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:
Contents for this Newsletter
About this Newsletter
Upcoming Meeting Schedule
October Meeting Minutes
Telecom Industry News
Amateur Radio News
DTV Comment Deadline Nears
BroadBand Networks; Part 8
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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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Wednesday November 20, 1996
TEKTRONIX - EDIT STAR - WKOW TV TOUR
Dinner and Meeting at 5:30pm at J.T. Whitney's
Program at WKOW-TV
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Tentative Program Subjects
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
Wed, Apr 16, 1997
Elections and NAB Review
Thu, May 22, 1997
Tue, Jun 17, 1997
Facility Tour - TBA
Sat, Jul 26, 1997
Annual Family Picnic
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OCTOBER BUSINESS MEETING MINUTES
Submitted by Neal McLain, Secretary
October Business Meeting Minutes
Chapter 24 of the Society of Broadcast Engineers met on Wednesday October 30, 1996 as part of an Upper Midwest Regional SBE meeting. This meeting was
being conducted in conjunction with the 1996 Broadcast Clinic held at the Ramada Inn Capital Conference Center in Madison, Wisconsin. Fifty-nine people were
present for the meeting and subsequent program.
The meeting was called to order at 7:13 PM by Chapter 24 Chair Paul Stoffel. Paul reminded Chapter 24 members to sign-up for the Chapter's holiday party
coming up on December 17th.
Paul then announced several National awards Chapter 24 recently received, and presented them to those Chapter members involved with each award. The
following awards were presented to the following individuals: Best Technical Article to Neal McLain, Best Chapter Newsletter to Mark Croom, Best Student
Paper or Article to Vicki Way and James Stellpflug, and Broadcast Engineer of the Year to Leonard Charles. Don Borchert then presented an award to Leonard
from the Broadcast Clinic acknowledging Leonard's hard work during the past year as well as his reception of the SBE Broadcast Engineer of the Year award.
Leonard then made a speech regarding these awards.
Terry Baun then spoke on a variety of issues regarding the SBE at a National level.
The meeting was then adjourned. Paul Stoffel then presented as well as moderated the evenings program entitled EAS Local Plan Case Studies. Chapter 24 hosted
this program involving the other SBE Chapters throughout Wisconsin as well as the SBE Chapter in Rockford, IL.
Minutes recorded by Fred Sperry, Vice Chair
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HOLIDAY PARTY PLANNED
By Denise Maney
Make Plans to Attend the SBE Chapter 24 Holiday Party
FUN FUN FUN (Oh, and great FOOD too!)
Concourse Hotel on December 17, 1996
Social Hour at Concourse bar at 5:30pm
Dinner promptly at 6:30pm
Reduced parking passes for under ground parking at the Concourse will be available the night of party. Please RSVP with your name and your guests name to Denise Maney no later than November 20th. (telephone/fax 277-8001 or e-mail to email@example.com)
After Dinner Fun: Just for Fun... We are asking the spouses and guests to share any funny or informative stories about their engineer. ie: Has your favorite engineer ever... while working on the transmitter... on-air oops... etc. Let's all have fun!
Sustaining members and guests are cordially invited to attend our Holiday gathering (Dutch Treat). Please plan on attending this enjoyable event.
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By Tom Smith
MM Docket No. 96-197, FCC 96-381 Waiver of the Newspaper/Broadcast Cross-Ownership Restriction
The FCC has issued a Notice of Inquiry to seek comments on whether it shall adopt new policies concerning requests for waivers of the newspaper/broadcast cross ownership restrictions.
In 1975 the FCC adopted rules which prohibited anyone from creating a cross-ownership of a newspaper and a broadcast station in the same market. The reason for this rule was to prevent undo concentration of the media by one owner in a market. This rule prohibited any new combinations and called for the break-up of any existing combinations when they were sold. Existing combinations were grandfathered and could remain intact if they remained under the ownership that existed when the rule took effect. The rules allowed for waivers under certain circumstances which were for mainly dire financial reasons. Waivers were allowed under the rules in cases when one entity would go bankrupt, the inability to sell one of the media or if due to the lack of buyers the price for one or both would be depressed below fair value. The FCC has only issued 2 permanent waivers, one was in New York City and one was in Chicago
The FCC is seeking comment on the existing criteria and how it should base these waivers on these economic issues. It is also seeking comment on the definition of what is a market and how to determine impact on a market. The includes how to figure the number of broadcasters and newspapers in a market by their location and overlap to each other. The FCC is also interested in what percentage of revenue by the proposed new combinations would create dominance of the market.
The FCC is prohibited by Congress from repealing the rule but is allowed to write rules that grant waivers.
