Newsletter edited on Pagemaker 5.0 by: Mike Norton

Chapter 24 World Wide Web Site http://www.broadcast.net/~sbe24
Leonard Charles is the editor for the Electronic Version of this Newsletter uploaded monthly onto SBE Chapter 24's web page.
Thanks to Chris Cain for his work on the Chapter 24 WWW page and electronic newsletter

© 1996 by SBE Chapter 24. Views expressed herein do not necessarily reflect the official positions of the Society, its officers, or its members. SBE Chapter 24 regrets, but is not liable for, any omissions or errors. The Chapter 24 Newsletter is published twelve times per year. Other SBE Chapters are permitted to use excerpts if attributed to the original author, sources, and SBE Chapter 24.

Contributors this month:
Leonard Charles
Denise Maney
Tom Smith
Tom Weeden
Neal McLain
Kevin Ruppert
Paul Stoffel

About this Newsletter
Meeting Announcement
Upcoming Meeting Schedule
September Meeting Minutes
Broadcast Clinic Happenings
Bake Off
Chapter 24 Holiday Party
TCI Completes Fiber
EAS Update
Local Legals
SBE Board Of Directors Meeting
Amateur Radio News
Latest DTV Information
Telecom Industry News
Broadband Networks (Pt 7)
Sustaining Members Listing

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NEWSLETTER EDITOR'S STATS

The Chapter 24 Newsletter is published monthly by Chapter 24 of the Society of Broadcast Engineers; Madison, Wisconsin. Submissions of interest to the broadcast technical community are welcome. You can make your submissions by e-mail to: Mike_Norton@went.pbs.org

Information and/or articles are also accepted by US Mail. Please address them to:

SBE Chapter 24 Newsletter Editor
46 Trillium Court
Madison, WI 53719-2308

Please submit text file on DOS or Windows 3.5" floppy diskette if possible.

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MEETING ANNOUNCEMENT

Wednesday October 30, 1996

EAS LOCAL PLAN CASE STUDIES

Ramada Inn Capital Conference Center
Highways 12 & 18 at I-90

The FCC has left the makeup of the Local EAS (Emergency Alert System) entirely up to broadcasters, working with the Local Emergency Government agencies and officials. Because of the lack of mandates, the complexity of your local EAS is limited only by your Local Area's collective resources, imagination, and hard work.

SBE Chapters and Local Area Emergency Communications Committees (LAECC) are invited to participate in the EAS LOCAL PLAN CASE STUDIES. "As-is" Local Plans from Areas around Wisconsin and Illinois will be shared. These Local Plans will be in various degrees of development. No Area is being asked to submit its final, pre-approved version. Areas without a Local Plan will have an opportunity to learn how to get started. It is hoped that everyone involved will have an opportunity to learn from each other's Local Plan and be able to ask questions.

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UPCOMING MEETINGS

Tentative Program Subjects

Wed, Nov 20, 1996
Newstar Edit System

Tue, Dec 17, 1996
Holiday Dinner Party

Wed, Jan 22, 1997
Part 1 - Digital Tape Options

Thu, Feb 20, 1997
Part 2 - Interfacing To Broadcast

Tue, Mar 18, 1997
Radio Automation

Wed, Apr 16, 1997
Elections and NAB Review

Thu, May 22, 1997
ATM Technology

Tue, Jun 17, 1997
Facility Tour - TBA

Sat, Jul 26, 1997
Annual Family Picnic

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SEPTEMBER BUSINESS MEETING MINUTES

Submitted by Neal McLain, Secretary

Chapter 24 of the Society of Broadcast Engineers met on Tuesday, September 24, 1996, at the studios of WHA-TV, in Madison. There were 20 persons in attendance, 15 of whom were certified. The meeting was chaired by Chapter 24 Chairman Paul Stoffel. Call to order: 7:22 pm.

The minutes of the August meeting were approved as published in the September Newsletter.

