RSS

Electronics Blog

Capacitors value

Q. I was looking at the schematic diagram of an amplifier and I noticed that there was a 0.01 µF capacitor connected in parallel with the filter capacitor, which was 5,000µF.

The 0.01 µF capacitors value is so much smaller than the filter capacitors that it doesn't make any difference in the capacitance there.

Do you think this is a mistake?

A. No. The smaller value capacitor is there for a good reason. The 5,000µF capacitor is big, physically. It is basically two foil plates and an insulator rolled up into a tube. As a result, the two plates have some inductance.

At higher audio frequencies, and above the audible range, this inductance has a significant amount of reactance. This counteracts the capacitive reactance of the filter capacitor at these frequencies.

This would lead to oscillations in the amplifier, as some of the output signal would get coupled back to the input stages by way of the common power supply line.

The 0.01 pµF capacitor is much smaller, physically, than the 5,000µF one. It has far less inductance, so it is still effective at the higher frequencies. So it has a low capacitive reactance at the higher frequencies, and short-circuits the filter capacitor for them. This prevents oscillations.

Q. How does AC flow through capacitors? I thought electrons couldn’t flow through the dielectric in a capacitor.

A. You’re right, Electrons can’t flow through the dielectric in a capacitor because it is an insulator. But the electric fields from the charged plates on either side of the dielectric do pass through the dielectric.

On one half-cycle of the AC, the AC source puts a positive voltage on one plate and a negative voltage on the other plate. Electrons are drawn off the positive plate and an equal number are pushed onto the negative plate. No electrons pass through the dielectric -only the electric fields.

Electrons move through the rest of the circuit, though, as the capacitor starts to charge. But before it does, the polarity of the AC reverses. Now electrons are moved in the opposite direction. They are pushed onto the plate that had been positive and pulled off the plate that had been negative. Again, current flows through the rest of the circuit, but not between the plates.

In circuits like RC coupled amplifiers there are resistors in the current path. As a result, most of the AC voltage appears across these resistors. The voltage across the capacitor doesn’t have time to change because the AC changes polarity too fast. This is another way of saying that the reactance of the capacitor is low at the frequency ofthe AC.

DC, on the other hand, can’t get through a capacitor. This is because the dielectric is an insulator. Current will flow when the capacitor is first charged by the DC, with electrons flowing onto the negative plate and an equal number flowing off the positive plate. But once the capacitor has charged, no further current will flow.

Boolean equation truth table

Q. I am studying Lesson 5516. In Appendix A-5 in the back of the lesson there is a sample problem. It starts with a Boolean equation you are supposed to use to fill out a truth table. I don’t know how to do this.

A. Each term in the Boolean equation describes where ones are to be put in the output column of the truth table. For example, the first term in the equation is AB. This means that Whenever A is zero and B is one, there is a 1 in the output column, regardless of the states of C and D. So you put a 1 in the output columns where A, B, C, and D are 0100, 0101, 0110, and 0111. The next term is ABCD, with all the terms NOT-ed. This means that where A, B, C, and D are all zeros, we have a 1 in the output column. Only one line of the truth table meets this condition: the first one, where ABCD are 0000.

The next term is ABCD. So We put a 1 in the output column for 1111. In the last term we have A,B, C, D. This means that we put a 1 in the output column for 1011. Note that where there are four letters in a term, it refers to only one line in the truth table. Where there are three letters, two lines of the table will be involved. Where there are two letter, it refers to four lines of the truth table. If there is a one-letter term, eight lines will have a l in their output column.

Regulator IC voltages

 

Q. I am an electronics hobbyist. I like to build circuits for different applications. I recently built a regulated 5V power supply for experimental use.

I used a 7805 regulator chip, which is supposed to supply 1A of output current. I tested it by putting different power resistors across the output to get different amounts of load current. I then measured the output voltage.