The FCC adopted this notice on May 9, 1996 and released it on May 20, 1996. It was published in the FEDERAL REGISTER on October 15, 1996, pages 53,694 through 53,698. Comments are due on December 9, 1996 and replies are due on January 8, 1997.
(From the FEDERAL REGISTER)
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TELECOM INDUSTRY NEWS
By Neal McLain
AREA CODE 414 RELIEF PLAN: NO DECISION YET
The Public Service Commission has postponed, once again, the decision it must eventually make concerning a relief plan for area code 414. As noted last month, the Commission is considering two plans: overlay and split. The overlay plan is favored by GTE and BellSouth Cellular (the Cellular One franchisee); the split plan is favored by Ameritech and several long-distance companies.
According to Albert F. Schams, a communications analyst with the Commission's office in Madison, the Commission is now taking the following course of action: it has asked the Commission staff to prepare a report, called the "Staff Draft," to be circulated for public comment for a ten-day period. The comments will become part of the record on which the Commission will base its decision.
Schams expects the Staff Draft to be issued "any day now." Approximately 350 copies will be printed; copies will be distributed to all persons who request it, and to all who attended one of the public hearings.
In the meantime, 414 continues to grow rapidly. As of October 1, 1996, 655 (out of a possible 792) central office codes were in service; six of these had been assigned during the previous three months. (1)
By contrast: in 608, only 293 central office codes are in service; in 715, 337 are in service.
CABLE TV ENTRY INTO TELEPHONY: THE FIRST FCC DECISION
The FCC has issued a preliminary decision in a case in which a municipal government refused to permit a cable television company to offer telephone service. The case involves two Kansas cities which had refused to issue telecommunications franchises to the local cable television operator. The FCC remanded the case to the cities for reconsideration.
This is the first case of its kind to come before the FCC since the passage of the Telecommunications Act of 1996. The Act encourages competition in the telecommunications marketplace, and directs the FCC to remove barriers which bar the entry of new competitors. The Act gives the FCC the authority to preempt state or local regulations which conflict with this goal.
In the case at hand, two small cities in northwestern Kansas, Hill City and Bogue, had refused to issue telecommunications franchises to Classic Telephone, a company affiliated with the local cable television operator Classic Cable. The reasons for the refusals are somewhat muddled (Bogue didn't even put its initial decision in writing); however, both decisions seem to be rooted in the "antagonistic relationships" which existed between Classic Cable and the cities.
The FCC's decision was firm and broad: it asserted that, antagonistic relationships notwithstanding, the goal of the Telecommunications Act is to allow the marketplace not government regulators to determine who will provide telecommunications services. As noted in a memorandum issued by the law firm which represented Classic, "This decision stands as a message to municipal authorities that telecommunications franchising can neither protect local telephone monopolies nor limit the number of service providers. The limited authority for municipalities to manage rights-of-way within their jurisdiction does not allow for the creation of barriers to entry, nor can any delegated authority to protect consumers stand as the basis for an outright denial of a franchise." (2)
As it happens, this wasn't a particularly good test case for the cable industry because of the rather unusual provision of Kansas law which permits cities to issue telecommunications franchises in the first place. In most states, telecommunications franchises are issued at the state level.
A more relevant case is still pending before the FCC: the City of Troy, Michigan has refused to issue certain construction permits to the local cable television operator, TCI. TCI needs the permits in order to construct a fiber optic network which, it claims, is part of an upgrade to its video distribution network. The city claims that, since the fiber network also can be used to support telecommunications (telephone and internet access) services, TCI must first obtain a "telecommunications franchise." TCI claims that, under Michigan law, it does not need a telecommunications franchise, and that, in any case, the city doesn't have the right to require such a franchise in the first place. TCI has appealed the issue to the FCC; the FCC has not yet issued a decision.
(1) John Cropper. "NPA-NXX Data for 4Q96." TELECOM DIGEST, Volume 16, #517, September 29, 1996.
(2) Cole, Raywid, & Braverman, L.L.P. Memorandum: Commission Preempts Two Municipal Telecommunications Franchise Denials. Washington, DC, October 1, 1996.
PSC STAFF RECOMMENDS SPLIT IN AREA CODE 414
Since this issue of Chapter 24 Newsletter went to press, the Wisconsin Public Service Commission has issued the Area Code 414 "Staff Draft" for public comment. The Staff Draft appears to be a tentative draft of the Commission's final order. Assuming that no significant changes result from the public comments, it will probably become the final order.
