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Fig. 2: Setup for testing cable TV receivers in line with SCTE 40. Linearity of amplifiers Testing amplifiers for cable networks presents similar challenges for test equipment. The challenge here is to generate a signal as close to ideal as possible. It must be ensured that any detrimental effect that the amplifier might have on the signal is kept as small as possible. Two types of measurements are performed for this: The first type requires feeding a full channel load to the amplifier. At the output, a test receiver determines the bit error ratio (BER). The deterioration of the BER provides a measure of the extent to which the amplifier distorts the signal. The second type of measurement test intermodulation behaviour. Again a full load is again fed to the amplifier. In this case the channel that is to be measured remains free. The input signal is a full load with a gap of one channel (Fig. 5). The intermodulation is measured at the amplifier output in the unassigned channel. When the full load that is being applied at the amplifier’s input consists exclusively of analog TV signals or CW carriers, aggregations of discrete noise lines appear. Since the second and third order products dominate, these noise lines are referred to as composite second-order / composite triple-beat (CSO/CTB). Fig. 3: Block diagram for an FPGA-based multichannel signal generator for simulating full channel load. Fig. 4: Complex cable network loading with 7 MHz channels and 8 MHz channels, PAL and DVB-C. In networks with digital QAM signals this parameter is increasingly losing its significance. As with all digital modulation signals, a QAM signal is noise-like. In a completely digital network intermodulation expresses itself as an increase in the noise in the measurement channel which is measured with the aid of a spectrum analyser. A cable TV amplifier must meet stringent linearity requirements. In order to measure the intermodulations the signal bearing a full channel load can only have very low intermodulations of its own. For this reason, it is necessary to reduce the signal source’s intermodulations in the useful channel by applying a bandstop filter at the amplifier’s input (Fig. 6). Using generators to simulate a cable network When performing simulations the number of channels within the network presents the largest challenge. State- of-the-art signal generators that are capable of producing many signals simultaneously are able to significantly reduce the amount of equipment required. That saves space in the lab, and lowers the cost of the devices. The generated signals are highly precise and provide reliable simulations. Signal generators such as the CLG help substantially reduce risks when developing cable TV components. With them, engineers can perform development projects quickly and efficiently. Fig. 5: Full channel load with a one-channel gap Cable Load Generator ~~ ~ ... Cable TV Amplifier (D.U.T.) Spectrum Analyzer / TV Test Receiver ... ... multiple QAM signals with deep notch ... additional noise introduced by DUT Fig 6: Setup for testing the linearity of cable TV TV-BAY MAGAZINE: ISSUE 79 JULY 2013 | 51 TV-BAY079JUL13.indd 51 09/07/2013 16:51