I found that the output voltage started dropping off at much less load current than 1A. Apparently the regulator went into shutdown, because when I disconnected the load, the output voltage slowly came back to 5V. What is going on?

 

A. Most regulator ICs contain an additional circuit that shuts them down when the IC gets too hot.

The 1A current rating applies only when the IC is at 25°C (77°F). To keep the IC cool, you need to attach it to a heat sink. This is a chunk of aluminum with cooling fins. Heat sinks are rated in degrees Celsius per Watt. In other words, this is the temperature rise per Watt above the temperature of the air in the device’s environment. You also have to take into account the degrees per Watt of the interface between the device and the heat sink, and add this to the heat sink rating.

Most regulator ICs go into thermal limiting at about 1250€ (257°F). The ratings for the IC you are using are listed in its spec sheets.

It also helps to use the minimum possible voltage for the unregulated supply. The regulator IC is basically connected in series with the load, so all the load current, plus a small amount for the regulator itself, flows through the IC. The Voltage across the IC is, from Kirchhoff’ s voltage law, the difference between the unregulated voltage and the regulated voltage.

The power the chip must handle is thus about equal to this voltage times the load current. Keeping the drop across the regulator chip low helps minimize the power it must handle.

Regulator ICs sometimes fail, and some types do so by shorting between the input and output. This places the unregulated voltage across the load. Since you are building a 5V supply, your load is probably TTL chips. If the unregulated voltage is 7V or more, if the regulator fails, it will destroy the TTL chips, too. There are ways to prevent this, such as using a “crowbar” circuit across the output, which you may want to investigate.

Q. What is HTML? I know it has to do with web pages, but that’s about all.

A. HTML stands for “hypertext markup language.” It is basically a computer programming language used to write the instructions for a web page.

In the past, you had to use it extensively to create a web page. But in recent years software has been developed that makes it much easier to do this, and much less actual programming is required.

Home Automation Industry Poised for Growth

Home Automation Industry poised for Growth
by Mary Rose Anderson

It was easier to travel to the moon 43 years ago than it is to fully automate the average homeowner’s abode today. That’s according to several leaders in the long-time-coming home automation industry.

At least everybody knew what they wanted when they were developing technologies to go to the moon in the late 1960s, says Rob Gerhardt of the Dallas based Electronic Environments, a high end home automation installation and service

“Getting a liability and quality level that is acceptable to a consumer market is not really that easy," agrees Tricia Parks of Parks Associates, a national marketing research firm which specializes in emerging household technologies.

At least when General Electric is testing a new high-tech jet engine, the pilot or “end-user” is highly-trained and wants to see the system work. Consumers at home are far different, says Parks. With home products, she says, “it’s not as easy as it looks.” Home products, according to her, have got to be easy-to use, incredibly durable, befitting of the consumer’s psychological - as well as practical needs, and affordable.

Failed attempts to introduce home automation to the marketplace by General Electric, Mitsubishi, and others in the 19805 proves the difficulty the industry has had getting off the ground. As a general rule, price and ease-of operation has played a major role in confining this market to commercial buildings and the homes of the very, very rich.

By definition, home automation involves a central computer which controls two or more household functions or subsystems. Using any combination of communication links wired and wireless the temperature, security, lights, appliances, even multi-room monitoring and entertainment audio/ video controls can be tied into one network.

With home automation, residents enter simple commands into the system from nearly anywhere with wall mounted keypads, hand-held remote controls, or computer touch screens throughout the house. Remote access  from outside the home is also possible with the use of the telephone. Upon receiving commands as “Away,” “Good Night,” “Good Morning” or “At Work,” the computer then adjusts household appliances and other subsystems accordingly.