Treasurer’s Report (reported by Stan Scharch, Treasurer): the chapter balance is in the black.

Newsletter Editor’s Report (reported Mike Norton): The deadline for the October Newsletter is midnight 10/11/96; the folding party is 5:30 pm 10/16/96 at WKOW-TV.

Sustaining Membership Report (reported by Fred Sperry): Five renewals have been received: Broadcast Communications, Comark, CTI, Norlight, and WMSN. The Chapter now has 22 sustaining members.

Programming Committee (reported by Steve Zimmerman, Paul Stoffel, and Denise Maney): The October Chapter meeting will take place during the annual Broadcasters’ Clinic; it will feature proposed EAS Local Plans. Future meetings include Newstar Edit System (November 20) and Holiday Dinner Party (December 17).

Special Events (reported by Kevin Ruppert): The chapter will sponsor a booth at the Broadcasters’ Clinic. Volunteers are needed to staff the booth and provide cookies.

Certification and Education (reported by Jim Hermanson): The next local examination period will be November 8-18; application deadline is September 26. One member has applied for recertification.

Frequency Coordination Report (reported by Tom Smith): Only one request has been received for the current football season. All stations are requested to submit coordination requests related to the forthcoming national election as soon as possible.

Special Events: no report.

National Liaison Report (reported by Leonard Charles and Paul Stoffel):
(1) A third EAS-equipment manufacturer has received FCC type acceptance: Holly Anne Corporation, 888-432-7463;
(2) The forthcoming World Media Expo in Los Angeles will be the last: the NAB has decided to buy out its contract with SBE (as well as other non-NAB participants);
(3) National Election results were announced.

Old business: none.

New business: Fred Sperry announced that the Chapter will be sponsoring a scholarship good for two day’s attendance at the Broadcasters’ Clinic.

The business meeting was adjourned at 7:45 pm. The program featured:
(1) A presentation by Jim Engeseth and Paul Stoffel about the proposed EAS Local Plan;
(2) A video report, produced by the PBS engineering department, about the pending DTV rules.

Submitted by Neal McLain, Secretary

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LOTS HAPPENING AT BROADCAST CLINIC

Here's reminder that the 42nd annual BROADCASTERS CLINIC 1996 will be held October 29-31 at the Ramada Inn I-90 Capital Conference Center in Madison. Radio presentations will be featured on Tuesday the 29th. Joint presentations will be held on the 30th. TV presentations will be given on the 31st. The cost is $135 for any two days, or $165 for all three. Continuing Education Units (CEUs) will be provided by the UW. To register by phone, call the Wisconsin Broadcasters Association (WBA) at 608-255-2600. Don’t forget to participate in the Chapter 24 Cookie Bake-Off by baking a batch of cookies! Then, stop by the SBE booth and vote for your favorite cookie. Also, contact Kevin Ruppert if you can help staff the booth during exhibit hours. Wednesday, October 30th, Chapter 24 is hosting the evening program titled “Local EAS Case Studies.” A Midwest SBE Chapters' Meeting will be held at 7:10 PM, followed by the program at 7:30. The evening’s agenda includes the following:

• 7:10 Chapters’ meeting
• 7:30 Program introductions
• 7:40 National EAS update from the FCC’s Frank Lucia (via the phone)
• 7:50 State EAS update from Gary Timm, Wisconsin SECC Chair
• 8:00 Local Plan Case Studies and Q&A.

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CHAPTER 24 BAKE OFF!!

By Kevin Ruppert

Just a reminder that we will be holding the first annual Chapter 24 Bake Off during the Broadcaster’s Clinic. Several people have already signed up. If you choose to participate, just bring your home made cookies to the Chapter 24 booth at the equipment exhibits during the Clinic. There will be a ballot box for people to use to vote for their favorite. The Special Events Committee has selected a special prize for the winner of the Bake Off.

