Extra Class question of the day: Semiconductor diodes

Diodes are the simplest semiconductor devices. In their simplest form, they have two terminals and conduct current in only one direction, from the cathode to the anode. By manipulating the characteristics of the semiconductor material, manufacturers can make diodes useful in a wide variety of applications.

Take, for example, the Zener diode. The most useful characteristic of a Zener diode is a constant voltage drop under conditions of varying current. (E6B01) This makes it useful in voltage regulator circuits.

Another example is the varactor diode. The varactor diode is a semiconductor device designed for use as a voltage-controlled capacitor. (E6B04) Varactor diodes are often used in tuning circuits.

A PIN diode is a semiconductor device that operates as a variable resistor at RF and microwave frequencies. One common use for PIN diodes is as an RF switch. (E6B12)The characteristic of a PIN diode that makes it useful as an RF switch or attenuator is a large region of intrinsic material. (E6B05) The forward DC bias current is used to control the attenuation of RF signals by a PIN diode. (E6B11)

Two types of diodes used in RF circuits are the tunnel diode and hot-carrier diode. The tunnel diode is a special type of diode is capable of both amplification and oscillation. (E6B03) Tunnel diodes are capable of operating well into the microwave region. A hot-carrier diode is commonly used as a VHF / UHF mixer or detector. (E6B06)

Metal-semiconductor junction is a term that describes a type of semiconductor diode. (E6B08) A Schottky diode is an example of a metal-semiconductor diode. An important characteristic of a Schottky diode as compared to an ordinary silicon diode when used as a power supply rectifier is that it has less forward voltage drop.

In Figure E6-3 (below), 5 is the schematic symbol for a light-emitting diode. (E6B10) Forward bias is required for an LED to emit light. (E6B13)

Figure E6-3

No matter what kind of diode you are using, it’s very important to not exceed the forward current specification. Doing so, will cause it to fail. Excessive junction temperature is the failure mechanism when a junction diode fails due to excessive current. (E6B07)

Extra Class question of the day: Receiver performance characteristics

In the past, sensitivity was one of the most important receiver performance specifications. Today, instead of sensitivity, we speak of a receiver’s minimum discernible signal, or MDS. The MDS of a receiver is the minimum discernible signal. (E4C07) This is the weakest signal that a receiver will detect.

One parameter that affects receiver sensitivity is the noise figure. The noise figure of a receiver is the ratio in dB of the noise generated by the receiver compared to the theoretical minimum noise. (E4C04) Lowering the noise figure of a receiver would improve weak signal sensitivity. (E4C08)

A related specification is the noise floor. When we say that the noise floor of a receiver has a value of -174 dBm/Hz, it is referring to the theoretical noise at the input of a perfect receiver at room temperature. (E4C05) If a CW receiver with the AGC off has an equivalent input noise power density of -174 dBm/Hz, the level of an unmodulated carrier input to this receiver would have to be -148 dBm to yield an audio output SNR of 0 dB in a 400 Hz noise bandwidth. (E4C06)

A receiver’s selectivity is the result of a lot of things, including the filters a receiver has. 300 Hz is a desirable amount of selectivity for an amateur RTTY HF receiver. (E4C10)2.4 kHz is a desirable amount of selectivity for an amateur SSB phone receiver.(E4C11)

In addition to a 300 Hz filter and a 2.4 kHz filter, high-end receivers also have filters called roofing filters. A narrow-band roofing filter affects receiver performance because it improves dynamic range by attenuating strong signals near the receive frequency. (E4C13)

Back in the day, when superheterodyne receivers had intermediate frequencies, or IFs, in the 400 – 500 kHz range, image rejection was a problem. If there was a strong signal present on a frequency about two times the IF away from the frequency your receiver was tuned to, you might hear that signal. Accordingly, 15.210 MHz is a frequency on which a station might be transmitting if is generating a spurious image signal in a receiver tuned to 14.300 MHz and which uses a 455 kHz IF frequency. (E4C14)

One solution to this problem is to select an IF higher in frequency. One good reason for selecting a high frequency for the design of the IF in a conventional HF or VHF communications receiver is that it is easier for front-end circuitry to eliminate image responses. (E4C09) A front-end filter or pre-selector of a receiver can also be effective in eliminating image signal interference. (E4C02)