“I could tell the house what I routinely do at night and then hit a button and say ‘Good Night House’ and the house would come back to me and say ‘Lights off, outdoor lights on, doors locked, alarms set, will wake you at up at seven,” explains Parks. The system could even start coffee brewing, raise the thermostat, and someday - draw your bath water upon your scheduled awakening

Likewise, a “At Work” command adjust might just shut off all appliances left on, adjust the thermostat, and monitor the house for such emergencies as attempted break-ins, a fire, a flood or frozen pipes. If a family member returns from work or school unexpectedly, inputting a simple code at the entryway could let them in. “It sounds scary to people now, but really it’s not. It’s not anymore scary than your car,” Parks says.

“We’re redefining the definition of comfort in the home to include peace of mind, security perhaps even energy savings,” says Steven Amoldt of the Minneapolis-based Honeywell. The point of home automation systems, says Amoldt, is to use electronic technology for improving a person’s life, not complicating it.

Parks agrees, saying if it isn’t really easy to operate these relatively expensive products, they won’t succeed. “Many people still don’t use most of the functions on their entertainment remotes because they don’t know what (the buttons) are and they care - even I don't care enough to take the time to out," she notes.

While home automation systems vary widely, they tend to fall into these broad categories: customized vs. standardize; professionally installed vs. do-it-yourself packages and interfacing technologies. Regardless of the category, several key factors have long-kept most home automation technologies out of the average resident’s home.

Protocol

Communication between such dramatically different subsystems is no easy task. “One of the issues the manufacturers have to solve is they’re trying to use this to create some common interfaces,” Parks says. “So you know your whole house would have the same philosophy,” Amoldt agrees, saying, “There’s been reluctance in the past for someone to step ahead and say I’ll coordinate the lighting, air conditioning, and heating. In other words, an HVAC contractor, and electrician and a security company.”

However, that’s a hurdle currently being overcome from many different angles. Completed just this year, CEBUS, the largest non-military standard ever published in the United States was written with home automation in mind. CEBUS is the result of an impressive array of corporate leaders i who met every six weeks for eight years, laboring over this comprehensive set of standards. With such notables as the director of Phillips laboratories and Panasonic’s top design engineer, it is likely most home automation manufacturers will heed this protocol.

Meanwhile, Honeywell grew tired of waiting for the CEBUS standards to finish and came up with it’s own protocol called H-BUS. “We have been designing these “BUS” systems for our larger commercial buildings for years. Because building automation been in existence for 20 or 30 years and Honeywell has been a leader in it, we had existing communication capabilities,” says Amoldt. “But we also plan to be compatible with whatever system, if any, become standard, whether CEBUS or Echelon.” Echelon, according to Amoldt, networks and homes subsystems together with transferable chips.

Price

Until Honeywell introduced its breakthrough “TotalHome” product this January, most home automation systems ran between $10,000 and $200,000. As Electronic Environment’s Gerhardt put it, these were systems for commercial industry and the homes of those “as rich as companies.” Because a pool of experienced technicians is needed to provide design, installation, programming and ongoing service, Gerhardt believes home automation can only evolve from the top down.

Most of the subsystems employed today in home automation started out as commercial technologies, Gerhardt says. Automated security, lighting and heating systems were used on commercial buildings while audio/video technology was used in theaters, he explained. Therefore, he assumes that the collection of these subsystems “will be commercially mastered first. Then it will be in the very wealthy and eventually it will drop down.”

However, Honeywell’s TotalHome system may be just the catalyst for the home which the slow-to-come home automation industry has been searching. The bare-bones package offers 10 points of lighting or appliance control; 10 points of security control (fire, burglary, etc.); temperature control; two command entry panels, and remote phone access for only $4,000. TotalHome, Amoldt says, is targeting both existing or new homes in the $180,000 and up range.

Workforce Resistance

It’s going to take some effort to convince most home builders that their customers want technology like this, says Jim Cain, State University Extention specialist who works closely with smaller regional home builders. Most home builders, he says, “don’t see that their customers are going to pay what it’d cost them to install these things."

“The building community is slow to change,” says Cain. “They are conservative because they’re marketing very closely. They don’t do much speculative construction. They aren’t going to go out and take much of a flier. So you’ve got to sell the end-use customer on the technology before you even sell the builders.”

