Theory and practice: two different worlds. In theory, the modulation, transmission, and demodulation of signals are simple tasks. In practice, however, errors and inaccuracies occur in each of these steps. To ensure that the consumer can receive a television service properly under these imperfect conditions, set-top boxes (STBs) must be designed to cope with them. This article provides a basic introduction to cable networks, the problems that occur, and how the Rohde & Schwarz (R&S) SFU broadcast test system, the R&S SFE broadcast tester, and the R&S SFE100 broadcast test transmitter can be used to help in the development and quality assurance of STBs by simulating impairments found in cable TV systems.

Terrestrial, satellite, and cable - these are the classic means of transmission for TV broadcasting. Each of them uses different technologies with respect to modulation, forward error correction, and system design adapted to the specific environment. Digital cable TV broadcasting utilizes single carrier quadrature amplitude modulation (QAM) with typically 16, 64, 256, 512, and now also 1024 valence. In the past, three main standards were implemented in different regions of the world. The three standards have been consolidated in Recommendation J.83 by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T).

To deliver the television signal to consumer households via cable, a reliable distribution system is required. Fig. 1 shows such a system. A cable headend is responsible for conditioning the actual RF signal that is output. Satellite, IP link, or directional radio are typically used to supply the headend with the digital video and audio data for the various television services. Mainly delivered as an MPEG-2 transport stream, the baseband feed depends on the requirements remultiplexed and input to the different modulators. A system combiner is used to merge all modulated channels to a single output for further distribution over the cable network.

Conventional coaxial cables are used to transmit this single output signal to consumer households. A signal carried over these copper cables is attenuated due to cable loss. To ensure that all cable service subscribers over a wide area are provided with signals of sufficient level, amplification needs to be performed along the transmission path. The rule of thumb here is one amplifier every 300 m.

In addition to any signal degradation that may be caused during conditioning at the source (program feed, forward error correction, IQ modulation, channel allocation), the transmission path itself can also have significant adverse effects on signal quality.

A major adverse effect originates from the amplifier stages. These amplifiers are nonlinear to a certain extent and thus create intermodulation products. Looking at a broadband cable, dozens of channels (i.e., carriers) exist. As a result of the nonlinear amplification process, carrier combinations create "beats". Of particular importance here are the composite second order (CSO) and composite triple beat (CTB) intermodulation products. Beats appearing in an occupied channel overlay with the useful signal and degrade its signal quality. Plus, each amplification step introduces a certain level of noise as well.

Ideally, the characteristic impedance of distribution networks for cable TV should be constant at every point along the transmission path starting at the source and ending at the actual load, e.g., the STB. Under real conditions, however, the presence of imperfect impedances on equipment such as connectors or cables in the overall system make mismatching a common occurrence. This causes part of the incident energy to be reflected back to the source, thus resulting in a standing wave. These kinds of microreflections cause amplitude and phase ripples on the broadcasted signals.

Last but not the least, receiving terminals are one of the most crucial components. Just as the STB must not introduce further interferences to the received signals, it must also be able to operate effectively outside the lab environment. In other words, the STB must function properly in different kinds of cable systems even under circumstances where the quality parameters mentioned above are not strong and where other signals (e.g., analog and digital ones, adjacent channels) are present.

Designing and assuring the quality of receiving equipment for cable TV applications is not an easy task. Indeed, as mentioned above, the receivers need to provide proper demodulation and decoding of television services under a wide variety of conditions.

Rohde & Schwarz offers a broad portfolio of test transmitters in the field of broadcasting that can handle numerous applications. The FPGA-based implementation of all relevant analog and digital terrestrial, satellite, and cable standards makes the products ideal for current and future use in R&D, quality assurance, and production.

The R&S SFU broadcast test system from Rohde & Schwarz allows the simulation of impairments on a cable TV signal as required in order to test STBs and other receiver terminals. Noncompliant modulators can be simulated via user-definable IQ impairments, phase noise generation, and internal baseband playout. A built-in arbitrary waveform generator enables users to add analog and digital adjacent channels to the useful signal (J.83/A/B/C) in order to check the filtering at the receiver input. Hum modulation as well as microreflections are reproduced by means of a powerful fading option. Furthermore, impulsive noise and white Gaussian noise generators allow further stress testing of the receiving equipment.

To check the influence of beats in an occupied channel as well as the behavior of the frontend, a full channel load first needs to be fed through an amplifier with nonlinear characteristics and then provided to the input of the receiving terminal. Instead of using modulated carriers, CW carriers are placed in each channel. The model of the single-standard R&S SFE100 test transmitter that includes an arbitrary waveform generator (ARB) provides a bandwidth of up to 84 MHz for outputting continuous wave (CW) carriers in 6 MHz, 7 MHz, and 8 MHz spacing. Several Rohde & Schwarz test transmitters can be combined to provide a solution for all relevant channels in the TV bands. Compared to existing solutions that require a dedicated signal generator for each CW carrier, this solution offers advantages in price, power consumption, and handling. If a more flexible solution is needed, the R&S SFE broadcast tester is a good alternative. It can support up to three different realtime modulations in addition to the ARB functionality in one box.

Fig. 2 shows a test setup with a possible interconnection. An amplifier is used to generate the CSO/CTB intermodulation products overlaid with the signal to be tested. Depending on the application, additional bandpass or band rejection filtering may need to be applied. This will prevent a CW carrier from impairing the useful signal as well as adjacent channels.

The Society of Cable Telecommunications Engineers (SCTE) has defined the Digital Cable Network Interface Standard (ANSI/SCTE 40), which serves as a guideline for ensuring the quality of STBs and other cable receiving equipment when operated in a real cable system.

The tests are a combination of noise (AWGN and phase noise), AM hum, microreflections, digital and analog adjacent channels, and other discrete interferences added to a J.83/B signal. By providing NTSC as well as J.83/B adjacent channel interferers and additional waveforms containing CW carriers in 6 MHz spacing, all necessary tests can be carried out in line with local requirements. Moreover, channel and power settings as well as noise, fading, and impairment options can be configured dynamically. The spectrum plot in Fig. 3 illustrates what an ANSI/SCTE 40 test case looks like.

For details on how to apply ANSI/SCTE 40 testing, refer to Application Note 7BM68 from Rohde & Schwarz. The document is available on the Rohde & Schwarz website (www.rohde-schwarz.com).

Providing reliable cable TV service for consumers is not an easy task due to various impairments that occur as the signal is passed from the headend to the receiver. To cope with these impairments, the receiving terminals must meet demanding requirements.

"The R&S SFU broadcast test system, the R&S SFE broadcast tester, and the R&S SFE100 test transmitter from Rohde & Schwarz are the key to achieving this objective in development, quality assurance, and production." says Harald Gsoedl, Product Marketing for Broadcasting T&M Products at Rohde & Schwarz.