Another way to get rid of image signals is to use a narrow IF filter. An undesirable effect of using too wide a filter bandwidth in the IF section of a receiver is that undesired signals may be heard. (E4C12)

Because most modern transceivers use digital techniques to generate a local oscillator signal to tune a receiver, synthesizer phase noise might be a problem. An effect of excessive phase noise in the local oscillator section of a receiver is that it can cause strong signals on nearby frequencies to interfere with reception of weak signals. (E4C01)

Finally, here are two miscellaneous questions on receiver performance characteristics. Atmospheric noise is the primary source of noise that can be heard from an HF receiver with an antenna connected. (E4C15) Capture effect is the term for the blocking of one FM phone signal by another, stronger FM phone signal. (E4C03)

Extra Class question of the day: Digital signals and communications modes

Morse Code is arguably the original digital mode. Morse code is a digital code consists of elements having unequal length. (E8C01) One advantage of using Morse Code is that it is very narrow bandwidth. The bandwidth necessary for a 13-WPM international Morse code transmission is approximately 52 Hz. (E8C05)

The next oldest digital mode is radioteletype, or RTTY. RTTY uses a five-bit code called Baudot. Most modern digital devices these days use ASCII, which is a 7-bit or 8-bit code. Some of the differences between the Baudot digital code and ASCII are that Baudot uses five data bits per character, ASCII uses seven or eight; Baudot uses two characters as shift codes, ASCII has no shift code. (E8C02) One advantage of using the ASCII code for data communications is that it is possible to transmit both upper and lower case text. (E8C03)

The reason that some ASCII transmissions have only seven bits, while others use eight bits is that the eighth bit is a parity bit. The advantage of including a parity bit with an ASCII character stream is that some types of errors can be detected. (E8C12)

The bandwidth needed for ASCII digital transmissions increases as the data rate increases. The bandwidth necessary for a 170-hertz shift, 300-baud ASCII transmission is 0.5 kHz. (E8C06) The bandwidth necessary for a 4800-Hz frequency shift, 9600-baud ASCII FM transmission is 15.36 kHz. (E8C07)

PSK has become a very popular digital mode. One reason for this is that it occupies a very narrow bandwidth – only 31 Hz. One technique used to minimize the bandwidth requirements of a PSK31 signal is the use of sinusoidal data pulses. (E8C04)

An up-and-coming digital mode is JT-65, named after its inventor, Nobel Prize winner and amateur radio operator, Joe Taylor, K1JT. It uses 65 different tones spread over a bandwidth of 175 Hz. One advantage of using JT-65 coding is the ability to decode signals which have a very low signal to noise ratio. (E8C13)

Spread-spectrum communication is a wide-bandwidth communications system in which the transmitted carrier frequency varies according to some predetermined sequence. (E8C08) Direct sequence is a spread-spectrum communications technique uses a high speed binary bit stream to shift the phase of an RF carrier. (E8C11) Frequency hopping is a spread-spectrum communications technique alters the center frequency of a conventional carrier many times per second in accordance with a pseudo-random list of channels. (E8C10) Spread-spectrum techniques causes a digital signal to appear as wide-band noise to a conventional receiver. (E8C09)

Extra Class question of the day: matching antennas to feedlines

For many types of antennas, matching the impedance of the antenna to the impedance of the feedline, normally coax, is essential. Mismatched lines create high SWR and, consequently, feedline losses. An SWR greater than 1:1 is characteristic of a mismatched transmission line. (E9E08)

When a feedline and antenna are mismatched, some of the power you are trying to transmit will be reflected back down the feedline. The ration of the amplitude of the reflected wave to the amplitude of the wave you are trying to send is called the reflection ratio, and it is mathematically related to SWR. Reflection coefficient is the term that best describes the interactions at the load end of a mismatched transmission line. (E9E07)

To match the impedance of the feedline to the impedance of the antenna, we use a variety of different techniques. The delta matching system matches a high-impedance transmission line to a lower impedance antenna by connecting the line to the driven element in two places spaced a fraction of a wavelength each side of element center. (E9E01)