Even if you do not enter, come to our booth to sample the cookies. See you there!

By Kevin Ruppert

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HOLIDAY PARTY PLANNED

The SBE Holiday Dinner Party will be December 17, 1996 at the Concourse Hotel. Warm up and Happy Hour will begin at 5:30 PM Dinner at 6:30. (Dutch Treat)

More details and menu selections will follow in this same space. Discounted underground parking will be arranged. Please send your RSVP no later than Nov. 8 by 3 PM to Denise Maney 608-277-8001 phone or Fax or send e-mail to Sloop26@aol.com

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TCI COMPLETES FIBER INTERCONNECT
SHUTS DOWN MICROWAVE STATION

TCI has completed the construction of a fiber-optic network to interconnect its Madison-area franchises. Microwave station WSA-43, which formerly transmitted programming to nine receive sites, has been shut down.

WSA-43 had been transmitting since the mid ’70s. It was first placed in service in 1973 when Complete Channel TV, Inc. (then 25% owned by TCI) originally built the Madison system. The first transmitter, a Hughes Microwave AML-132, transmitted 13 signals to three receive sites, all within Madison: 131 West Wilson Street, in downtown Madison; the Oscar Mayer processing plant on the northeast side; and a 100-foot tower near Pflaum Road and Stoughton Road on the southeast side.

Over the years, the transmitter was upgraded many times, as new channels and new paths were added. In 1979, a path to Deforest was added; in 1980, the entire transmitter was replaced with a Hughes AML-141 to provide higher output power and increased channel capacity.

In 1984, Complete Channel TV purchased the former Viking Media systems serving Cambridge, Deerfield, Marshall, McFarland, Monona, Oregon, Stoughton, and Sun Prairie. In the wake of that purchase, WSA-43 was expanded once again. New paths were added to Cambridge, Cross Plains, Oregon, Stoughton, and Sun Prairie, bringing the total number of paths to nine. About the same time, channel capacity was expanded to 36 video channels.

In the years since, several more channels have been added, but the number of paths has remained the same. WSA-43 was permanently shut down at 11:30 am, September 10, 1996, by TCI technician Will Elmendorf.

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EAS UPDATE

By Leonard Charles

The EAS certified manufactures count it now up to four. HollyAnne Corporation and Gorman-Redlich are the latest manufacturers to join TFT and SAGE in acquiring certification. Each of the four designs have distinctively different characteristics giving broadcasters an opportunity to match an EAS unit to their physical and budgetary needs.

The SBE’s Internet Home Page provides links to available manufacturers sites for detailed descriptions of each unit.

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LOCAL LEGALS

Compiled by Tom Smith

PROPOSED

WMSN-TV, Channel 47, Madison, WI. Sullivan Broadcasting License Corp. seeks to change ERP to 5,000 kw and antenna height to 451 meters. Filed July 5, 1996

WJNW-TV, Channel 57, Janesville, WI. Harish Puri seeks to change ERP to 3,980 kw , antenna height to 342 meters and transmitter location to 5.6 kilometers NNW of Footville, WI. Filed August 21, 1996

New TV Station, Channel 45, Richland Center, WI. Fant Broadcast Development LLC (Anthony J. Fant, 98% owner) seeks channel 45 with ERP of 5,000 kw with antenna at 306 meters. Transmitter location on Tower Rd, 1.3 km west of Freedom Road, 7.2 kilometer south of the village of North Freedom, WI. Fant Development has applications to construct 4 new TV stations. Anthony Fant owns all or part of 7 TV stations and has applications to construct 4 more TV stations. He is in the process of selling 3 of the TV stations and an AM-FM station in Duluth. Filed July 22, 1996. (Compiled from BROADCASTING and CABLE)

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SBE BOARD OF DIRECTORS MEETING

By Leonard Charles

The Fall 1996 SBE Board meeting was held in Los Angeles during the World Media Expo (WME). The Board heard national committee reports and took action on a few issues.

