Biblio5

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15.2 EVOLUTION OF CATV

15.2.1 Beginnings

Broadcast television, as we know it, was in its infancy around 1948. Fringe area problems were much more acute in that period. By fringe area, we mean areas with poor or scanty signal coverage. A few TV users in fringe areas found that if they raised their antennas high enough and improved antenna gain characteristics, an excellent picture could be received. These users were the envy of the neighborhood. Several of these people who were familiar with RF signal transmission employed signal splitters so that their neighbors could share the excellent picture service. It was soon found there was a limit on how much signal splitting could be done before signal levels got so low that they were snowy or unusable.

Remember that each time a signal splitter (even power split) is added, neglecting insertion losses, the TV signal drops 3 dB. Then someone got the bright idea of amplifying the signal before splitting. Now some real problems arose. One-channel amplification worked fine, but two channels from two antennas with signal combining became difficult. Now we are dealing with comparatively broadband amplifiers. Among the impairments we can expect from broadband amplifiers and their connected transmission lines (coaxial cable) are the following:

•Poor frequency response. Some part of the received band had notably lower levels than other parts. This is particularly true as the frequency increases. In other words, there was fairly severe amplitude distortion. Thus equalization became necessary.

•The mixing of two or more RF signals in the system caused intermodulation products and “beats” (harmonics), which degraded reception.

•When these TV signals carried modulation, cross-modulation (Xm) products degraded or impaired reception.

Several small companies were formed to sell these “improved” television reception services. Some of the technicians working for these companies undertook ways of curing the ills of broadband amplifiers.

These were coaxial cable systems, where a headend with a high tower received signals from several local television broadcasting stations, amplified the broadband signals, and distributed the results to CATV subscribers. A subscriber’s TV set was connected to the distribution system, and the signal received looked just the same as if it were taken off the air with its own antenna. In fringe areas, signal quality, however, was much better than own-antenna quality. The key to everything was that no changes were required in the user’s TV set. It was just an extension of her/ his TV set antenna. This simple concept is shown in Figure 15.2.

Note in Figure 15.2 that home A is in the shadow of a mountain ridge and receives a weakened diffracted signal off the ridge and a reflected signal off the lake. Here is the typical multipath scenario resulting in ghosts in A’s TV screen. The picture is also snowy, meaning noisy, as a result of poor carrier-to-noise ratio. Home B extended the antenna height to be in line-of-sight of the TV transmitting antenna. Its antenna is of higher gain; thus it is more discriminating against unwanted reflected and diffracted signals. Home B has an excellent picture without ghosts. Home B shares its fine signal with home A by use of a 3-dB power split (P) and a length of coaxial cable.

Figure 15.1 An early CATV distribution system.

15.2.2 Early System Layouts

Figure 15.1 illustrates an early CATV distribution system (ca. 1968). Taps and couplers (power splits) are not shown.1 These systems provided from 5 to 12 TV channels. An LOS microwave system might bring in channels from distant cities. We had direct experience with an Atlantic City, NJ, system where channels were brought in by microwave from Philadelphia and New York City. A 12-channel system was derived and occupied the entire assigned VHF band (i.e., channels 2–13).

Figure 15.2 CATV initial concept.

1A coupler or power split is a passive device that divides up the incoming power in various ratios. A 3-dB split divides the power in half. A 10-dB split divides off one-tenth of the power, leaving approximately nine-tenths of the power on the main route.

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As UHF TV stations began to appear, a new problem arose for the CATV operator. It was incumbent on that operator to keep the bandwidth as narrow as possible. One approach was to convert UHF channels to vacant VHF channel allocations at the headend.

Satellite reception at the headend doubled or tripled the number of channels that could be available to the CATV subscriber. Each satellite has the potential of adding 24 channels to the system. Note how the usable cable bandwidth is “broadened” as channels are added. We assume contiguous channels across the band, starting at 55 MHz. For 30 channels, we have 55–270 MHz; for 35 channels, 55–300 MHz; for 40 channels, 55–330 MHz; for 62 channels, 55–450 MHz; and for 78 channels, 55–550 MHz. These numbers of channels were beyond the capability of many TV sets of the day. Set-top converters were provided that converted all channels to a common channel, an unoccupied channel, usually channel 2, 3, or 4, to which the home TV set is tuned. This approach is still very prevalent today.

