Extra Class question of the day: field-effect transistors

A field-effect transistor (FET) is a device that uses an electric field to control current flow through the device. Like the bipolar transistor, a FET normally has three terminals. The names of the three terminals of a field-effect transistor are gate, drain, source. (E6A17)

FETs are normally made with a technology called Complementary Metal-Oxide Semiconductor, or CMOS. The initials CMOS stand for Complementary Metal-Oxide Semiconductor. (E6A13) FETs made with CMOS technology are sometimes call MOSFETs.

In Figure E6-2 (below), schematic symbol 1 is the symbol for a P-channel junction FET. (E6A11) In Figure E6-2 (below), schematic symbol 4 is the symbol for an N-channel dual-gate MOSFET. (E6A10)

One characteristic of the MOSFET is that they have a high input impedance. This makes them more attractive for use in many test equipment applications than bipolar transistors. How does DC input impedance at the gate of a field-effect transistor compare with the DC input impedance of a bipolar transistor? An FET has high input impedance; a bipolar transistor has low input impedance. (E6A14)

One disadvantage of using MOSFETs is that they are very sensitive to electrostatic discharge (ESD). Sometimes, they are damaged by static discharges so low that you never even see the spark or feel the shock. To reduce the chance of the gate insulation being punctured by static discharges or excessive voltages many MOSFET devices have internally connected Zener diodes on the gates. (E6A12)

Most FETs are enhancement-mode devices. When using an enhancement-mode FET, you must apply a voltage to the gate to get current to flow from source to drain. Some FETs are, however, depletion mode devices. A depletion-mode FET is an FET that exhibits a current flow between source and drain when no gate voltage is applied. (E6A09)

Extra Class question of the day: Aurora propagation, selective fading; radio-path horizon; take-off angle over flat or sloping terrain; effects of ground on propagation; less common propagation modes

One of the most interesting propagation phenomena is Aurora propagation. To make use of this phenomenon, radio amateurs actually bounce their signals off of the Aurora Borealis, also known as the “Northern Lights.” All of these choices are correct when talking about effects Aurora activity has on radio communications (E3C01):

  • SSB signals are raspy
  • Signals propagating through the Aurora are fluttery
  • CW signals appear to be modulated by white noise

The cause of Aurora activity is the interaction of charged particles from the Sun with the Earth’s magnetic field and the ionosphere. (E3C02) Aurora activity occurs in the E-region of the ionosphere. (E3C03) CW is the emission mode that is best for Aurora propagation. (E3C04) From the contiguous 48 states, an antenna should be pointed North to take maximum advantage of aurora propagation. (E3C11)

Normally, we think of the ionosphere as a mirror, reflecting HF signals back to Earth at the same angle at which the signal hits the ionosphere. While this is normally the case, sometimes the ionosphere does not get refracted sufficiently to return directly to Earth, but instead travels for some distance in the F2 layer before finally being returned. The name of the high-angle wave in HF propagation that travels for some distance within the F2 region is called the Pedersen ray. (E3C08)

While we say that VHF/UHF communications is “line of sight,” the distance that a VHF/UHF radio wave will travel is slightly longer than the line-of-sight distance. We call this distance the “radio horizon” or “radio-path horizon.” The VHF/UHF radio-path horizon distance exceeds the geometric horizon by approximately 15% of the distance. (E3C06) The radio-path horizon distance exceeds the geometric horizon because of downward bending due to density variations in the atmosphere. (E3C14)

Another phenomenon that sometimes makes VHF signals beyond the line of sight is tropospheric ducting. Tropospheric ducting is usually responsible for causing VHF signals to propagate for hundreds of miles. (E3C09)

One of the most frustrating propagation phenomena is selective fading. Selective fading is partial cancellation of some frequencies within the received pass band. (E3C05) It is frustrating because it sometimes makes portions of an otherwise perfectly readable signal unreadable.

Amateur radio operators may sometimes use ground-wave propagation to communicate. One important thing to know about this type of propagation is that the maximum distance of ground-wave propagation decreases when the signal frequency is increased. (E3C12) Vertical polarization is the best type of polarization for ground-wave propagation. (E3C13) So, if you really want to make a contact via ground wave, use a vertical antenna on the 160m band.