The gamma match is the name of an antenna matching system that matches an unbalanced feed line to an antenna by feeding the driven element both at the center of the element and at a fraction of a wavelength to one side of center. (E9E02) The purpose of the series capacitor in a gamma-type antenna matching network is to cancel the inductive reactance of the matching network. (E9E04)The gamma match is an effective method of connecting a 50-ohm coaxial cable feed line to a grounded tower so it can be used as a vertical antenna. (E9E09)

The stub match is the name of the matching system that uses a section of transmission line connected in parallel with the feed line at or near the feed point. (E9E03) What the stub does is to add reactance at the feed point. By varying the length of the stub, you can change the reactance that the stub provides to whatever value is needed. An effective way of matching a feed line to a VHF or UHF antenna when the impedances of both the antenna and feed line are unknown is to use the universal stub matching technique. (E9E11)

Inserting a 1/4-wavelength piece of 75-ohm coaxial cable transmission line in series between the antenna terminals and the 50-ohm feed cable is an effective way to match an antenna with a 100-ohm feed point impedance to a 50-ohm coaxial cable feed line. (E9E10) Note that this will only work on one band as the length of 75-ohm coax you use will only be 1/4 of a wavelength on one band.

Many directly-fed Yagi antennas have feedpoint impedances of approximately 20 to 25 ohms. One technique often use to match these antennas to 50-ohm coaxial cable is the hairpin match. To use a hairpin matching system to tune the driven element of  a 3-element Yagi, the driven element reactance must be capacitive. (E9E05) The equivalent lumped-constant network for a hairpin matching system on a 3-element Yagi is an L network. (E9E06)

Some beam antennas use multiple driven elements in order to make them multi-band antennas. The primary purpose of a phasing line when used with an antenna having multiple driven elements is that it ensures that each driven element operates in concert with the others to create the desired antenna pattern. (E9E12)

I’m not sure that Wilkinson dividers are used much in antenna systems, or why this question is in the section on feedline matching, but here it is. The purpose of a Wilkinson divider is that it divides power equally among multiple loads while preventing changes in one load from disturbing power flow to the others. (E9E13)

Extra Class question of the day: frequency synthesizers

Most modern amateur radio transceivers use digital frequency synthesizers instead of analog oscillators to generate RF signals. On reason for this is that they are much more stable than analog oscillators. The two main types of digital frequency synthesizers are the direct digital synthesizer and the phase-locked loop synthesizer

A direct digital synthesizer is the type of frequency synthesizer circuit that uses a phase accumulator, lookup table, digital to analog converter and a low-pass anti-alias filter. (E7H09) The phase accumulator is a principal component of a direct digital synthesizer (DDS). (E7H12) The information is contained in the lookup table of a direct digital frequency synthesizer is the amplitude values that represent a sine-wave output. (E7H10)

Both the direct digital synthesizer and the phase-locked loop synthesizer have issues with spectral purity. The major spectral impurity components of direct digital synthesizers are spurious signals at discrete frequencies. (E7H11)

For a more detailed explanation of how direct digital synthesizers work, see the electric druid’s  Synth DIY page.

Another type of frequency synthesizer that’s popular are those that use a phase-locked loop. A phase-locked loop circuit is an electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator. (E7H14) 

A phase-locked loop is often used as part of a variable frequency synthesizer for receivers and transmitters because it makes it possible for a VFO to have the same degree of frequency stability as a crystal oscillator. (E7H17) Frequency synthesis, FM demodulation are two functions that can be performed by a phase-locked loop. (E7H15)

An important specification for phase-locked loop circuits is the short-term stability of the reference oscillator. The short-term stability of the reference oscillator is important in the design of a phase locked loop (PLL) frequency synthesizer because any phase variations in the reference oscillator signal will produce phase noise in the synthesizer output. (E7H16) Phase noise is the major spectral impurity components of phase-locked loop synthesizers. (E7H18)

Another important specification is capture range. The capture range of a phase-locked loop circuit is the frequency range over which the circuit can lock. (E7H13)