The reports included many enhancements to the services provided to the membership. The SBE World Wide Web home page is being utilized heavily by members and other www browsers. Since last July, there have been 34,300 visits to the site. Recent Home Page enhancements include a search engine for members, the ability to request membership and certification applications on line, and the addition of the SBE store merchandise listing and order form on line.

The EAS Committee utilizes the SBE www site to update the membership with the most recent details of the new system. Currently there are four manufacturers certified to sell EAS equipment to broadcasters. These four manufacturers are featured in the most recent EAS Committee page updates along with hot links to available manufacturer’s www sites. The committee was asked about concerns that TV station’s weather alert crawls are being covered by Closed Captioning text when that receiver feature is activated by hearing impaired individuals. The result is those viewers being unaware of the alert situation. Though not directly an EAS issue, the committee was asked by the board to work with the Industry Relations Committee to formulate suggestions on how this information may co-exist on the video screen.

The FCC Liaison Committee is watching closely a petition filed with the FCC on June 10th by Wireless Fixed Access Local Loop Services. In it, the company requests 1990 through 2110 MHz and 2450 through 2483.5 MHz of the 2GHZ band for wireless connection to residences and businesses to provide phone and Internet service. If approved, the action would preclude 2GHZ spectrum now available on a case by case spill over basis. The committee has also filed a formal letter in support of a petition to codify the existing use of the 2GHZ band by NASA in hopes of protecting the band from further pillaging.

The Futures committee released its report resulting from the strategic planning session earlier this year. It sited a very sound organization and suggested only a few minor tweaks and expansions. The first of those to be acted upon came from the Certification Committee. It was announced that the committee will investigate expanding the levels of certification to be offered. The first of those expansions will be into the areas of audio and video engineer. The certification requirements will be similar to those of Broadcast Engineer levels without the RF and regulatory questions. Since people in these careers have no way to be certified in their chosen field, the SBE hopes to fill a niche. Additionally, the board suggested that the Certification Committee draft a frequency coordination handbook as a guide to new or existing coordinators.

SBE President Terry Baun announced that the NAB has decided to discontinue the World Media Expo with the 1996 event being the last. This move was characterized by Baun as unfortunate because the show had been working out very well for the SBE. As part of a re-negotiated agreement, the SBE will continue to assist in the planning and execution of the Engineering Conference at the Spring NAB show and has proposed moving the day-long Ennes workshops to Las Vegas adjacent to that show. In addition, the SBE will retain booth space at the Spring event. Since the WME was the site of the SBE fall conference and associated meetings, the board needed to act on a direction to pursue for 1997. After considerable debate of possible options, a resolution was passed to take the Fall board meeting, membership meeting, and awards banquet to a different SBE regional show each year. Along with that will come the Ennes Foundation’s assistance for programming that regional show’s educational seminars.

The Board was informed of the resignation of Peggy Hall from the National Office staff. Peggy has served many years as the organization’s membership administrator as well as resident computer software expert. Peggy is leaving to accompany her husband in a career move to Florida.

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AMATEUR RADIO NEWS

By Tom Weeden, WJ9H

• In response to what he says are nearly 45,000 complaints to the FCC every year, Wisconsin Senator Russ Feingold has introduced Senate bill S.2025 which would give state, county and local governments the ability to prohibit the use of unauthorized CB transmitters or amplifiers in an attempt to regulate radio frequency interference (RFI) to consumer electronics. CB interference to television reception and telephones in Beloit prompted the city council there to unanimously approve a measure that allows police to cite CB operators who operate at higher-than-FCC-authorized power.

Although the Feingold legislation would not affect amateur radio operators, the “Badger State Smoke Signals” newspaper calls on Feingold to withdraw the bill, saying it would set a very risky precedent if passed. “Smoke Signals” says the proposed law “delegates authority to any governmental entity without any engineering or other qualification requirements of personnel enforcing CB over-power operators: Radio signals do not respect city limits, county lines, state borders...radio is interstate by its very nature.”

