© 2010 cisco systems, inc. all rights reserved.cisco public digital proofs 1 digital proofs or not?...

© 2010 Cisco Systems, Inc. All rights reserved. Cisco PublicDigital Proofs 1

Digital Proofs or Not?

Ron Hranac



Did you know the FCC has required digital signals on most cable networks to meet certain technical performance parameters, and that this requirement has been on the books for several years?

Graphics source: Sunrise Telecom



§76.640(b)(1)(i) is where you’ll find the rules for digital signals:(1) Digital cable systems with an activated channel capacity of 750 MHz or greater shall comply with the following technical standards and requirements:

(i) SCTE 40 2003 (formerly DVS 313): “Digital Cable Network Interface Standard” (incorporated by reference, see §76.602), provided however that with respect to Table B.11, the Phase Noise requirement shall be −86 dB/Hz, and also provided that the “transit delay for most distant customer” requirement in Table B.3 is not mandatory.



This definitely causes a lot of confusion!

§76.640 of the FCC rules states that digital signals in 750 MHz and greater plants MUST meet the technical parameters in SCTE-40.

The FCC rules don’t say when, how, or how often to verify that digital signals are in compliance, nor do the rules say how to document that compliance.

But those signals must comply—that part is clear.



The good news is that most of the QAM signal technical parameters in SCTE-40 can be measured with commonly available QAM analyzers (also known as digital signal analyzers), spectrum analyzers, and signal level meters (SLMs).

As we go through the requirements in SCTE-40, you’ll notice that some of them apply to analog TV channels. For the most part, those parameters mirror what is required in §76.605 and other sections of the FCC rules.

This presentation highlights measurement techniques applicable to QAM signals, since it is assumed that measurements of analog TV channels are already being done as part of FCC-required proof-of-performance testing.


SCTE-40

6.4 Downstream Transmission Characteristics

The Downstream Transmission Characteristics are contained in Table B. Analog and FAT1 Channel: RF Transmission Characteristics and Table C. FDC2 Channel: RF Transmission Characteristics as provided below. Analog and FAT signals shall meet the characteristics specified in Table B when measured on the subscriber's premises at the end of a properly terminated drop cable and OOB FDC signals shall meet the characteristics specified in Table C when measured on the subscriber's premises at the end of a properly terminated drop cable.

Note 1. Forward Application Transport (FAT) channels: 64- and 256-QAM channels that comply with ITU-T J.83 Annex B, and are carried in the 54-864 MHz range.

Note 2. Forward Data Channels (FDC): Out-of-band QPSK signals located between 70 and 130 MHz


SCTE-40 Table B


SCTE-40 Table B (cont’d)


RF Channel Spacing

This is pretty much a yes or no requirement. If in doubt, channel spacing is easily determined using a spectrum analyzer.


RF Frequency Range

The first portion of this requirement is largely a fill-in-the-blanks question. Specify the frequency range used for downstream transmission.

The second portion of this requirement can be fulfilled by stating the CEA 542-C channel plan—STD, IRC, or HRC—in use for downstream transmission.

ChannelSTD (Analog visual)

IRC (Analog visual)

HRC (Analog visual)

STD(QAM CF)

IRC(QAM CF)

HRC(QAM CF)

2 55.2500 55.2625 54.0027 57.0000 57.0000 55.7500

3 61.2500 61.2625 60.0030 63.0000 63.0000 61.7500

4 67.2500 67.2625 66.0033 69.0000 69.0000 67.7500

5 77.2500 79.2625 78.0039 79.0000 81.0000 79.7500

6 83.2500 85.2625 84.0042 85.0000 87.0000 85.7500

Frequencies in table from CEA 542-C


Transit Delay (not required by FCC)

The SCTE-40 one-way transit delay specification is ≤0.800 millisecond (ms or msec)

Signals traveling one way from the headend to the subscriber through, say, 18 km (59,055 feet) of fiber and 1 km (3,281 feet) of coax: about 92 microseconds (μs or μsec) transit delay. This is the same as 0.092 msec.

Headend Fiber Node

Coax serving area

18 km 1 km

~92 μs



One can calculate the approximate one-way transit delay (propagation delay) through a cable network. The following pieces of information are required: the length of transmission medium (fiber or coax) through which the signals travel, and the velocity of propagation of the transmission medium.