In the next section we discuss CATV transmission impairments and measures of system performance. In Section 15.4, hybrid-coaxial systems are addressed. The fiber replaced coaxial cable trunks, which made a major stride toward better performance, greater system extension, and improved reliability/ availability.

15.3 SYSTEM IMPAIRMENTS AND PERFORMANCE MEASURES

15.3.1 Overview

A CATV headend places multiple TV and FM (from 30 to 125) carriers on a broadband coaxial cable trunk and distribution system. The objective is to deliver a signal-to-noise ratio (S/ N) of from 42 dB to 45 dB at a subscriber’s TV set. From previous chapters we would expect such impairments as the accumulation of thermal and intermodulation noise. We find that CATV technicians use the term beat to mean intermodulation (IM) products. For example, there is triple beat distortion, defined by Grant (Ref. 1) as “spurious signals generated when three or more carriers are passed through a nonlinear circuit (such as a wideband amplifier).” The spurious signals are sum and difference products of any three carriers, sometimes referred to as “beats.” Triple-beat distortion is calculated as a voltage addition.

The wider the system bandwidth is and the more RF carriers transported on that system, the more intermodulation distortion, “triple beats,” and cross-modulation we can expect. We can also assume combinations of all of the above, such as composite triple beat (CTB), which represents the pile up of beats at or near a single frequency.

Grant (Ref. 1) draws a dividing line at 21 TV channels. On a system with 21 channels or fewer, one must expect Xm to predominate. Above 21 channels, CTB will predominate.

15.3.2 dBmV and Its Applications

We define 0 dBmV as 1 mV across 75 Q impedance. Note that 75 Q is the standard impedance of CATV, of coaxial cable, and TV sets. From Appendix A, the electrical power law, we have:

where Pw is the power in W, E the voltage in V, and R the impedance, 75 Q . Substituting the values from the preceding, then:

If a signal level is 1 V at a certain point in a circuit, what is the level in dBmV?

dBmV c 20 log(1000/ 1) c +60 dBmV.

If we are given a signal level of +6 dBmV, to what voltage level does this correspond?

+6 dBmV c 20 log(XmV/ 1 mV).

Divide through by 20:

6/ 20 c log(XmV/ 1 mV) antilog(6/ 20) c XmV

XmV c 1.995 mV, or 2 mV, or 0.002 volt

These signal voltages are rms (root mean square) volts. For peak voltage, divide by 0.707. If you are given peak signal voltage and wish the rms value, multiply by 0.707.

15.3.3 Thermal Noise in CATV Systems

We remember from Section 3.3.3 that thermal noise is the most common type of noise encountered in telecommunication systems. In most cases, it is thermal noise that sets the sensitivity of a system, its lowest operating threshold. In the case of a CATV system, the lowest noise levels permissible are set by the thermal noise level—at the antenna output terminals, at repeater (amplifier) inputs, or at a subscriber’s TV set—without producing snowy pictures.

Consider the following, remembering we are in the voltage domain: Any resistor or source that looks resistive over the band of interest, including antennas, amplifiers, and long runs of coaxial cable, generates thermal noise. In the case of a resistor, the thermal noise level can be calculated based on Figure 15.3.

To calculate the noise voltage, en, use the following formula:

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Figure 15.3 Resistor model for thermal noise voltage, en.

where en c rms noise voltage;

R c resistance in ohms (Q );

B c bandwidth (Hz) of the measuring device (electronic voltmeter, V); and k c a constant equal to 40 × 10−16 at standard room temperature.2

Let the bandwidth, B, of an NTSC TV signal be rounded to 4 MHz. The open circuit noise voltage for a 75-Q resistor is

en c (4 × 75 × 4 × 10−16)1/ 2 c 2.2mV rms.

Figure 15.4 shows a 2.2-mV noise generating source (resistor) connected to a 75-Q (noiseless) load. Only half of the voltage (1.1 mV) is delivered to the load. Thus the noise input to 75 Q is 1.1 mV or −59 dBmV. This is the basic noise level, the minimum that will exist in any part of a 75-Q CATV system. The value −59 dBmV will be used repeatedly in the following text (Ref. 2). The noise figure of a typical CATV amplifier ranges between 7 dB and 9 dB (Ref. 3).