• A pair of amateurs who work for Wegener Communications, maker of digital video transmission systems for the broadcast industry, made what could be the first two-way digital amateur television contact in North America. The contact between Mike McCombs, KM4YW, and Ned Mountain, WC4X, was made with 32 mW over a 3-mile obstructed path on 1290 MHz.

Equipment used was a Wegener DVT2000 MPEG-2 encoder/modulator, a Wegener DVR2000 MPEG-2 IRD, and an L-band line amplifier. In the spirit of amateur radio, the antennas were made from coffee cans.

(Excerpted from August & September Badger State Smoke Signals and October QST)

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DTV INFORMATION

By Tom Smith

With the November 22nd comment due date on the Sixth Notice of Inquiry on DTV coming up, now is the time to gather up information. Even if you are not going to file comments, anyone in the TV industry should gather up as much information as possible, as the FCC’s decision on this docket will determine the future of television.

There are a number of good resources, with the Internet having the greatest amount of information. The FCC web site (www.fcc.gov) has a DTV page which has links to download the text for the 4th, 5th, and the current 6th Notice of Inquiries. There is also the text of the En Banc hearings on DTV held last December. Speeches of the Chairman and various commissioners of the FCC are also available.

Another good resource is Doug Lung’s RF web page (www.transmitter.com). Doug Lung writes a weekly column on his web page (RF Currents) along with a number of other features pertaining to TV and RF subjects. Many of his web pages contain links to other websites containing DTV information. Doug also writes a column for TV TECHNOLOGY, of which he has back issues available on his web page.

Also TV TECHNOLOGY has been running a series of articles by Charles Rhodes on DTV allocation and interference issues. These articles give a good background on how the proposed FCC table for DTV allocations will work. Mr. Rhodes will be speaking Wednesday afternoon at Broadcasters Clinic 1996.

Another good Internet resource is the Broadcast Net (www.broadcast.net) which has links to the FCC, broadcast organizations and manufacturers.

Also, check Television Broadcast and Broadcast Engineering for articles on the technology and Broadcasting and Cable, Electronic Media and newspapers such as the NY Times for articles on the politics of DTV and spectrum auctions.

Having and keeping up with all the information concerning the changes in technology and politics will help you understand, anticipate, and survive the transition to DTV.

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TELECOM INDUSTRY NEWS

By Neal McLain

PSC CONDUCTS HEARINGS ON AREA CODE 414 RELIEF PLAN

The Wisconsin Public Service Commission continues its efforts the finalize a relief plan for Area Code 414. During the past month, it conducted public hearings in four cities: Green Bay, Oshkosh, Waukesha, and Watertown. Each of these hearings addressed the same familiar issues: Split or overlay? And if a split, where?

Each hearing has followed a fixed format: opening remarks by a PSC staff member; presentations by industry representatives; and questions and comments from the audience with responses by industry representatives. The questions and comments will become part of the record on which the Commission will base its decision.

I attended the hearing in Watertown. At this hearing, the industry representatives were William Brown, State Regulatory Manager for BellSouth Cellular (the Cellular One franchisee, arguing for an overlay) and Michael Klasen, from Ameritech (arguing for a split).

According to Klasen, the Industry Team studying the issue has tentatively agreed to a split line (see map). This line follows telephone exchange boundaries; since exchange boundaries bear no relationship to any other geopolitical boundaries, it inevitably splits some geopolitical entities: townships, counties, and school districts. Klasen asserted, however, that no incorporated village or city would be split.

I counted a total of 21 people in attendance at the hearing. That number includes PSC personnel, the two industry representatives, the court reporter, and me, wearing my newsletter-reporter hat. So that leaves, at most, 15 members of the general public. Of these, only three spoke, and only two of them addressed the basic issues.

