TV-BAY Issue 79 July 13

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A complete cable network – out of a hat Testers employ a TV signal generator that modulates the wanted signal and overlays noise interference. The signal generator’s fading simulator creates the microreflections as well as the AC. The AC hum is created by two additional paths with Doppler displacements of +/-50Hz. by Peter Lampel, Rohde & Schwarz A new way to perform simulations of cable TV networks! “ Time-to-market” – a familiar buzz word for any engineer. Meeting these demands requires early concept testing where simulations have become an indispensable tool. This approach is particularly useful when developing cable TV components, which must cope with many different types of interference. Simulating complete cable TV network presents challenges for test equipment. Each of the traditional methods of television broadcasts – terrestrial, satellite and cable – is flawed by characteristics that have a detrimental effect on transmission. In the case of terrestrial broadcasting it’s the multipath propagation caused by reflections. With satellite reception it’s the high levels of signal attenuation resulting from the 36,000 km between the satellite and the receiving dish. In cable television, the difficulty lies in the large number of tightly packed channels. In the cable band from 47 MHz to 1000 MHz (fig 1), this can be hundreds of channels. Ensuring error-free reception requires the receiver to employ a channel filter that features selectivity in order to avoid the interference. In addition, the characteristics of the components that are involved in the transmission must exhibit the highest levels of linearity in order to keep the intermodulation products small. Interference in cable networks Many different types of interference effects play a role in cable TV transmissions. To start with, the cable is subject to white noise. This is then joined by the phase noise from the modulators which creates errors in the constellation diagram. Electromagnetic fields cause a third type of interference, for example electric motors, light switches, microwave loaded network. SCTE 40 requires that all interference effects occur at the same time. This is not only a problem for the receiver; it is also challenge for the test equipment. Laboratory simulation Fig. 1: Fully loaded cable TV network with NTSC and QAM signals. ovens etc... This interference appears as impulsive noise. Other types of interference are caused by connections between the components. Mismatches can cause microreflections and create echoes. If these connections do not have proper RF shielding, interfering signals can make their way into the cable. The opposite case can happen where cable TV signal emissions disturb frequency ranges being used for other purposes. This is becoming increasingly significant because mobile communications employ the UHF TV range in the “digital dividend.” AC hum is yet another form of interference. Aging components in the amplifier can cause overlaying of the AC supply frequency. To this we can add the AC supply frequency’s harmonics, which arise in rectifier circuits. The AC hum adds further amplitude modulation to the TV signal. Testing tuners In order to prove a cable TV receiver functions properly under realistic conditions testers must overlay the received signal with the above mentioned forms of interference. Standardization bodies establish the values for this in their test specifications. The Digital Cable Standard published by the U.S. Society of Cable Telecommunication Engineers (SCTE), better known by the abbreviation “SCTE 40,” is probably the best-known. It defines a worst-case scenario with fixed values for white noise, phase noise, microreflections and an AC hum. It also establishes the levels for a fully Simulating full channel load presents the greatest challenge. Until now this required using a tremendous amount of equipment. Advanced methods reduce these requirements considerably. The new CLG from Rohde & Schwarz is the first integrated compact device capable of generating hundreds of signals simultaneously. These signals can be digital or analogue or they can contain unmodulated continuous wave (CW) carriers. This significantly simplifies the test setup that SCTE 40 requires (Fig. 2). The CLG takes advantage of a technique that is increasingly being employed in the QAM modulators for cable headends. The generator creates multiple QAM signals in a field programmable gate array (FPGA). The device employs this process so that a single FPGA generates all signals. This makes it possible to achieve the high precision and reproducible results. The CLG generates ten “blocks”, each of which generates up to 16 channels simultaneously (Fig. 3). Each block can generate its own type of modulation; analogue, QAM or CW. The frequencies and levels can be set individually per channel, providing tremendous flexibility. This enables laboratory simulation of networks. Manufacturers of cable TV tuners and receivers generally operate one development centre where they develop for the global market. In order to analyse the problems from around the world, they have to be able to simulate their customer’s network in a lab. These network configurations can differ greatly. Fig. 4 shows the simulation of a configuration in Germany, with many different analogue and digital channels.. 50 | TV-BAY MAGAZINE: ISSUE 79 JULY 2013 TV-BAY079JUL13.indd 50 09/07/2013 16:52