15.3.4 Signal-to-Noise (S/ N) Ratio versus Carrier-to-Noise (C/ N) Ratio in

CATV Systems

We have been using S/ N and C/ N many times in previous chapters. In CATV systems S/ N has a slightly different definition as follows (Ref. 2):

This relationship is expressed by the “signal-to-noise ratio,” which is the difference between the signal level measured in dBmV, and the noise level, also measured in dBmV, both levels being measured at the same point in the system.

S/ N can be related to C/ N on CATV systems as:

Figure 15.4 Minimum noise model.

2This value can be derived from Boltzmann’s constant (Chapter 3) at room temperature (688F or 290 K).

This is based on Carson (Ref. 4), where the premise is “noise just perceptible” by a population of TV viewers, with an NTSC 4.2-MHz TV signal. Adding noise weighting improvement (6.8 dB), we find:3

It should be noted that S/ N is measured where the signal level is peak-to-peak and the noise level is rms.4 For C/ N measurement, both the carrier and the noise levels are rms. These values are based on a VSB-AM (vestigial sideband, amplitude modulation) with an 87.5% modulation index.

The values for S/ N should be compared with those derived by the Television Allocations Study Organization (TASO) and published in their report to the U.S. FCC in 1959. Their ratings, corrected for a 4-MHZ bandwidth, instead of the 6-MHZ bandwidth that was used previously, are shown in Section 14.4.2.

Once a tolerable noise level is determined, the levels required in a CATV system can be specified. If the desired S/ N has been set at 43 dB at a subscriber TV set, the minimum signal level required at the first amplifier would be −9 dBmV + 43 dB or −16 dBmV, considering thermal noise only. Actual levels would be quite a bit higher because of the noise generated by subsequent amplifiers in cascade.

It has been found that the optimum gain of a CATV amplifier is about 22 dB. When the gain is increased, IM/ Xm products become excessive. For gains below this value, thermal noise increases, and system length is shortened or the number of amplifiers must be increased—neither of which is desirable.

There is another rule-of-thumb of which we should be cognizant. Every time the gain of an amplifier is increased 1 dB, IM products and “beats” increase their levels by 2 dB. And the converse is true: every time gain is decreased 1 dB, IM products and beat levels are decreased by 2 dB.

With most CATV systems, coaxial cable trunk amplifiers are identical. This, of course, eases noise calculations. We can calculate the noise level at the output of one trunk amplifier. This is

where NF is the noise figure of the amplifier in dB.

In the case of two amplifiers in cascade (tandem), the noise level (voltage) is

If we have M identical amplifiers in cascade, the noise level (voltage) at the output of the last amplifier is

3Weighting (IEEE, Ref. 10): The artificial adjustment of measurements in order to account for factors that in normal use of the device, would otherwise be different from the conditions during measurement. In the case of TV, the lower baseband frequencies (i.e., from 20 Hz to 15 kHz) are much more sensitive to noise than the higher frequencies (i.e., >15 kHz).

4Peak-to-peak voltage refers, in this case, to the measurement of voltage over its maximum excursion, which is the voltage of the “sync tips.” (See Figures 14.3 and 14.4.)

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This assumes that all system noise is generated by the amplifiers, and none is generated by the intervening sections of coaxial cable.

Example 1. A CATV system has 30 amplifiers in tandem; each amplifier has a noise figure of 7 dB. Assume that the input of the first amplifier is terminated in 75 Q resistive. What is the thermal noise level (voltage) at the last amplifier output?

Use Eq. (15.8):

NV c −59 dBmV + 7 dB + 10 log 30

c−59 dBmV + 7 dB + 14.77 dB

c−37.23 dBmV.

For carrier-to-noise ratio (C/ N) calculations, we can use the following procedures. To calculate the C/ N at the output of one amplifier,

Example 2. If the input level of a CATV amplifier is +5 dBmV and its noise figure is 7 dB, what is the C/ N at the amplifier output?

Use Eq. (15.9):

C/ N c 59 dBmV − 7 dB + 5 dBmV c 57 dB.

With N cascaded amplifiers, we can calculate the C/ N at the output of the last amplifier, assuming all the amplifiers are identical, by the following equation:

Example 3. Determine the C/ N at the output of the last amplifier with a cascade (in tandem) of 20 amplitifers, where the C/ N of a single amplifier is 62 dB.