And address them they did: they both objected strenuously to the split plan, at least it if follows the line proposed by the Industry Team.

Joseph St. Thomas, Chairman of the Town of Oconomowoc, stated that the proposed split line would split the township. He objected to the argument that “incorporated cities and villages will not be split”, asserting that since the Town of Ocomonowoc is an “urban town”, it should be accorded the same rights as a city or village. Continuing this argument, he asserted that, if a split is inevitable, it should follow county lines. Klasen, the Ameritech representative, rebutted this argument by noting that a county-line split line would split many incorporated cities, including the City of Watertown.

Thomas Sloan, of Tom Sloan Associates, argued forcefully in favor of an overlay. Mr. Sloan owns a private employment agency serving the food service industry; his clients include food producers, food distributors, and food-industry personnel nationwide. He asserted that the split plan would force him to change area codes, resulting in possible loss of business.

So there you have it, the two basic arguments against a split: “I don’t want to change my phone number” and “I don’t want the split line in my backyard.”

Will these arguments affect the PSC’s decision? I don’t know what happened at the other three hearings, but if the results were similar to what I heard in Watertown, I wouldn’t be surprised if Wisconsin turns out to be the fourth state to approve an overlay.

AREA CODE 312 SPLITS AGAIN

Meanwhile, the Illinois Commerce Commission’s decisions favoring splits over overlays are still being felt in the Chicago area: area code 312 is splitting again. Downtown Chicago retains 312; the rest of the city gets a new code, 773. Permissive dialing began October 12; mandatory dialing begins January 11, 1997. This brings to five the total number of area codes carved out of what was one area code just eight years ago.

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BROADBAND NETWORKS PART 7
AMPLIFIER NOISE PERFORMANCE

By Neal McLain

(Editor's Note: any graphics accompanying this article are not included in this electronic version. Contact Chapter 24 for a hard copy if desired)

This is the seventh in a series of articles about coaxial broadband networks. In this article, we’ll begin a discussion about broadband amplifiers.

WHY HAVE AMPLIFIERS?

Broadband distribution systems use amplifiers for two reasons: to overcome the signal attenuation occurring in the cable, and to provide sufficient signal level to drive each tap port at the specified DTL.

The placement of amplifiers within the distribution system is a result of several factors: amplifier performance characteristics (noise and distortion), cascade depth (number of amplifiers connected end-to-end), signal loading (number of channels), cable attenuation (loss per specified distance), physical distances, specified performance objectives, and cost. All of these factors interact; the system designer must consider them all.

Let’s begin with a discussion of performance objectives.

PERFORMANCE OBJECTIVES

Like all amplifiers, broadband amplifiers do three things: they amplify the signal, they add noise, and they add distortion. Accordingly, we must specify the performance objectives of an amplifier in terms of gain, noise level, and distortion level. We define these terms as follows:

• Noise means the thermal, or Gaussian, noise generated within the amplifier circuitry. Noise places a lower limit, or “floor,” under the allowable range of signal levels. Noise is worst when the signal levels are lowest; this condition occurs at the amplifier input. Thus, noise establishes the minimum input signal level.

• Distortion is a catchall term used to identify a variety of signal impairments which vary as a function of signal level. Distortion places an upper limit, or “ceiling,” above the allowable range of signal levels. Because distortion is a function of signal level, it is worst when the signal levels are highest; this condition occurs at the amplifier output. Thus, distortion establishes the maximum output signal level.

• Gain means the gain in signal level of one amplifier; it equals output level minus input level.

As we noted in Part 1 of this series (1), the so-called trunk portion of the distribution system is optimized to carry signals over long distances. To maximize distance, we must cascade amplifiers, or place them end-to-end. As a general rule, amplifiers are installed at intervals so that the gain of each amplifier equals the attenuation in the previous span of cable. All cable spans have equal attenuation and all amplifiers have equal gain, so the net gain is zero.