Typical velocity of propagation for single mode optical fiber at 1310 nm is about 68%, and for hardline coaxial cable the value is about 87%

The free space value of the speed of light is 299,792,458 meters per second, or 983,571,056.43 feet per second



RF travels through hardline feeder cable at 87% of the free space value of the speed of light, or 983,571,056.43 x 0.87 = 855,706,819.09 feet per second. The time it takes for RF to travel through 1 foot of cable is 1.17x10-9 second (1.17 nanosecond)

Surprisingly, light travels through fiber slower than RF travels through coax! For example, the published effective group index of refraction for Corning’s SMF-28e+ optical fiber is 1.4676 at 1310 nanometers. That puts the velocity factor at 1/1.4676 = 0.6814, and the velocity of propagation at just over 68%.

The light makes its way through the fiber at the leisurely pace of 670,190,144.75 feet per second, or through 1 foot of fiber in 1.49x10-9 second (1.49 nanosecond).

See “Velocity of Propagation” in the March 2010 issue of Communications Technology magazine: http://www.cable360.net/ct/sections/columns/bullpen/40153.html


Carrier-to-Noise Ratio

The first part of this requirement applies to downstream QAM signals.

The second part of this requirement applies to downstream analog TV channels, and is the same as what is specified in §76.605(a)(7) of the FCC rules:

(7) The ratio of RF visual signal level to system noise shall not be less than 43 decibels. For class I cable television channels, the requirements of this section are applicable only to:

(i) Each signal which is delivered by a cable television system to subscribers within the predicted Grade B contour for that signal;

(ii) Each signal which is first picked up within its predicted Grade B contour;

(iii) Each signal that is first received by the cable television system by direct video feed from a TV broadcast station, a low power TV station, or a TV translator station.


Carrier-to-Noise Ratio (cont’d)

When using a spectrum analyzer to measure the carrier-to-noise ratio of a QAM signal, the CNR is simply the signal’s height above the noise floor in dB.

Make certain that the spectrum analyzer is displaying the cable system’s noise floor, and not the test equipment’s noise floor!

CNR ≈ 15 dB

This is the test equipment’s noise floor, not the system’s noise floor. This is not a valid

CNR measurement!


Carrier-to-Noise Ratio (cont’d)

This example shows a CNR of about 34 dB, which exceeds the SCTE-40 requirements for both 64- and 256-QAM

CNR ≈ 34 dB


CTB and CSO

Measuring composite triple beat and composite second order in a QAM channel can be done one of two ways:

Turn off the QAM channel at the headend, and measure CTB and CSO beat cluster levels in the field the same as is done to comply with 76.605(a)(8)(i) and (ii) of the FCC rules (note that SCTE-40’s parameter is -53 dBc compared to the -51 dBc in the FCC rules). This method is service disruptive.

Perform a non-disruptive measurement using test equipment that can display the noise floor beneath an active QAM signal (example on next slide).

Remember that SCTE-40’s CTB/CSO ratio measurements are relative to analog TV channel levels


CTB and CSO

Graphics source: JDSU

QAM signal’s suppressed

carrier

CSO

CTB

IntermodulationCSO

In a network with a lot of analog TV channels, CTB beat clusters will appear on visual carrier frequencies, and CSO will appear ±0.75 and ±1.25 MHz relative to visual carrier frequencies.

As the number of QAM signals increases, CTB and CSO take on a noise-like appearance called composite intermodulation noise (CIN), which cannot be differentiated from thermal noise.


Carrier-to-Interference (Ingress)

One can measure this SCTE-40 parameter using the same method(s) used for CTB and CSO. What’s unclear is the reference for the -53 dBc spec.

Graphics source: JDSU

QAM signal’s suppressed

carrier

CSO

CTB

IntermodulationCSO


AM Hum Modulation

While AM hum modulation can be measured on a CW carrier, some QAM analyzers support automatic measurement of hum on an active QAM channel.



Group Delay Variation

A QAM signal’s in-channel group delay is most easily measured using a QAM analyzer that supports this parameter. The group delay is derived from the test equipment’s QAM receiver adaptive equalizer coefficients.