To take advantage of some of these phenomena, or to just make your antenna work better, you should know how antenna’s performance changes with changes in its design or installation. For example, the radiation pattern of a horizontally polarized 3-element beam antenna varies as the height above ground changes. What happens is the the main lobe takeoff angle decreases with increasing height. (E3C07)

The performance of a horizontally polarized antenna mounted on the side of a hill will be different from the performance of same antenna mounted on flat ground. Specifically, the main lobe takeoff angle decreases in the downhill direction. (E3C10)

Extra Class question of the day: system noise; electrical appliance noise; line noise; locating noise sources; DSP noise reduction; noise blankers

Noise is often a real problem for radio amateurs. Fortunately, by understanding how noise is generated and how to reduce or eliminate it, noise can be tamed.

Atmospheric noise is naturally-occurring noise. Thunderstorms are a major cause of atmospheric static. (E4E06) There’s not much you can do to eliminate, but you can often use a receiver’s noise blanker to help you copy signals better. Signals which appear across a wide bandwidth (like atmospheric noise) are the types of signals that a receiver noise blanker might be able to remove from desired signals. (E4E03) Ignition noise is one type of receiver noise that can often be reduced by use of a receiver noise blanker. (E4E01)

One undesirable effect that can occur when using an IF noise blanker is that nearby signals may appear to be excessively wide even if they meet emission standards. (E4E09)

Many modern receivers now use digital signal processing (DSP) filters to eliminate noise. All of these choices are correct when talking about types of receiver noise can often be reduced with a DSP noise filter (E4E02):

  • Broadband white noise
  • Ignition noise
  • Power line noise

One disadvantage of using some types of automatic DSP notch-filters when attempting to copy CW signals is that the DSP filter can remove the desired signal at the same time as it removes interfering signals. (E4E12)

While filters can be very effective at reducing noise, it is often better to figure out what is  generating the noise and taking steps to reduce or eliminate the amount of noise generated in the first place. For example, one way you can determine if line noise interference is being generated within your home is by turning off the AC power line main circuit breaker and listening on a battery operated radio. (E4E07) If by doing this you determine that an electric motor is a problem, noise from an electric motor can be suppressed by installing a brute-force AC-line filter in series with the motor leads. (E4E05)

All of these choices are correct when it comes to the cause of a loud roaring or buzzing AC line interference that comes and goes at intervals (E4E13):

  • Arcing contacts in a thermostatically controlled device
  • A defective doorbell or doorbell transformer inside a nearby residence
  • A malfunctioning illuminated advertising display

Sometimes your own equipment may be the cause of received noise. A common-mode signal at the frequency of the radio transmitter is sometimes picked up by electrical wiring near a radio antenna. (E4E08)

The main source of noise in an automobile is the alternator. Conducted and radiated noise caused by an automobile alternator be suppressed by connecting the radio’s power leads directly to the battery and by installing coaxial capacitors in line with the alternator leads. (E4E04)

Personal computer and other digital devices can also generate noise. One type of electrical interference that might be caused by the operation of a nearby personal computer is the appearance of unstable modulated or unmodulated signals at specific frequencies. (E4E14) All of these choices are correct when talking about common characteristics of interference caused by a touch controlled electrical device (with an internal microprocessor) (E4E10):

  • The interfering signal sounds like AC hum on an AM receiver or a carrier modulated by 60 Hz hum on a SSB or CW receiver
  • The interfering signal may drift slowly across the HF spectrum
  • The interfering signal can be several kHz in width and usually repeats at regular intervals across a HF band

Noise can even be generated by the most unlikely things. For example, it is mostly likely that nearby corroded metal joints are mixing and re-radiating the broadcast signals if you are hearing combinations of local AM broadcast signals within one or more of the MF or HF ham bands. (E4E11)

Extra Class question of the day: modulation methods; modulation index and deviation ratio; pulse modulation; frequency and time division multiplexing

In FM modulation, the two primary parameters of interest are deviation ratio and modulation index. Deviation ratio is the ratio of the maximum carrier frequency deviation to the highest audio modulating frequency. (E8B09) The deviation ratio of an FM-phone signal having a maximum frequency swing of plus-or-minus 5 kHz when the maximum modulation frequency is 3 kHz is 1.67. (E8B05)The deviation ratio of an FM-phone signal having a maximum frequency swing of plus or minus 7.5 kHz when the maximum modulation frequency is 3.5 kHz is 2.14. (E8B06)