But this leads to a further complication: noise and distortion are cumulative. If amplifiers are cascaded, noise and distortion accumulate.

Thus, we face a paradox: we can improve the performance of an individual amplifier by increasing its input level and/or by reducing its output level. But doing so reduces its gain; therefore, more amplifiers must be cascaded in order to cover distance. But increasing the number of cascaded amplifiers results in the accumulation of more noise and distortion, thereby negating (at least partially) our original attempt to improve performance.

Similarly, if we attempt to improve the performance of the cascade by increasing amplifier gain (reducing input levels and/or increasing output levels), we can reduce the number of amplifiers in the cascade. But now, each amplifier contributes more noise and distortion, again negating our original objective.

All of this leads to a question: is there some optimum tradeoff among the various parameters of noise, distortion, gain, and cascade depth which yields maximum distance?

Indeed there is. But to answer this question in detail, we must start at the bottom (the noise floor), and work up.

NOISE

We have defined noise as the thermal noise generated within the amplifier circuitry. Noise places a floor under the allowable range of signal levels.

Where is this floor? In other words, what is the level (in volts or dBmV) of the noise floor?

Let’s start with the input circuitry of a theoretically ideal broadband amplifier. The equivalent circuit can be represented by a 75-ohm resistor:

This is an “ideal” resistor: it complies with all of the theoretical laws of molecular physics.

Now let’s measure the voltage across this resistor: For this measurement, we use an “ideal” voltmeter: it measures any voltage from zero to infinity, it places no load across the resistor, it adds no noise of its own, and it gives us the results in volts and dBmV. We also insert a 4-MHz bandpass filter to restrict the bandwidth to 4 MHz, the bandwidth of an NTSC video signal.

Now let’s put the whole thing into a refrigerator and reduce the temperature down to -273° Celsius. We recognize that temperature as Absolute Zero, the temperature at which all molecular motion ceases, and everything -- even the oxygen and nitrogen in the air -- freezes solid.

Under these conditions, what do we measure on our ideal voltmeter?

We measure:

Volts = 0.0000000

dBmV = Minus Infinity.

Well, this shouldn’t be a surprise: the electrons in the resistor are frozen solid.

Now let’s open the refrigerator and let the resistor warm up to about 20°C, or 68°F:

Now what do we measure? We measure (approximately):

Volts = 0.0000011, or about one microvolt.

dBmV = -59.

Where did this voltage come from? It’s generated by the thermal motion of the electrons within the materials which make up the resistor.

It’s possible to derive this value theoretically, although doing so is way beyond the scope of this article. A full derivation may be found in a number of engineering reference books (2, 3).

This measurement tells us that the noise floor at the input of a theoretically ideal 75-ohm amplifier, across a 4-MHz bandwidth, at room temperature, is -59 dBmV. Remember that number.

Now, let’s use our same test setup to look at a real-world amplifier.

Here, we measure the noise floor at the output of the amplifier. An attenuator is inserted after the amplifier to simulate a span of cable; as noted earlier, cable loss equals amplifier gain, so the net gain is zero.

Now what do we measure? We measure (approximately):

Volts = 0.0000025.

dBmV = -52.

Now what’s happening? The real-world amplifier appears to generate more noise than the ideal 75-ohm resistor.

Indeed it does: real-world amplifiers generate more noise than ideal amplifiers. The difference between the ideal and the actual noise floor of a given amplifier is called the noise figure. In this example, the noise floor falls at -52 dBmV, so the noise figure is 7 dB.

The best real-world broadband amplifiers available today have a specified noise figure of around 7 db. This figure is probably about as good as it’s ever going to be: it hasn’t changed much in the past twenty years, in spite of fierce competition among amplifier manufacturers.