0.25 µsec is the same as 250 ns


Chroma/Luma Delay

This is a parameter for analog TV channels that is required to be measured in accordance with §76.605(a)(11)(i):

(11) As of June 30, 1995, the following requirements apply to the performance of the cable television system as measured at the output of the modulating or processing equipment (generally the headend) of the system:

(i) The chrominance-luminance delay inequality (or chroma delay), which is the change in delay time of the chrominance component of the signal relative to the luminance component, shall be within 170 nanoseconds.

(ii) The differential gain for the color subcarrier of the television signal, which is measured as the difference in amplitude between the largest and smallest segments of the chrominance signal (divided by the largest and expressed in percent), shall not exceed ±20%.

(iii) The differential phase for the color subcarrier of the television signal which is measured as the largest phase difference in degrees between each segment of the chrominance signal and reference segment (the segment at the blanking level of O IRE), shall not exceed ±10 degrees.


Phase Noise

The FCC in §76.640(b)(1)(i) relaxed this requirement 2 dB from what is specified in SCTE-40, to -86 dB/Hz

Phase noise measurement is a service-disruptive measurement that requires removing a QAM signal’s modulation, leaving just a CW carrier on the channel’s center frequency

A detailed procedure for phase noise measurement can be found in NCTA Recommended Practices for Measurements on Cable Television Systems, 3rd Ed. (“Section 3.7 Phase Noise”)

Recommended Practices, 3rd Ed. is available from SCTE


In-Channel Frequency Response

The first part of this requirement applies to downstream QAM signals.

The second part of this requirement applies to analog TV channels, and is largely the same as what is specified in §76.605(a)(6) of the FCC rules:

(6) The amplitude characteristic shall be within a range of ±2 decibels from 0.75 MHz to 5.0 MHz above the lower boundary frequency of the cable television channel, referenced to the average of the highest and lowest amplitudes within these frequency boundaries. The amplitude characteristic shall be measured at the subscriber terminal.


In-Channel Frequency Response (cont’d)

One can determine the approximate in-channel flatness of a QAM signal by observing the flatness of the top of the “haystack” on a spectrum analyzer.

A more accurate method is to use test equipment that supports automated in-channel flatness measurement, in which the results are typically derived from the test equipment’s QAM receiver adaptive equalizer coefficients.

Examples are included on the next two slides.



Graphics source: Agilent and Sunrise Telecom

1.2 dB p-p1.5 dB p-p




This example shows about 1 dB peak-to-peak amplitude variation across the QAM signal’s symbol rate bandwidth


Micro-Reflections

A QAM analyzer’s adaptive equalizer graph (“equalizer stress” graph) can be used to characterize micro-reflections.

An equalizer graph’s vertical axis usually shows relative amplitude in decibels, and the horizontal axis shows time, typically µsec.


Micro-Reflections (cont’d)



Micro-Reflections (cont’d)


0.5 µs 1.0 µs 1.5 µs 4.5 µs

The adaptive equalizer’s DFE taps (those to the right of the main tap) should be

below the thresholds shown

-10 dB @ ≤0.5 µs-15 dB @ ≤1.0 µs-20 dB @ ≤1.5 µs-30 dB @ ≤4.5 µs


Burst Noise

Section 3.6 of NCTA Recommended Practices for Measurements on Cable Television Systems, 3rd Ed., describes three methods for measuring upstream impulse noise. Method B or C should be able to be adapted to downstream measurement of burst (impulse) noise.

Note that SCTE-40 does not include an amplitude reference for burst noise, per Table B’s Note 5: “Burst noise is statistical in nature and a reference level should be defined. Studies on this are continuing.”

Note: Recommended Practices, 3rd Ed. is available from SCTE


Signal Levels at Subscriber Terminal

The first two parameters of this requirement apply to downstream QAM signals

The second two parameters of this requirement apply to analog TV channels. Make sure the instrument used for analog TV channel levels measures peak envelope power (PEP). A properly calibrated SLM, QAM analyzer with SLM functionality, or spectrum analyzer can be used to measure analog TV channel signal levels.



When we measure the RF level of QAM signals carried on cable networks, we measure the entire signal’s average power, also known as digital channel power.

Most modern SLMs, QAM analyzers, and some spectrum analyzers support digital channel power measurement, which removes the potential for error when performing a manual measurement that requires correction factors.

Examples are shown on the next slide.