The term for the ratio between the frequency deviation of an RF carrier wave, and the modulating frequency of its corresponding FM-phone signal is modulation index. (E8B01) The modulation index is equal to the ratio of the frequency deviation to the modulating frequency. The modulation index of a phase-modulated emission does not depend on the RF carrier frequency. (E8B02)

The modulation index of an FM-phone signal having a maximum frequency deviation of 3000 Hz either side of the carrier frequency, when the modulating frequency is 1000 Hz is 3. (E8B03) The modulation index of an FM-phone signal having a maximum carrier deviation of plus or minus 6 kHz when modulated with a 2-kHz modulating frequency is 3. (E8B04)

Some amateur radio communications are pulse-width modulated. That is to say that the information being sent is proportional to the time the carrier is on. When using a pulse-width modulation system, the transmitter’s peak power greater than its average power because the signal duty cycle is less than 100%. (E8B07)

Some signals are pulse-position modulated. That is to say, what is significant is when the pulse occurs. The time at which each pulse occurs is the parameter that the modulating signal varies in a pulse-position modulation system. (E8B08)

Frequency division multiplexing is one method that can be used to combine several separate analog information streams into a single analog radio frequency signal. (E8B10) When a system uses frequency division multiplexing, two or more information streams are merged into a “baseband,” which then modulates the transmitter. (E8B11)

When a system uses digital time division multiplexing, two or more signals are arranged to share discrete time slots of a data transmission. (E8B12)

Extra Class question of the day: Television practices: fast scan television standards and techniques; slow scan television standards and techniques

Although we are called “radio” amateurs, we can also send and receive television signals.  There are several ways that amateurs communicate by television. Perhaps the two most popular ways are standard fast-scan television and slow-scan television (SSTV).

The video standard used by North American Fast Scan ATV stations is called NTSC.(E2B16) The NTSC, or National Television Systems Committee, is the body that set standards for the analog television system that was used in the U.S. and many other parts of the world. After nearly 70 years of using the analog NTSC system, U.S. broadcasters switched over to a digital broadcasting system on June 12, 2009.

A fast-scan (NTSC) television frame has 525 horizontal lines (E2B02), and a new frame is transmitted 30 times per second in a fast-scan (NTSC) television system. (E2B01) NTSC systems use an interlaced scanning pattern. An interlaced scanning pattern is generated in a fast-scan (NTSC) television system by scanning odd numbered lines in one field and even numbered ones in the next. (E2B03)

In order for the scanning beam to only show the picture, a technique called blanking is used. Blanking in a video signal is turning off the scanning beam while it is traveling from right to left or from bottom to top. (E2B04)

NTSC signals are amplitude modulated (AM) signals, but use a technique called vestigial sideband modulation. Vestigial sideband modulation is amplitude modulation in which one complete sideband and a portion of the other are transmitted. (E2B06) The reason that NTSC TV uses vestigial modulation is to conserve bandwidth. Even using this technique, an NTSC signal is 6 MHz wide. One advantage of using vestigial sideband for standard fast- scan TV transmissions is that vestigial sideband reduces bandwidth while allowing for simple video detector circuitry. (E2B05)

Amateurs can transmit color TV as well as black-and-white TV. The name of the signal component that carries color information in NTSC video is chroma. (E2B07)

There are a number of different ways to transmit audio with an NTSC signal. The following are common methods of transmitting accompanying audio with amateur fast-scan television:

  • Frequency-modulated sub-carrier
  • A separate VHF or UHF audio link
  • Frequency modulation of the video carrier

All of these choices are correct. (E2B08)

Slow-scan TV
Because of the bandwidth requirements, amateurs can only transmit fast-scan TV above 440 MHz. FM ATV transmissions, for example, are likely to be found on 1255 MHz. (E2B18) In fact, one special operating frequency restriction imposed on slow scan TV transmissions is that they are restricted to phone band segments and their bandwidth can be no greater than that of a voice signal of the same modulation type. (E2B19) The approximate bandwidth of a slow-scan TV signal is 3 kHz. (E2B17)

SSTV images are typically transmitted on the HF bands by varying tone frequencies representing the video are transmitted using single sideband. (E2B12) The tone frequency of an amateur slow-scan television signal encodes the brightness of the picture. (E2B14)