CARRIER TO NOISE RATIO

The carrier-to-noise (C/N) ratio is the difference, in decibels, between a desired signal and the noise floor. C/N is a crucial figure of merit in the design of broadband networks: poor C/N causes signal impairments such as visible noise (“snow”) in video images, audible hiss in audio signals, and degraded data-transmission performance. The minimum acceptable C/N for vestigial-sideband NTSC video signals is somewhere between 40 and 50 dB, depending on which research paper you read; the FCC spec for cable television systems is 43 dB (4).

The C/N of an amplifier is something we can control. Although we can’t control the noise floor, we can control C/N by proper specification of the input signal level. Two examples:

• Example #1: Given a broadband amplifier with a 7-dB noise figure, determine the minimum input signal level required to achieve a C/N of 43 dB. First, calculate the noise floor: -59 + 7 = -52 dBmV. For a C/N of 43 dB, the input signal must be 43 dB above the noise floor. Therefore: -52 + 43 = -9 dBmV.

• Example #2: Given a 1950’s-vintage television set with 20-dB noise figure, determine the minimum signal input level required to achieve a C/N of 40 dB. Again, first calculate the noise floor: -59 + 20 = -39 dBmV. For a C/N of 40 dB, the input signal must be 40 dB above the noise floor. Therefore: -39 + 40 = +1 dBmV.

Back in Part 2 of this series (5), we noted that one millivolt was selected as the zero reference for the dBmV scale because it represents the signal level at which a good-quality 1950’s-vintage television receiver could recover a snow-free picture. Example #2 shows us the arithmetic reason for this choice.

CASCADED AMPLIFIERS

Now, let’s look at what happens when we cascade amplifiers. We’ll start with the amplifier we discussed earlier:

Since the noise figure is 7 db, the measured noise floor is -52 dBmV.

Now let’s add a second amplifier in cascade:

We now measure the noise floor at -49 dBmV.

Notice what happens: with two amplifiers in cascade, the noise floor increases 3 dB. The reason: we now have two noise sources, each contributing a level of -52 dBmV. When we combine these sources, we double the power. We know from the definition of a decibel that if we double the power, the level increases 3 dB. We also know that if the power increases 3 dB, voltage (which is what we’re measuring with our ideal voltmeter) also increases 3 dB(6).

If we add more amplifiers to our cascade, the noise floor goes up another 3 dB each time we double the number of amplifiers (Figure 1).

We can now state a general rule: in a cascade of identical amplifiers, the noise floor at the output of the last amplifier increases 3 dB when the number of amplifiers in the cascade is doubled.

Next month, we’ll see what happens to distortion when we cascade amplifiers.

(1) “Broadband Networks” Part 1. SBE Chapter 24 Newsletter, April, 1996, p. 4.

(2) K. Blair Benson. Television Engineering Handbook, revised edition. New York: McGraw Hill, Inc., 1992, p. 13.32.

(3) Wilbur R. Vincent. “Radio Noise and Interference.” Reference Data for Engineers: Radio, Electronics, Computer, and Communications. Ed. Edward C. Jordan. Indianapolis: Howard W. Sams & Company, 1988, pp. 34-10 – 34-12.

(4) Code of Federal Regulations 47 CFR 76.605(a)(8)(iii).

(5) “Broadband Networks” Part 2. SBE Chapter 24 Newsletter, May, 1996, p. 4.

(6) Ibid, p. 5.

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CHAPTER 24 SUSTAINING MEMBERS

Recent Renewals:

Comark Communications
WMSN-TV 47

Thanks to all our Sustaining Members:

Broadcast Communications
CCA Electronics
CTI
Clark Wire and Cable
Emmons Associates
Fuji Film
I&I
Harris Corporation
Maney Logic
Norlight Telecommunications
Panasonic Broadcast
Richardson Electronics
Roscor Wisconsin
Scharch Electronics
Sony Broadcast
Skyline Communications
Teleport Minnesota
Video Images
WISC-TV 3
WKOW-TV 27
WMTV-TV 15