The Staff Draft recommends a split instead of an overlay. The split line is approximately as shown in the map in last month's Newsletter, with one exception: the split line is adjusted northerly so that Washington and Ozaukee Counties "remain whole and not split between the two area codes." The actual split line still puts part of Ozaukee County in the new code, but it appears to be as close to the county line as possible.
Comments on the Staff Draft are due November 19, 1996 at 4:30pm. The Commission is expected to reach a final decision in December.
A copy of the entire Staff Draft (44 pages) or a summary (8 pages) can be obtained from the PSC office at 610 North Whitney Way, Madison.
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AMATEUR RADIO NEWS
By Tom Weeden, WJ9H
--- The November issue of QST Magazine reports that amateur radio operators have given so much attention to Senate Bill 2025, introduced by Wisconsin Senator Russ Feingold, that the Senator's office contacted the American Radio Relay League (ARRL) for help in making the bill more acceptable.
In its original form, the bill carved out an exception to the current total federal preemption of telecommunication matters by permitting state and local officials to go after CB operators using illegal linear amplifiers and other unauthorized equipment thought to be responsible for interference in consumer devices. Other services were exempted.
The ARRL told Feingold's office that if the bill passed, local law enforcement officials, without proper equipment or any technical knowledge, might find themselves in the position of trying to differentiate between illegal CB operators and amateurs operating legally. And, the bill did not address the actual cause of many interference problems: poorly designed consumer electronic devices.
ARRL attended a Senate Commerce Committee meeting along with a member of Feingold's staff and FCC representatives. The Senator's staff agreed to work with the ARRL to craft a bill that could help address the problems of local illegal CB operation without fundamentally altering the federal preemption concept or jeopardizing amateur operations.
--- The League has also petitioned the FCC to reconsider and reverse portions of the Commission's Report and Order that imposes RF-emission safety standards on amateur radio, effective January 1, 1997. ARRL has asked the FCC to increase the flat 50 watt threshold to 150 watts at a distance of 10 meters from all parts of the antenna (for HF operation), or to scale the evaluation threshold by frequency to match the maximum permissible exposure limits, directly corresponding to the way the regulations scale the levels with frequency.
(Excerpted from November 1996 "QST")
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By Tom Smith
With last hour objections from the computer industry and parts of the movie industry, it looks like the final rules setting the DTV standards by the FCC will be delayed. The Chairman of the FCC, Reed Hundt, and some of the commissioners have stated the they will not vote on a final standard until the broadcast industry and all other parties reach some kind of compromise on DTV standards. The Clinton administration is also supporting a compromise.
The computer industry is claiming that the standards are not compatible with those of the computer industry. There biggest compliant is that the proposed DTV standard allows for the use of non-squared pixels, 60 Hz vertical rate, and the use of interlaced scanning in some formats. The computer industry has also proposed a new compression system. Bill Gates met with Hundt last summer to voice his objections to the standards.
Some members of the film industry are objecting to the 16/9 picture ratio which they believe should be set at 2/1 for maximum wide screen display.
The cable industry is also voicing concerns over the use of VSB modulation versus the proposed cable QAM modulation method.
Their have been a number of heated articles in both computer and broadcast publications and on the internet. The lead story in the October 21st issue of BROADCASTING and CABLE was titled "Is The Digital TV Standard Going Down The Drain". With the new attacks, the broadcast industry is increasing its efforts to lobby for the Grand Alliance standard. Broadcasters are also questioning why the other industries are raising these questions at this time, as this proceeding has be going on since 1987. Some commentators have stated this debate has started to look a lot like the AM Stereo debacle.
(From TV TECHNOLOGY and BROADCASTING and CABLE)
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COMMENT DEADLINE NEAR
By Tom Smith
This is a reminder that the deadline for comments for the 6th Notice of Rulemaking on DTV is due on November 22, 1996. This notice concerns the allocation of DTV channels and will affect the coverage and future of all TV stations in one way or another.
Even if you are not making comments, find out what your trade organizations are doing and add your input. There is much information on this docket in the various trade journals and on the Internet.
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BROADBAND NETWORKS PART 8
AMPLIFIER DISTORTION PERFORMANCE
By Neal McLain
Electronic Editor's note: The diagrams and figures referred to in this article were not reproduced for this electronic version of the Chapter 24 newsletter. For a complete hard copy of the newsletter, contact the newsletter editor; Mike Norton.
This is the eighth in a series of articles about coaxial broadband networks. In this article, we'll continue with our discussion of broadband amplifiers.
NOISE VS. DISTORTION
In last month's article, we defined three new terms noise, distortion, and gain as follows:
Noise: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: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:The gain in signal level of one amplifier, equal to output level minus input level. In a trunk of cascaded amplifiers, amplifier gain equals the loss in the intervening spans of cable, so the net gain is zero.