Extra Class question of the day: operating HF digital modes; error correction

Perhaps the most popular digital mode these days is PSK31. PSK stands for “phase shift keying.” One of its main advantages is that it had a very narrow bandwidth—only 31 Hz. In fact, PSK31 is the digital communications mode that has the narrowest bandwidth. (E2E10)

One of the ways is achieves this narrow bandwidth is that uses variable length coding. That is to say, characters have different numbers of bits, depending on how frequently they appear in normal text. PSK31 is an HF digital mode that uses variable-length coding for bandwidth efficiency. (E2E09)

Another type of modulation commonly used on the HF bands is frequency-shift keying, or FSK. RTTY, for example uses FSK modulation. FSK is a type of modulation that is common for data emissions below 30 MHz. (E2E01) One type of FSK modulation is MFSK16. The typical bandwidth of a properly modulated MFSK16 signal is 316 Hz. (E2E07)

Amateur transceivers use two different methods to modulate a signal using FSK: direct FSK and audio FSK. The difference between direct FSK and audio FSK is that direct FSK applies the data signal to the transmitter VFO. (E2E11) When using audio FSK, audio, typically from a computer sound card, is used to shift the frequency of the transmitted signal.

To tune an FSK signal, one often uses a crossed-ellipse display. You have properly tuned a signal when one of the ellipses is as vertical as possible, and the other is as horizontal as possible. When one of the ellipses in an FSK crossed-ellipse display suddenly disappears, selective fading has occurred. (E2E04)

PACTOR is one digital mode that uses FSK. It also uses the ARQ protocol to detect errors. Because of this, PACTOR is an HF digital mode that can be used to transfer binary files. (E2E08) How does ARQ accomplish error correction? If errors are detected, a retransmission is requested. (E2E05)

Another way to detect and correct errors in a data transmission is forward error correction. The letters FEC mean Forward Error Correction when talking about digital operation. (E2E02) Forward Error Correction is implemented by transmitting extra data that may be used to detect and correct transmission errors. (E2E03)

No matter what type of modulation you use, data transmission over an HF radio link is very slow. 300 baud is the most common data rate used for HF packet communications. (E2E06) In fact, due to bandwidth limitations, 300 baud is the maximum data rate.

Many of the digital modes were designed to allow keyboard-to-keyboard operation. That is to say, that operators can type messages back and forth to one another, almost as if they were having a conversation using SSB. Winlink, however, does not support keyboard-to-keyboard operation. (E2E12)

Extra Class question of the day: VHF and UHF digital modes; APRS

One of the most commonly understood concepts in digital communications is the baud. A baud is not equal to a bit per second, except for very simple systems. Rather, the definition of baud is the number of data symbols transmitted per second. (E2D02) A data symbol may represent multiple bits.

The baud rate is a measure of how fast a digital communications system can transmit data. Under clear communications conditions, 300-baud packet is the digital communication mode that has the fastest data throughput. (E2D09)

In the past ten years or so, we’ve had an explosion of digital modes become available. JT65 is one example. JT65 is a digital mode especially useful for EME communications. (E2D03) JT65 improves EME communications because it can decode signals many dB below the noise floor using FEC.(E2D12) FSK441 is a digital mode especially designed for use for meteor scatter signals. (E2D01)

One of the most popular digital modes is the Automatic Packet Reporting System, or APRS. AX.25 is the digital protocol used by APRS. AX.25 is more commonly known as packet radio. (E2D07) Unnumbered Information is the type of packet frame used to transmit APRS beacon data. (E2D08)

APRS stations can be used to help support a public service communications activity. An APRS station with a GPS unit can automatically transmit information to show a mobile station’s position during the event. (E2D10) Latitude and longitude are used by the APRS network to communicate your location. (E2D11) 144.39 MHz is a commonly used 2-meter APRS frequency. (E2D06)

Amateurs that enjoy satellite communications also use digital modes. For example, store-and-forward is a technique normally used by low Earth orbiting digital satellites to relay messages around the world. (E2D05) The purpose of digital store-and-forward functions on an Amateur Radio satellite is to store digital messages in the satellite for later download by other stations. (E2D04)

