Extra Class question of the day: Optical devices

Cathode-ray tubes (CRTs) used to the be most common type of display. They were not only used in television sets, but also computer terminals. They have an electron gun which shoots electrons onto a screen which then glows where the electron hits the screen. By sweeping this “beam” both horizontally and vertically, you can display an image on the screen.

To sweep the beam across the CRT, you deflect it by passing it though a set of plates. Varying the voltage will change the angle at which the beam is deflected. Electrostatic deflection is the type of CRT deflection that is better when high-frequency waveforms are to be displayed on the screen. (E6D13)

To accelerate the electron towards the screen, you apply a relatively high anode voltage to it. The higher the voltage, the brighter the CRT will glow. You don’t want to make that voltage too high, however. Exceeding the anode voltage specification can cause a cathode ray tube (CRT) to generate X-rays. (E6D02)

A spot on the CRT screen will glow even after the beam moves onto another spot. Cathode ray tube (CRT) persistence is the length of time the image remains on the screen after the beam is turned off. (E6D01) This characteristic is useful in many different applications.

A more modern type of display is the liquid-crystal display. A liquid-crystal display (LCD) is a display using a crystalline liquid which, in conjunction with polarizing filters, becomes opaque when voltage is applied. (E6D05) The principle advantage of liquid-crystal display (LCD) devices over other types of display devices is that they consume less power. (E6D15)

Unlike the CRT or LCD display, which transform electrical signals into an image, a charge-coupled device is used to transform an image into electrical signals. A charge-coupled device (CCD) samples an analog signal and passes it in stages from the input to the output. (E6D03) One of the things a charge-coupled device (CCD) does in a modern video camera is that it stores photogenerated charges as signals corresponding to pixels. (E6D04)One thing that is NOT true of a charge-coupled device (CCD) is that it is commonly used as an analog-to-digital converter. (E6D14)

Extra Class question of the day: Modulation and demodulation

Modulation is the process of adding some kind of information, including voice and digital information, to a carrier signal. The most common types of modulation that we use in amateur radio are amplitude modulation (AM) and frequency modulation (FM). Single-sideband, or SSB, is a form of amplitude modulation.

To frequency modulate a carrier, a transmitter will sometimes us a modulator that varies the phase of the signal. This is sometimes called phase modulation (PM). One way to generate FM phone emissions is to use a reactance modulator on the oscillator. (E7E01) The function of a reactance modulator is to produce PM signals by using an electrically variable inductance or capacitance. (E7E02) An analog phase modulator functions by varying the tuning of an amplifier tank circuit to produce PM signals. (E7E03)

To boost the higher audio frequencies, a pre-emphasis network is often added to an FM transmitter. (E7E05) For compatibility with transmitters using phase modulation, de-emphasis is commonly used in FM communications receivers. (E7E06)

Amplitude modulation and single-sideband signals are produced using mixer circuits. The carrier frequency and the baseband signals are input to the mixer circuit which produces an amplitude modulated output. The term baseband in radio communications refers to the frequency components present in the modulating signal. (E7E07) The principal frequencies that appear at the output of a mixer circuit are the two input frequencies along with their sum and difference frequencies. (E7E08)

When using a mixer, you must take care not to use too high of a signal at the inputs. Spurious mixer products are generated when an excessive amount of signal energy reaches a mixer circuit. (E7E09)

Single sideband is most often used for phone transmission on the HF bands and for weak-signal operation on the VHF and UHF bands. One way a single-sideband phone signal can be generated is by using a balanced modulator followed by a filter.  (E7E04) A balanced modulator is a type of mixer.  The output of a balanced modulator, however, does not contain the carrier frequency, only the two sidebands.