User-Interface

A large chunk of people can’t use their VCR. How do you expect them to program off of this stuff? asks Cain, who believes it may take us twenty or thirty years before we see home automation in most homes. “The kids are the ones doing that stuff now,” he says, so it may take an entire generation.

But TotalHome does not require homeowners to program their systems. “We think we’ve removed these barriers by designing what we call lifestyle modes,” he says. The homeowner “actually designs the program by verbally describing what he or she wants lo the salesperson. All of the programming is done by Honeywell,” says Amoldt.

“They design these initial modes and we program them in, typically on-sight. We just load them into a PC and then download that into our system,” he explains. A simple phone call to Honeywell’s central station can get it reprogrammed, which is also done over the phone line. “It’s really simplified the human interchange,” says Amoldt.

Consumer Perceptions

Consumers have got to understand and value the advantages of this technology. “What we see in the mass public is there’s a lot of resistance,” Cain says. “When you talk about making life easier, a lot of things that are not promoted by the home automation industry do things which people don’t to be onerous. So you push a button and lock the doors, or check on them, or tum off the lights. Most people don’t that to be onerous,” he says. He says when you offer technology which performs such simple tasks, “most people fear it’ll make them appear lazy or even snobbish.”

In addition, while people have learned to expect sophisticated instruments in their cars, they don’t see their homes the same way. “If you walked into a car dealership and the new car didn’t have a thing that told you the gas was running out, the oil was low, the door was open, or the engine was not functioning, you would not buy it,” Parks said. “Yet people spend thousands everyday on systems for their home and they have no idea when (the products) are going to break except to out once it’s already broken.”

“There’s no technological reason in the world they shouldn’t be warning you and informing you of the efficiency levels they’re operating at, of whether the filters needed changing," Parks said. Once home automation opens up the theory that home products should ‘talk to you’ and share information with each other, she says, “you’re going to get a system that is greater than the sum of its parts.”

 

FCC Inspection Doesn’t Have to be a Painful Experience

FCC Inspection Doesn’t Have to be a Painful Experience
by Tim McCartney

Geographically isolated from nearby cities, Boise, Idaho, was frequently chosen to test a variety of consumer products. The closest large market, Salt Lake City, provides 335 miles over which to minimize “advertising interference” from radio and TV stations. And, visits from the Federal Communications Commission (FCC) to inspect radio and TV stations were understandably infrequent.

One such visit occurred by an inspector from the FCC’s Field Operations Bureau in Seattle, located over 500 miles from Boise. The last such FM and TV inspections in this mountain/desert community had been four years earlier, although inspections of cable TV and AM radio facilities had occurred in the interim.

Field Measurements

Before visiting any station in Boise, the inspector had already studied all of the local TV and FM broadcast signals using monitoring equipment in an FCC field van and had noted the obvious discrepancies. Two photographic “maps” from the spectrum analyzer examined the entire FM band. The photos were taken at two separate locations one about 30 miles outside of Boise and the other inside city limits.

So, this monitoring meant that the inspector knew a lot, but not everything, about the performance of these transmission plants.

First Up

When an FCC inspector is in town, word travels very quickly. But, in the case of the station to be inspected, it was a true surprise. The unexpected am. arrival at the studios opened with an introduction and identification which included a badge, clearly setting the tone for an official visit.

For this particular broadcast facility, such a visit was uneasy, The station was reached him of the increase in power. And, the 3500 feet increase in transmission elevation turned out to be less than everyday discussion among FCC personnel in Seattle.

This experience demonstrated that the regional FCC inspector does not always have current information from the Washington, D.C. offices. While the information was available via computer, it was usually easier to review station documents during inspections and discuss them with station officials.

Despite the surprise, this station was pleased to have the opportunity to demonstrate compliance with FCC requirements. So, here are summaries of the inspections studio and transmitter.