Extra Class question of the day: Impedance plots and coordinate systems

Rectulangar and Polar Coordinates

Most often when we plot values on a graph, we use the rectangular, or Cartesian, coordinate system. The two numbers that are used to define a point on a graph using rectangular coordinates are the coordinate values along the horizontal and vertical axes. (E5C11) In the graph above, point P is at x,y. Rectangular coordinates are often used to display the resistive, inductive, and/or capacitive reactance components of an impedance. (E5C13)

When thinking about how capacitive reactances, inductive reactances, and resistance combine, it’s useful to think in terms of polar coordinates. Polar coordinates are often used to display the phase angle of a circuit containing resistance, inductive and/or capacitive reactance. (E5C14) In a polar-coordinate system, each point on the graph has two values, a magnitude (shown by r in the figure above) and an angle (shown by θ in the figure above).

When using rectangular coordinates to graph the impedance of a circuit, the vertical axis represents the reactive component. (E5C10) To figure out the impedance of a circuit, you first plot the inductive reactance on the positive y-axis and the capacitive reactance on the negative y-axis. The net reactance, X, will be the sum of the two reactances.

When using rectangular coordinates to graph the impedance of a circuit, the horizontal axis represents the resistive component. (E5C09) After you’ve computed the net reactance, you plot the resistance on the x-axis and compute the magnitude of the impedance, shown by r in the graph above. If you consider that r is the third side of a right triangle made up of the sides r, x, and y, r is equal to the square root of x2 and y2.

Let’s take a look at an example. In polar coordinates, is the impedance of a network consisting of a 100-ohm-reactance inductor in series with a 100-ohm resistor is 141 ohms at an angle of 45 degrees. (E5C01) In this example, x=100 and y=100, so

r = sqrt (X2 + R2) = sqrt (1002 + 1002) = sqrt (20000) = 141 ohms.

The cosine of the phase angle θ is equal to x/r, or 100/141, or .707.  If you look up this value in a table of cosines, you’ll find that the angle is 45 degrees.

Here’s another thing to notice. When the value of the reactance is equal to the value of the resistance, the angle will be either 45 degrees or -45 degrees, depending on whether the net reactance is inductive or capacitive.

Now, let’s look at an example with both inductive and capacitive reactance. In polar coordinates, the impedance of a network consisting of a 100-ohm-reactance inductor, a 100-ohm-reactance capacitor, and a 100-ohm resistor, all connected in series is 100 ohms at an angle of 0 degrees. (E5C02) In this case, the inductive reactance and the capacitive reactance are the same, meaning that there is no net reactance. If you plot the impedance of a circuit using the rectangular coordinate system and find the impedance point falls on the right side of the graph on the horizontal axis, you know that the circuit impedance is equivalent to a pure resistance. (E5C12)

Here’s an example with unequal inductive and capacitive reactances. In polar coordinates, the impedance of a network consisting of a 300-ohm-reactance capacitor, a 600-ohm-reactance inductor, and a 400-ohm resistor, all connected in series is 500 ohms at an angle of 37 degrees. (E5C03) Here’s how we got that result:

X = 600 – 300 = 300 ohms

r = sqrt (X2 + R2) = sqrt (3002 + 4002) = sqrt (250000) = 500 ohms

θ = cos-1(x/r) = cos-1(400/500) = 37 degrees

Here are some more examples. I’ll leave the solutions up to you:

  • In polar coordinates, the impedance of a network consisting of a 400-ohm-reactance capacitor in series with a 300-ohm resistor is 500 ohms at an angle of -53.1 degrees. (E5C04)
  • In polar coordinates, the impedance of a network consisting of a 400-ohm-reactance inductor in parallel with a 300-ohm resistor is 240 ohms at an angle of 36.9 degrees. (E5C05)
  • In polar coordinates, the impedance of a network consisting of a 100-ohm-reactance capacitor in series with a 100-ohm resistor is 141 ohms at an angle of -45 degrees. (E5C06)
  • In polar coordinates, the impedance of a network comprised of a 100-ohm-reactance capacitor in parallel with a 100-ohm resistor is 71 ohms at an angle of -45 degrees. (E5C07)
  • In polar coordinates, what is the impedance of a network comprised of a 300-ohm-reactance inductor in series with a 400-ohm resistor is 500 ohms at an angle of 53 degrees. (E5C08)
  • In polar coordinates, the impedance of a series circuit consisting of a resistance of 4 ohms, an inductive reactance of 4 ohms, and a capacitive reactance of 1 ohm is 5 ohms at an angle of 37 degrees. (E5C18)