128 or 256 lines are commonly used in each frame on an amateur slow-scan color television picture. (E2B13) Specific tone frequencies signal SSTV receiving equipment to begin a new picture line. (E2B15)

There are a number of different SSTV modes. The function of the Vertical Interval Signaling (VIS) code transmitted as part of an SSTV transmission is to identify the SSTV mode being used. (E2B11)

Digital Radio Mondiale is one way to send and receive SSTV signals. No other hardware is needed, other than a receiver with SSB capability and a suitable computer, is needed to decode SSTV using Digital Radio Mondiale (DRM). (E2B09) Just like any SSTV transmission, 3 KHz is an acceptable bandwidth for Digital Radio Mondiale (DRM) based voice or SSTV digital transmissions made on the HF amateur bands. (E2B10)

Extra Class question of the day: 60m operation

The 60 m band is one of the oddest amateur radio bands. One of the reasons for this is that the 60 meter band is the only amateur band where transmission on specific channels rather than a range of frequencies is permitted. (E1A07) Also, the rules for operation on the 60 meter band state that operation is restricted to specific emission types and specific channels. (E1A06)

The rules for power output are also a bit arcane. The maximum power output permitted on the 60 meter band is 100 watts PEP effective radiated power relative to the gain of a half-wave dipole. (E1A05) The rules are written this way to minimize interference between amateur radio operators, who are secondary users of this band, and the primary users, which are primarily government radio stations.

Extra Class question of the day: Frequency priviledges

When using a transceiver that displays the carrier frequency of phone signals, the highest frequency at which a properly adjusted USB emission will be totally within the band is 3 kHz below the upper band edge. (E1A01) So, with your transceiver displaying the carrier frequency of phone signals, you hear a DX station’s CQ on 14.349 MHz USB. Is it legal to return the call using upper sideband on the same frequency? No, the sidebands will extend beyond the band edge. (E1A03)

The reason for this is that the USB signal extends from the carrier frequency, which is the frequency that the transceiver is displaying, up 3 kHz. When you set the transceiver to 14.349 kHz, the upper sideband will extend up to 14.352 MHz, and because the amateur radio band stops at 14.350 MHz, some of the transmission will fall outside the band.

A similar thing happens, but in reverse, when you operate lower sideband, or LSB. When using a transceiver that displays the carrier frequency of phone signals,the lowest frequency at which a properly adjusted LSB emission will be totally within the band is 3 kHz above the lower band edge. (E1A02) With your transceiver displaying the carrier frequency of phone signals, you hear a DX station calling CQ on 3.601 MHz LSB. Is it legal to return the call using lower sideband on the same frequency? No, my sidebands will extend beyond the edge of the phone band segment. (E1A04)

The lower sideband will extend down 3 kHz from the carrier frequency. So, when your transceiver is set to 3.601 Mhz, your signal will extend down to 3.598 MHz, which is outside the phone band.

This is also a consideration when operating CW because a CW signal occupies a finite bandwidth. (C) With your transceiver displaying the carrier frequency of CW signals, if you hear a DX station’s CQ on 3.500 MHz, it is not legal to return the call using CW on the same frequency because the sidebands from the CW signal will be out of the band. (E1A12)

Extra Class question of the day: Skin effect

At RF frequencies, the current in a conductor tends to flow near the surface of that conductor. This is the reason this phenomenon is called skin effect. The result of skin effect is that as frequency increases, RF current flows in a thinner layer of the conductor, closer to the surface. (E5D01)

Because the RF current flows in a smaller cross-sectional area of a conductor than a DC current, the RF current will experience more resistance than a DC current. In other words, the resistance of a conductor is different for RF currents than for direct currents because of skin effect. (E5D02)

Extra Class question of the day: AC and RF energy in real circuits: electrostatic and electromagnetic fields; reactive power; power factor

AC circuits–and RF circuits are just a type of AC circuit–capacitors and inductors store and release energy as the voltages and currents change. Because of this calculating power and energy in an AC circuit is not as straightforward as it is for DC circuits.

For example, a capacitor is a device is used to store electrical energy in an electrostatic field. (E5D03) During the positive portion of an AC cycle, the capacitor stores energy in its electrostatic field, but during the negative portion of the cycle, it returns that energy to the circuit.