Studio Inspection EBS

The station was asked to produce two weeks of logs which showed the sending and receiving of Emergency Broadcast System (EBS) tests. The inspector said that he usually asks for the last month’s logs. If the recent logs appear in compliance, he asks for no more. However, is problems surface, the last three to six months of logs may be reviewed.

He noted the EBS test receive times from the designated EBS origination radio station in Boise so as to enable subsequent comparisons with logs from other local stations.

Then, he asked the on-ajr operator to conduct an EBS test, in order to ascertain if the operator had been properly trained. Also, he was then able to determine that the equipment was functional and if the two tones comprising the attention oriented EBS sound sufficiently modulated the transmitter so that listeners could readily hear the sound.

He also asked to see the FCC supplied authenticator envelope, which contain authentication words to be verified in case of an emergency.

The FCC inspector explained since FCC deregulation in the 1980’s, many stations­ incorrectly assumed that EBS requirements were less stringent. He also pointed out that the station must conduct an investigation if even one weak passes during which an EBS test has not been received and logged.

So, the station was asked to show how the logs with EBS entries were being checked each week. This was the mechanism by which stations would log their investigations into EBS problems; Is the equipment malfunctioning? Are the tests being done but not logged? Are the tests not being sent by the primary station?

At least one station in Boise was cited for a six-month failure to maintain a functional EBS receiver. In such cases, FCC Field Operations Bureaus were required to issue a fine. However, the inspector said he would not issue such a mandatory for an occasional mistake.

If a station missed one week’s of logging, there would be no fine. However, he indicated that if a station’s` EBS apparatus was not operational at the time of the inspection and its nonfunctional status not so logged, a fine was certain.

Public File

The station was asked to produce from its required public file the “Public and Broadcasting” manual, the most recent station license renewal and the last two community issues/programs in response summarizations.

The FCC required that the April-May-June issues/programs reports be filed by July l0. Since his visit was just four days after this deadline, it was a joyful moment to find the report in the file as required. While the visit to Boise was not planned with this deadline in mind, surprise inspections always pose situations of this type.

Current FCC Authorizations

The inspector asked to see the FCC station license (in this case, a construction permit and license) and the operator licenses. He asked the on-air operator to point out his restricted operator permit.

Remote Control

The station engineer explained how the remote control worked. This system controls the transmitter from the studio site, such as turning the transmitter on/ off, raising/lowering power, and taking remote meter readings.

He observed while the on-air operator called the transmitter and took readings. He studied the instructions posted next to the remote control point with various headings: Sign-On, Sign-Off, How to Put the Transmitter Back on the Air, etc.

At this stage, the inspector asked how the station could be certain that 100% of its FCC-authorized transmitter power was actually being transmitted. The station engineer explained that the transmitter power output was set up and calibrated to the authorized level, which equaled 100% as demonstrated on the transmitter power meter and simultaneously on the studio remote control power meter.

Modulation With SCAs

The inspector observed the levels on the station’s frequency-agile studio monitor. Then, he asked to see two other Boise FM stations which he had already noted where exceeding their authorized modulation levels. As it turned out, the studio monitor agreed with the results noted on the equipment in the FCC van. Since each of these two stations had two subcarriers (SCAs, he said they should reach only 110% modulation. With no subcarriers, 100% was the maximum authorized level).

He indicated that modulation levels were commonly misunderstood. Just how much modulation was allowed with a given number of subcarriers confused many broadcasters. A typical, proper level would be a stereo FM station using subcarriers at' 67 KHz SCA with 10% pilot injection, for a modulation of 110%.

Two rules on SCA injection levels were checked. The rule was that the total of all SCA injections must be 20% or less. The second rule was that those SCAs greater than 75 KHz cannot exceed 10% injected.

The inspector indicated that many stations were unclear on how to measure their SCA injection levels under SCA modulation conditions. He, too, had this difficulty in his remote monitoring van. So, he checked to establish that the total modulation limit was not to exceed. If the limit were exceeded, then he looked in the baseband for a reason.