We also stated a general rule about noise: 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.
In this article, we'll explore what happens to distortion when we cascade amplifiers.
TYPES OF DISTORTION
For the purpose of this discussion, we have defined distortion as "signal impairments which vary as a function of signal level." These impairments all arise from a single source: spurious signals generated by non-linearity's in the transistor circuitry used in the amplifiers.
Three types of distortion fall within this definition: second order, third-order, and cross-modulation. We will discuss each type separately.
This form of distortion arises from interaction between two carriers. If two carriers F1 and F2 are introduced into an amplifier, spurious signals are generated at F1+F2 and F1-F2. These signals appear at the output along with the original carriers; if one of these spurious signals falls within the passband of a desired signal, it can impair the desired signal.
This form of distortion also includes second harmonics generated by any single carrier.
These impairments are manifested in many ways, depending on the relative signal levels and the types of modulation involved. A common example familiar to every cable TV technician: a "thumbprint" pattern displayed in an NTSC video signal. This pattern is usually caused by a single interfering carrier.
A case in point: consider the interaction between the channel 3 and 4 visual carriers:
61.25 + 67.25 = 128.5 MHz.
This combination produces a spurious signal at 128.5 MHz, which falls 1.25 MHz above the visual carrier of cable channel 15, right in the middle to the video sidebands.
This form of distortion can be effectively suppressed by the use of push-pull amplifier circuitry. Indeed, virtually all broadband amplifiers manufactured within the past 20 years employ push-pull circuitry.
This form of distortion arises from interaction among three carriers. If three carriers F1, F2, and F3 are introduced into an amplifier, several additional spurious signals in the form F1±F2±F3 are generated internally and appear at the output along with the original carriers. Again, if one of these spurious signals falls within the passband of another desired signal, it can impair the desired signal.
Obviously, a great number of possible spurious signals can arise if several carriers are present: every combination of three carriers generates several different combinations of F1±F2±F3 products.
If the original carriers are incrementally related (as they are in a conventional cable television system), the spurious signals tend to fall in "clusters" surrounding the original carriers. Example: consider the combination of the channel 7, 8 and 9 visual carriers:
175.25 + 181.25 - 187.25 = 169.25 MHz
This combination produces a distortion product at 169.25 MHz, in cable channel 22. If the original three carriers are locked to a single master oscillator, the distortion product falls precisely on top of the visual carrier, effectively hiding it. But if the three carriers are generated by independent oscillators, the distortion product usually falls close to, but not precisely on, the visual carrier. If the oscillators drift, the distortion product wanders around.
Dozens of other XX.25 ± XX.25 ± XX.25 combinations also produce the same result, all clustered around the channel 22 visual carrier. The resulting clustering is sometimes called "Composite Triple Beat", or CTB.
In a cable television system, CTB manifests itself in an overall "grainy" or "wormy" pattern in the displayed picture. To an unpracticed eye, it's sometimes confused with noise; however, with experience, it's easy to distinguish the difference. Noise produces white spots ("snow") which are only one pixel in size, appear for only a single field, and are stationary. By contrast, CTB produces lighter (and darker) spots which tend to be larger than one pixel, are present for several successive fields, and sometimes appear to move ("wiggle") with respect to time.
Cross-modulation is a form of third-order distortion which arises from the interaction among the modulation sidebands of two or more carriers. The result: the signal modulating one carrier appears as modulation on one or more other carriers. In a cable television system, cross-mod appears as sync bars in the picture. It could be argued that cross-mod is not a separate form of distortion, but that it is instead simply a manifestation of third-order distortion. This is a valid argument; however, cross-mod is generally recognized as a separate form of distortion because of the distinctive sync-bar pattern which it produces.
SPECIFICATION OF DISTORTION LEVELS
Distortion levels, if known absolutely, can be specified in dBmV. However, distortion levels are usually specified in relation to the level of the desired signal carriers. Two terms are in common use:
dBc, defined as the level of the distortion with respect to the carriers. Assuming that distortion is lower than the carriers, dBc is a negative number.
C/D (carrier-to-distortion ratio), defined as the difference, in decibels, between the carriers and the distortion levels. C/D is an absolute value, expressed without sign.
In cable television systems, distortion is measured with respect to the visual carriers because visual carriers operate at higher levels than other carriers; thus, they produce the highest distortion levels (worst-case C/D ratio).