Modern transceivers use digital signal processing to generate SSB signals. The quadrature method describes a common means of generating an SSB signal when using digital signal processing. (E7E13)

At the receiving station, a modulated signal has to be demodulated. Amplitude modulated signals are often demodulated using a diode detector circuit. A diode detector functions by rectification and filtering of RF signals. (E7E10)

For demodulating SSB signals, you want something a little more sophisticated. A product detector is a type of detector that is well suited for demodulating SSB signals. (E7E11) A product detector is actually a frequency mixer. It takes the product of the modulated signal and a local oscillator, hence the name. In an FM receiver, the circuit for detecting FM signals is a frequency discriminator. (E7E12)

Some modern receivers demodulate a signal entirely in software. These receivers are called software-defined receivers. When referring to a software defined receiver, direct conversion means incoming RF is mixed to “baseband” for analog-to-digital conversion and subsequent processing. (E7E14)

Extra Class question of the day: Volunteer examiner program

The Volunteer Examiner program started in the early 1980s, and has been a boon for amateur radio. Exam sessions are now more accessible than when tests were given by the FCC, meaning that it is much easier to obtain an amateur radio license, and that more people can now enjoy our hobby.

As the name implies, volunteer examiners (VEs) are volunteers. They may not accept any payment for administering tests. They may, however, be reimbursed for some expenses. Preparing, processing, administering and coordinating an examination for an amateur radio license are the types of out-of-pocket expenses that Part 97 rules state that VEs and VECs may be reimbursed. (E1E14)

The organizations that are responsible for accrediting and administering the exams are called Volunteer Examiner Coordinators (VECs). A Volunteer Examiner Coordinator is an organization that has entered into an agreement with the FCC to coordinate amateur operator license examinations. (E1E03) There are currently 14 VECs in the U.S. The procedure by which a VEC confirms that the VE applicant meets FCC requirements to serve as an examiner is the phrase that describes the Volunteer Examiner accreditation process. (E1E04)

The National Conference of Volunteer Examiner Coordinators (NCVEC) is a group made up from representatives of the 14 VECs. The NCVEC is responsible for maintaining the question pools for the three examinations. The questions for all written US amateur license examinations are listed in a question pool maintained by all the VECs. (E1E02)

The rules and procedures for administering the tests are written so that everything is on the up and up. For example, 3 is the minimum number of qualified VEs required to administer an Element 4 amateur operator license examination. (E1E01) Each administering VE is responsible for the proper conduct and necessary supervision during an amateur operator license examination session. (E1E06) Having several VEs, and making them all responsible, leaves very little room for cheating.

VEs are not to show any favoritism. To minimize the chance of this happening, a VE may not administer an examination to relatives of the VE as listed in the FCC rules. (E1E08)

The penalty for a VE who fraudulently administers or certifies an examination can be revocation of the VE’s amateur station license grant and the suspension of the VE’s amateur operator license grant. (E1E09)

Before administering a test, the VEs instruct the candidates of the rules. For example, the candidates are not allowed to consult any books during the test. They may use a calculator, but only if they can demonstrate to a VE that all of the calculator’s memories have been cleared. If a candidate fails to comply with the examiner’s instructions during an amateur operator license examination, a VE should immediately terminate the candidate’s examination. (E1E07)

After the test, three VEs must correct each test sheet. This minimizes the chance for making a scoring mistake. On amateur operator license examinations, there is a minimum passing score of 74%. (E1E05) If an examinee scores a passing grade on all examination elements needed for an upgrade or new license, three VEs must certify that the examinee is qualified for the license grant and that they have complied with the administering VE requirements. (E1E11)

After the administration of a successful examination for an amateur operator license, the VEs must submit the application document to the coordinating VEC according to the coordinating VEC instructions. (E1E10) If the examinee does not pass the exam, the VE team must return the application document to the examinee. (E1E12)

From time to time, a licensee may be asked to re-take a test. The consequences of failing to appear for re-administration of an examination when so directed by the FCC are that the licensee’s license will be cancelled. (E1E13)

Extra Class question of the day: Digital integrated circuits

Integrated circuits (ICs) are now an integral part (pun intended) of amateur radio electronics. The two main technologies used to manufacture IC are transistor-transistor logic, or TTL, and complementary metal-oxide semiconductor, or CMOS.

CMOS is arguably the most common type of digital IC. An advantage of CMOS logic devices over TTL devices is that the have lower power consumption. (E6C05) CMOS digital integrated circuits also have high immunity to noise on the input signal or power supply because the input switching threshold is about one-half the power supply voltage. (E6C06)

TTL is the other common digital logic IC technology. 5 volts is the recommended power supply voltage for TTL series integrated circuits. (E6C01) The inputs of a TTL device assume a logic-high state if they are left open. (E6C02)

BiCMOS logic is an integrated circuit logic family using both bipolar and CMOS transistors. (E6C12) An advantage of BiCMOS logic is that it has the high input impedance of CMOS and the low output impedance of bipolar transistors. (E6C13)

Tri-state logic devices are logic devices with 0, 1, and high impedance output states. (E6C03) These devices can be made with either TTL or CMOS technology. The primary advantage of tri-state logic is the ability to connect many device outputs to a common bus. (EC604) When a device’s outputs are in the high-impedance state, they act as if they are disconnected.