Station Responsibility

Frequently, the inspector finds that stations have designated a chief operator who cannot address the issues raised during inspections. Since this person must understand all of the various requirements, ignorance would place the station at considerable risk.

Transmitter Inspection

Upon completion of the studio inspection, arrangements were made for a visit to the transmitter site at Boise-area mountain top for the following week. Usually in an inspection, the transmitter site was visited immediately after the studio.

All stations on the mountain were to be represented: FMS in the morning, TVs in the afternoon.

Tower Lighting/Painting

This issue turned out to need resolution. Most of the 13 users on one tower learned that their FCC licenses required that the tower they use to be painted the usual orange and white. But, this tower was not painted at all.

Subsequent discussions turned up the possibility that the Federal Aviation Administration (FAA) might exempt stations on the tower from the painting requirement. Since four painted towers closely located on the mountain top surrounded the unpainted tower, the FAA was contacted in an effort to see if this was an acceptable situation.

The question of responsibility of the required, daily tower light checks also surfaced. While one user on the tower assumed the responsibility, no formal agreement of this arrangement had ever been made.

These tower lighting and painting requirements actually reach beyond the FCC and FAA, to the U.S. Forest Service, which was charged to manage the mountain site. So, a can of bureaucratic worms was opened.

Antenna

Each station’s antenna was compared to the authorization descriptions on their station construction permit or licenses. Since there were so many FM antennas on this tower, the inspector asked that this station’s be pointed out.

Remote Control

The inspector wrote down three primary transmitter readings: plate volts, current and power forwarded. He then asked for verification of the remote control readings of these parameters.

The on-air operator at the studio then took meter readings and called back to read them directly to the inspector. The plate voltage reading was a little off, but well within the required 2% tolerance. The other two were accurate.

Power Level

The inspector asked how Transmitter Power Output (TPO) was determined on the basis of the construction permit authorization, which was noted only in Effective Radiated Power (ERP).

He was shown a copy of the computer program results for these calculations, which began with the ERP figure and worked backwards factoring in antenna power gain and line loss to arrive at TPO.

In addition, an in-line power wattmeter and dummy load mounted above the transmitter were also used for power level calibrations.

Ground Radiation

As it turned out, the required ground radiation standards were not being enforced by the FCC. Rather, such responsibility fell under the auspices of the Environmental Protection Agency (EPA).

This was confusing, since the FCC certainly addressed ground radiation standards in its licensing and renewal process. But, these standards were not enforced by FCC regional field offices.

When asked about the proposed limited-attention overnight operation of the station, the inspector explained that such authorizations came from the policy branch of the FCC. Thus, the role was to ensure that stations comply with the terms of their FCC licenses, not to evaluate the quality of those terms. So, the field offices became involved in' interpretation and enforcement, but not in policy-making. The lesson, then, was that the station authorization (license) needed to spell out critical information as much as possible. If in doubt, approval should be sought from the policy makers So that the inevitable FCC inspection can examine current, accurate authorizations.

I’m from the Government and I’m here to help you. True or False?

Well, this visit was certainly one in which most Boise broadcasters answered m. The visit seized the attention of station management to areas of concern by their own engineering departments. It cleaned up modulation levels. It began discussions on tower lighting and painting. And, it served to remind everyone of easily overlooked FCC regulations.

And, then there was the matter of attitude. Did the FCC inspector relish his power and wave it around? Or, did the inspector treat broadcasters as colleagues? In Boise, even those cited for violations felt they were treated fairly.

So, in this series of inspections, yes the government helped.

Because of government cut backs and deregulation, some thought that remotely-located Boise would be forgotten by the FCC Field Operations Bureau. Since then, with the FCC’s interest established and considerable consumer test marketing underway, will locals there still have to spell and pronounce Boise for everyone east of the Mississippi?