Distortion levels vary according to the following laws:
Second-order distortion varies according to a 1:1 law: for every 1-dB change in carrier levels, there is a corresponding 1-dB change in distortion levels relative to the carriers. Thus, if the level of the carriers is doubled (increased 3 dB), the C/D ratio degrades 3 dB.
Third-order (including cross-mod) distortion varies according to a 1:2 law: for every 1-dB change in carrier levels, there is a corresponding 2-dB change in distortion level relative to the carriers. Thus, if the level of the carriers is doubled (increased 3 dB), the C/D ratio degrades 6 dB.
All three forms of distortion are present in any distribution system; the dominant form depends on "channel loading"; i.e., the number of carriers present. Again using a cable television network as an example:
Second-order distortion is dominant in systems carrying a limited number of channels, usually no more than a dozen.
Cross-modulation is dominant in systems carrying up to about 30 channels.
CTB becomes dominant in systems carrying more than about 30 channels.
The minimum acceptable C/D ratio for vestigial-sideband NTSC video signals is somewhere between 50 to 60 dB. The FCC spec for cable television systems is 51 dB when measured with modulated carriers; (1) advocates of "good engineering practice" frequently specify a minimum C/D of at least 55 dB.
When amplifiers are cascaded, C/D ratio degrades according to the following laws:
Second-order distortion varies according to a 3-dB law: for every doubling in the number of amplifiers, C/D degrades 3 dB.
Third-order distortion (including cross-mod) varies according to a 6-dB law: for every doubling in the number of amplifiers, C/D degrades 6 dB.
Since third-order distortion is dominant in most distribution systems, we generally ignore second-order distortion and concentrate our efforts on controlling third-order.
We now know two facts:
If we increase carrier levels by 3 dB, C/D degrades 6 dB.
If we double the number of amplifiers in a cascade, C/D degrades 6 dB.
It logically follows that if we reduce the carrier levels by 3 dB, we can double the number of amplifiers with no net change in C/D. Or, stated another way, the allowable "distortion ceiling" is reduced by 3 dB when we double the number of amplifiers.
We can now state a general rule: in a cascade of identical amplifiers, the distortion ceiling at the output of the last amplifier decreases 3 dB when the number of amplifiers in the cascade is doubled.
THE TOLERANCE CHART
We have stated two general rules:
The Noise 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.
The Distortion Rule: in a cascade of identical amplifiers, the distortion ceiling at the output of the last amplifier decreases 3 dB when the number of amplifiers in the cascade is doubled.
Figure 1 illustrates these two rules graphically. Note that:
The noise floor rises at 3 dB/double.
C/N ratio is specified at 43 dB. In order to maintain this ratio at the output of the last amplifier, the minimum input level also rises at 3 dB/double.
The distortion ceiling drops at 3 dB/double.
C/D ratio is specified at 56 dB. In order to maintain this ratio at the output of the last amplifier, the allowable amplifier output level also drops at 3 dB/double.
For any given number of amplifiers in a cascade, amplifier input and output levels (and hence, gain) can be read directly from the chart. One example, for a cascade of 32 amplifiers, is indicated on the chart.
This chart is called a "tolerance chart" because it indicates the allowable input and output levels which can be tolerated for any given cascade depth. By manipulating the various parameters appearing in this chart, it's possible to study the interactions among cascade depth, amplifier operating levels, channel loading, required C/N performance, and required C/D performance. Several spreadsheet packages are available which allow a designer to play "what if?" games to determine the optimum tradeoffs for a given application.
Unlike the noise floor, the distortion ceiling is not related to any underlying physical law; it is constrained solely by two factors: carrier loading and the linearity of the amplifier output transistors. Improved transistor performance allows us to raise the distortion ceiling, resulting in a corresponding increase in allowable amplifier output. Similarly, reducing the number of carriers allows us to raise the distortion ceiling.
The values shown in Figure 1 are typical for production-line trunk amplifiers available today when used with carrier loads ranging from 36 to 54 NTSC video channels.
Barring some new technological breakthrough, transistor performance is probably about as good as it's ever going to get. The two major transistor manufacturers TRW and Motorola have been battling each other for years, each trying to eke out another tenth of a dB. Even though there's no theoretical limit to distortion performance, both manufacturers seem to be facing the practical limitations of manufacturing mass quantities at competitive prices.
Next month, we'll take a vacation from Broadband Networks. We'll resume in January, continuing our discussion of amplifiers.
(1) Code of Federal Regulations 47 CFR 76.605(a)(9)(i).
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CHAPTER 24 SUSTAINING MEMBERS
Welcome to our new sustaining member:
Thanks to all our Sustaining Members:
Wire and Cable
Fuji Film I&I