Extra Class question of the day: Station control

An important concept in the rules governing amateur radio is the concept of station control and the control operator. The control operator is the licensed radio amateur who is responsible for the transmissions of a station, and the location of that operator is called the control point. There are three ways that a control operator can control a station: local control, remote control, or automatic control.

Local control means direct manipulation of the transmitter by a control operator. (E1C07) So, when you were sitting in front of your radio, you are using local control.

A remotely controlled station is a station controlled indirectly through a control link. (E1C01) A control operator must be present at the control point is the true statement about remotely controlled amateur stations. (E1C06) This is, of course, true for local control as well. 3 minutes is the maximum permissible duration of a remotely controlled station’s transmissions if its control link malfunctions. (E1C08)

Automatic control of a station means the use of devices and procedures for control so that the control operator does not have to be present at a control point. (E1C02) The control operator responsibilities of a station under automatic control differs from one under local control. Under automatic control the control operator is not required to be present at the control point. (E1C03)

Most repeaters are operated with automatic control. Only auxiliary, repeater or space stations are the types of amateur stations that may automatically retransmit the radio signals of other amateur stations. (E1C10) 29.500 – 29.700 MHz is the frequency band available for an automatically-controlled repeater operating below 30 MHz. (E1C09) No repeaters are allowed on any other HF band.

An automatically controlled station may retransmit third party communications only when transmitting RTTY or data emissions. (E1C04) An automatically controlled station may never originate third party communications. (E1C05)

Extra Class question of the day: Station restrictions and special operations

Part 97 places many different restrictions on how amateurs can use their stations and specifies technical standards that amateur radio station must meet. For example, some rules set standards for spurious emissions A spurious emission is an emission outside its necessary bandwidth that can be reduced or eliminated without affecting the information transmitted. (E1B01) The rules also state that permitted mean power of any spurious emission relative to the mean power of the fundamental emission from a station transmitter or external RF amplifier must be at least 43 dB below for transmitters or amplifiers installed after January 1, 2003, and transmitting on a frequency below 30 MHZ is. (E1B11)

There are also restrictions on erecting antennas. One factor that might cause the physical location of an amateur station apparatus or antenna structure to be restricted is if the location is of environmental importance or significant in American history, architecture, or culture. (E1B02) If you are installing an amateur station antenna at a site at or near a public use airport, you may have to notify the Federal Aviation Administration and register it with the FCC as required by Part 17 of FCC rules. (E1B06)

The 60m band is one band that has a lot of weird restrictions not found on other ham bands. For example, the maximum bandwidth for a data emission on 60 meters is 2.8 kHz. (E1B05) The carrier frequency of a CW signal must be set at the center frequency of the channel to comply with FCC rules for 60 meter operation. (E1B07)

Because RACES operation is quasi-governmental, there are some rules about RACES operations. Any FCC-licensed amateur station certified by the responsible civil defense organization for the area served may be operated in RACES.  (E1B09) All amateur service frequencies authorized to the control operator are authorized to an amateur station participating in RACES. (E1B10)

Finally, there are some questions about random rules in this section:

  • The distance at which an amateur station must protect an FCC monitoring facility from harmful interference is 1 mile. (E1B03)
  • An Environmental Assessment must be submitted to the FCC must be done before placing an amateur station within an officially designated wilderness area or wildlife preserve, or an area listed in the National Register of Historical Places. (E1B04)
  • The amateur station must avoid transmitting during certain hours on frequencies that cause the interference if its signal causes interference to domestic broadcast reception, assuming that the receiver(s) involved are of good engineering design. (E1B08)
  • The highest modulation index permitted at the highest modulation frequency for angle modulation is 1.0. (E1B12)