Use Eq. (15.10):

C/ NL c 62 dB − 10 log 20

c62 dB − 13.0 dB

c49 dB.

15.3.5 Problem of Cross-Modulation (Xm)

Many specifications for TV picture quality are based on the judgment of a population of viewers. One example was the TASO ratings for picture quality given earlier. In the case of cross-modulation (cross-mod or Xm) and CTB (composite triple beat), acceptable levels are −51 dB for Xm and −52 dB for CTB. These are good guideline values (Ref. 1).

Xm is a form of third-order distortion so typical of a broadband, multicarrier system. It varies with the operating level of an amplifier in question and the number of TV channels being transported. Xm is derived from the amplifier manufacturer specifications.

The manufacturer will specify a value for Xm (in dB) for several numbers of channels and for a particular level. The level in the specification may not be the operating level of a particular system. To calculate Xm for an amplifier to be used in a given system, using manufacturer’s specifications, the following formula applies:

We spot the “2” multiplying factor and relate it to our earlier comments, namely, when we increase the operating level 1 dB, third-order products increase 2 dB, and the contrary applies for reducing signal level. As we said, Xm is a form of third-order product.

Example 1. Suppose a manufacturer tells us that for an Xm of −57 dB for a 35-channel system, the operating level should be +50.5 dBmV. We want a longer system and use an operating level of +45 dBmV. What Xm can we expect under these conditions?

Use Eq. (15.11):

Xma c −57 dB + 2(+ 45 dBmV − 50.5 dBmV)

c−68 dB.

CATV trunk systems have numerous identical amplifiers. To calculate Xm for N amplifiers in cascade (tandem), our approach is similar to that of thermal noise, namely,

where N c number of identical amplifiers in cascade; Xma c Xm for one amplifier; and

Xmsys c Xm value at the end of the cascade.

Example 2. A certain CATV trunk system has 23 amplifiers in cascade where Xma is

−88 dB. What is Xmsys? Use Eq. (15.12):

Xmsys c −88 dB + 20 log 23

c−88 + 27

c−61 dB

15.3.6 Gains and Levels for CATV Amplifiers

Setting both gain and level settings for CATV broadband amplifiers is like walking a tightrope. If levels are set too low, thermal noise will limit system length (i.e., number of amplifiers in cascade). If levels are set too high, system length will be limited by excessive CTB and Xm. On trunk amplifiers available gain is between 22 dB and 26 dB (Ref. 1). Feeder amplifiers will usually operate at higher gains, trunk systems at

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lower gains. Feeder amplifiers usually operate in the range of 26 –32-dB gain with output levels in the range of +47 dBmV. Trunk amplifiers have gains of 21–23 dB, with output levels in the range of +32 dBmV. If we wish to extend the length of the trunk plant, we should turn to using lower loss cable. Using fiber optics in the trunk plant is even a better alternative (see Section 15.4).

The gains and levels of feeder systems are purposefully higher. This is the part of the system serving customers through taps. These taps are passive and draw power. Running the feeder system at higher levels improves tap efficiency. Because feeder amplifiers run at higher gain and with higher levels, the number of these amplifiers in cascade must be severely limited to meet CTB and cross-modulation requirements at the end user.

15.3.7 Underlying Coaxial Cable System

The coaxial cable employed in the CATV plant has a nominal characteristic impedance (Zo) of 75 Q . A typical response curve for such cable ( 78 -in., air dielectric) is illustrated in Figure 15.5. The frequency response of coaxial cable is called tilt in the CATV industry. This, of course, refers to its exponential increase in loss as frequency increases.

For 0.5-in. cable, the loss per 100 ft at 50 MHz is 0.52 dB; for 550 MHz, 1.85 dB. Such cable systems require equalization. The objective is to have a comparatively “flat” frequency response across the entire system. An equalizer is a network that presents a mirror image of the frequency response curve, introducing more loss at the lower frequencies and less loss at the higher frequencies. These equalizers are often incorporated with an amplifier.

Equalizers are usually specified for a certain length of coaxial cable, where length is

Figure 15.5 Attenuation–frequency response for 78 -in coaxial cable, Zo ≈75 Q , Andrew HJ series Helix. (Courtesy of Andrew Corp.)