Electronic ignitions explained

Q. Cars now have electronic ignitions. What is the difference between an electronic ignition and the old kind?

A. The old kind used a set of switch contacts in series with the primary of the ignition coil. The contacts, called “points,” were opened and closed by a cam linked to the distributor shaft. When the points closed, current flowed through the primary of the ignition coil, building up a magnetic field in its core.

Then the contacts were opened so that the magnetic field began to collapse quickly. This induced a high voltage in the secondary of the ignition coil, producing the spark to fire each cylinder at the right time. A capacitor, which mechanics usually call a “condenser,” was connected in parallel with the points. Its purpose was to resonate with the inductance of the primary of the ignition coil when the points opened. This made the spark voltage an oscillating Waveform of decreasing amplitude. Instead of being one brief pulse, the spark voltage continued for a short time, helping to ignite the fuel / air mixture more effectively.

In an electronic ignition, the points are replaced by a transistor that is operated as a switch. It still sends on/ off pulses to the ignition coil’s primary in the same way as the points had. In some cars, there is a separate spark coil and transistor for each pair of cylinders. 

Various methods are used to control the timing of the transistor`s pulses, most of them being either magnetic or optical devices operated by a mechanism connected to the crankshaft. The points in the old kind of ignition would gradually burn up as they operated, so that they had to be replaced periodically. The mechanism which operated them would also wear a bit, so that periodic adjustments were necessary. This was the purpose of the “tune-up” The capacitor was usually replaced in a tune-up, too.

Since the electronic ignition has a transistor instead of points, the problem of burned points has been eliminated. The transistor can also deliver a more energetic spark, making ignition more efficient. For these reasons, modern automobiles do not require this kind of a tune-up. Usually, all that needs to be done in a tune-up now is to make minor adjustments to the ignition timing, and sometimes replacement of the spark plugs.

Colorful Language

Q. The lessons refer to the “video” signal in a TV receiver. Just what is the video signal, and why is it separate from the color signals?

A. When the standards for color TV were established, there were already many blackand-white receivers in use. To make the new signal for color compatible with these existing receivers, a black-and-white signal was included inthe color system.

It is derived from the three primary colors by mixing them in speciñc amounts: 11% blue, 30% red, and 59% green. This is properly called the “luminance” signal, since it is the brightness content of the picture. The author of the TV lessons usually calls it the “video” signal.

The color content of the picture is contained in the “chroma” signal, from which the three color difference signals are derived in the receiver. When these are mixed with the luminance signal, the original three color signals, red, blue, and green, are produced. These are then used to drive the cathodes of the three electron guns in the CRT.

A standard black-and-white receiver uses only the luminance signal in the broadcast. A color receiver basically takes the luminance signal and adds color to it.

Scope the Situation

Regular oscilloscope versus digital oscilloscope

Q. I have seen advertisements for digital oscilloscopes. What is the difference between a regular oscilloscope and a digital one?

A. In a conventional oscilloscope, the signal to be viewed is simply amplified and applied to the deflection plates in the CRT. A digital scope has a very fast electronic switch that is connected in series with the input. The switch is turned on for very short periods of time to “sample” the input signal. While the switch is on, a very fast analog-to-digital converter takes the sample and produces a digital signal.

The digital signal is a binary number whose value corresponds to the voltage of the sample. Successive samples are taken, so that the waveform of the signal is converted into a stream of binary numbers. Digital circuits in the CRT scanning electronics are used to light up individual dots on the screen called “pixels” Each pixel corresponds to a sample of the waveform. The value of the binary number for each sample determines the height of the pixel that represents it. The pixels are displayed in succession across the screen of the CRT. Together, the pixels forrn a line which represents the waveform of the signal.

There are also oscilloscopes which have electronic switches that break the signal up into analog samples that are instead simply amplified and applied to the deflection plates in a conventional scope CRT. These are called “sampling” scopes. They are used for high frequency signals.