sub-continent for prompt product service and customer support.

Source:
http://cablequest.org/articles/technical/item/1316-noises-in-a-catv-reverse-path.htmlurce: http://cablequest.org/articles/technical/item/1316-noises-in-a-catv-reverse-path.htm

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