Graphics source: Sunrise Telecom & Agilent


Some Parting Thoughts

§76.640 has been on the books for several years. In 750 MHz and greater plants, downstream digital signals must comply with SCTE-40. §76.640 does not specify how often or when to measure QAM signal performance, how many channels to test, or how to make specific measurements. That section of the rules says only:

“(1) Digital cable systems with an activated channel capacity of 750 MHz or greater shall comply with the following technical standards and requirements:

(i) SCTE 40 2003 (formerly DVS 313): “Digital Cable Network Interface Standard” (incorporated by reference, see §76.602)…”



My suggestion: Carefully read all of §76.640 and SCTE-40, and adopt the necessary procedures to ensure that you comply with what is stated.

Note that §76.640 refers to the 2003 version of SCTE-40. The latter has since been updated, although the FCC rules do not reference later versions.

Follow the test equipment manufacturers’ recommendations for appropriate factory calibration requirements, warm-up time and field calibration before performing all tests. These steps should be taken immediately prior to the commencement of testing.



Follow the test equipment manufacturers’ instructions for specific digital measurements. Actual setup and measurement procedures will vary among different makes/models of test equipment.

Refer to NCTA Recommended Practices for Measurements on Cable Television Systems, 3rd Ed. for detailed how-to descriptions of several digital measurement procedures.

Document everything!


Q and A


Backup slides


Distortions in an All-Digital Network

Distortions such as composite triple beat distortion, composite second order distortion, and common path distortion don’t go away in an all-digital network

Rather than clusters of discrete beats that occur in a network carrying large numbers of analog TV channels, the digital distortions are noise-like.

Those noise-like distortion products are variously known as composite intermodulation noise (CIN), composite intermodulation distortion (CID) or intermodulation noise (IMN)—none of which should be confused with thermal noise.



Confusion does occur, though. We know that raising RF levels in the plant improves the carrier-to-noise ratio, where “noise” is thermal noise. But in a system with a lot of digital signals, raising levels improves CNR to a point, then the noise floor starts to increase and the CNR appears to get worse.

That seems counterintuitive, but the now-elevated noise floor no longer is just thermal noise. It’s a combination of thermal noise and the previously mentioned noise-like distortions. When characterizing plant performance in the presence of thermal noise and CIN, the term “carrier-to-composite noise (CCN) ratio” commonly is used. Indeed, CCN is a much more appropriate measurement metric than is CNR under these circumstances, because there is no practical way to differentiate thermal noise from CIN.

Examples on the next three slides illustrate this


Distortions in an All-Analog Network

For each 1 dB increase in system carrier levels:

• CTB ratio degrades by 2 dB• CSO ratio degrades by 1 dB• CNR improves by 1 dB

CTB CSO

Thermal noise

Visual carriers Aural carriers


Distortions in an Analog + Digital Network

Thermal noise


• CNR, CTB, & CSO ratios behave as before with all-analog operation• CIN ratio degrades by 1 to 2 dB (mix of 2nd & 3rd order components)• CCN ratio degradation depends on CIN and CNR values

Composite intermodulation noise

Composite noise

Visual carriers

QAM signals

CTB CSO




• CNR behaves as before with all-analog operation• CIN ratio degrades by 1 to 2 dB (mix of 2nd & 3rd order components)• CCN ratio degradation depends on CIN and CNR values

Thermal noise

Composite noise

Composite intermodulation noise


Signal Leakage in an All-Digital Network

Leaking digital signals can cause harmful interference to over-the-air services under the right conditions

Despite the fact that a QAM signal’s power is spread across most of the 6 MHz channel bandwidth, moderate to high field strength leaks involving those noise-like QAM signals can indeed cause harmful interference.

6 MHz bandwidth

Communications transceiver’s S9+15 dB S-meter reading caused by 400 µV/m digital leak at 10 ft.


Signal Leakage in an All-Digital Network

The tens of thousands of existing leakage detectors out in the field today cannot be used to measure leaking digital signals

The good news is that manufacturers are working on digital-compatible leakage-detector technology, and one manufacturer recently introduced a digital-compatible leakage detection product.

Until new digital-compatible leakage detection gear becomes widely available, the only way to comply with the FCC’s existing leakage rules and maintain compatibility with existing leakage detectors is to use an analog TV channel or continuous wave (CW) carrier when measuring leakage.

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