An inductor is a device used to store electrical energy in a magnetic field. Electric current creates a magnetic field. (E5D05) The amount of current determines the strength of a magnetic field around a conductor. (E5D07) The direction of the magnetic field oriented about a conductor in relation to the direction of electron flow runs in a direction determined by the left-hand rule. (E5D06)

A similar thing happens to the magnetic field created by the current flow through an inductor that happens to the electrostatic field in a capacitor. When the current flows in one direction, a magnetic field is created. When the current changes direction, the energy stored in that magnetic field gets returned to the circuit.

The type of energy is stored in an electromagnetic or electrostatic field is potential energy. (E5D08) The unit that we use to measure the electrical energy stored in an electrostatic field is the Joule. (E5D04)

When talking about the power consumed by AC circuits, an important concept is reactive power. Reactive power is wattless, nonproductive power. (E5D14)

As noted above, during some portions of an AC cycle, inductors and capacitors will draw current and store energy, but during other portions of the cycle, it will return that energy to the circuit. So, what happens to reactive power in an AC circuit that has both ideal inductors and ideal capacitors is that it is repeatedly exchanged between the associated magnetic and electric fields, but is not dissipated. (E5D09) In other words, the net power dissipation is zero.

Of course, very few circuits contain only capacitors and inductors. In AC circuits where there is a resistance, that resistance will dissipate real power. For example, in a circuit consisting of a 100 ohm resistor in series with a 100 ohm inductive reactance drawing 1 ampere, the power consumed is 100 Watts. (E5D13) (P = I2 × R = 1A2 × 100 ohms = 100 W.)

In an AC circuit with inductors and capacitors, the voltage is out of phase with the current. You determine the true power an AC circuit where the voltage and current are out of phase by multiplying the apparent power times the power factor. (E5D10) For example, if a circuit has a power factor of 0.71 and the apparent power is 500 VA, the watts consumed is 355 W. (E5D18)

The power factor, or PF,  is the cosine of phase angle between the voltage and current. For example, if an R-L circuit has a 60 degree phase angle between the voltage and the current, the power factor is the cosine of 60 degrees, or 0.5 (E5D11) The power factor of an RL circuit having a 45 degree phase angle between the voltage and the current is the cosine of 45 degrees, or 0.707. (E5D15) The power factor of an RL circuit having a 30 degree phase angle between the voltage and the current is the cosine of 30 degrees, or 0.866. (E5D16)

Let’s look at a few examples:

  • If a circuit has a power factor of 0.2, and the input is 100-V AC at 4 amperes, the watts consumed is V × I × PF = 100V × 4A × 0.2 = 80 watts. (E5D12)
  • If a circuit has a power factor of 0.6 and the input is 200V AC at 5 amperes, the watts consumed is V × I × PF = 200V × 5A × 0.6 = 600 watts. (E5D17)

Extra Class question of the day: Optical components and power systems: photoconductive principles and effects, photovoltaic systems, optical couplers, optical sensors, and optoisolators

The photovoltaic effect is the conversion of light to electrical energy. (E6F04) In a device called a photovoltaic cell, electrons absorb the energy from light falling on a photovoltaic cell. (E6F12) The electrons then become free electrons.

The most common type of photovoltaic cell used for electrical power generation is silicon. (E6F10) The approximate open-circuit voltage produced by a fully-illuminated silicon photovoltaic cell is 0.5 V. (E6F11) The efficiency of a photovoltaic cell is the relative fraction of light that is converted to current. (E6F09)

Photoconductivity is a similar phenomenon. Photoconductivity is the increased conductivity of an illuminated semiconductor. (E6F01) The conductivity of a photoconductive material increases when light shines on it. (E6F02) A crystalline semiconductor is the material that is affected the most by photoconductivity. (E6F06)

A device that uses the phenomenon of photoconductivity is the optoisolator. The most common configuration of an optoisolator or optocoupler is an LED and a phototransistor. (E6F03) Optoisolators are often used in conjunction with solid state circuits when switching 120 VAC because  optoisolators provide a very high degree of electrical isolation between a control circuit and the circuit being switched. (E6F08)

A similar device is the solid-state relay. A solid state relay is a device that uses semiconductor devices to implement the functions of an electromechanical relay. (E6F07)

Optical shaft encoders are another device that rely on photoconductivity. An optical shaft encoder is a device which detects rotation of a control by interrupting a light source with a patterned wheel. (E6F05) Optical shaft encoders are used to detect when an operator turns a knob on an amateur radio transceiver.