Digital Logic Schematic Symbols

When working with digital ICs, it is important to recognize the various symbols for the different types of logic gates. In Figure E6-5, 1 is the schematic symbol for an AND gate. (E6C07) In Figure E6-5, 2 is the schematic symbol for a NAND gate. (E6C08) In Figure E6-5, 3 is the schematic symbol for an OR gate. (E6C09) In Figure E6-5, 4 is the schematic symbol for a NOR gate. (E6C10) In Figure E6-5, 5 is the schematic symbol for the NOT operation (inverter). (E6C11)

Extra Class question of the day: Receiver performance characteristics

One of the most commonly mentioned HF receiver specifications is blocking dynamic range. The blocking dynamic range of a receiver is the difference in dB between the noise floor and the level of an incoming signal which will cause 1 dB of gain compression. (E4D01) Cross-modulation of the desired signal and desensitization from strong adjacent signals are two problems caused by poor dynamic range in a communications receiver. (E4D02)

Another specification commonly bandied about is third-order intercept level. A third-order intercept level of 40 dBm with respect to receiver performance means a pair of 40 dBm signals will theoretically generate a third-order intermodulation product with the same level as the input signals. (E4D10) Compared to other products, third-order intermodulation products created within a receiver are of particular interest because the third-order product of two signals which are in the band of interest is also likely to be within the band. (E4D11)

The term for the reduction in receiver sensitivity caused by a strong signal near the received frequency is desensitization. (E4D12) Strong adjacent-channel signals can cause receiver desensitization. (E4D13) One way to reduce the likelihood of receiver desensitization is to decrease the RF bandwidth of the receiver. (E4D14)

A preselector might help in some cases. The purpose of the preselector in a communications receiver is to increase rejection of unwanted signals. (E4D09)

When operating a repeater, one thing that can occur is intermodulation interference, or simply intermod. Intermodulation interference is the term for unwanted signals generated by the mixing of two or more signals. (E4D06) Nonlinear circuits or devices cause intermodulation in an electronic circuit. (E4D08)

Intermodulation interference between two repeaters occurs when the repeaters are in close proximity and the signals mix in the final amplifier of one or both transmitters. (E4D03) The transmitter frequencies would cause an intermodulation-product signal in a receiver tuned to 146.70 MHz when a nearby station transmits on 146.52 MHz are 146.34 MHz and 146.61 MHz. (E4D05) We get this in the following way:

2 x 146.52 MHz – 146.34 MHz = 146.70 MHz and

2 x 146.61 MHz – 146.52 MHz = 146.70 MHz

A properly terminated circulator at the output of the transmitter may reduce or eliminate intermodulation interference in a repeater caused by another transmitter operating in close proximity. (E4D04) The circulator reduces intermodulation distortion because it helps to reduce the amount of energy from nearby transmitters that might get into a repeater’s final amplifier.

Cross modulation is a form of intermodulation. Cross modulation occurs when a very strong signal combines with a weaker signal and actually modulates the weaker signal. The most significant effect of an off-frequency signal when it is causing cross-modulation interference to a desired signal is that the off-frequency unwanted signal is heard in addition to the desired signal. (E4D07)

Extra Class question of the day: Waveforms and measurements

An electromagnetic wave is a wave consisting of an electric field and a magnetic field oscillating at right angles to each other. (E8D07) Changing electric and magnetic fields propagate the energy is a phrase that best describes electromagnetic waves traveling in free space. (E8D08)

The polarization of an electromagnetic wave is related to the orientation of the wave’s electric field. If, for example, the electric field is oriented vertically, we say that the electromagnetic wave is vertically polarized. Waves with a rotating electric field are called circularly polarized electromagnetic waves.(E8D09)

Peak-to-peak voltage is the easiest voltage amplitude parameter to measure when viewing a pure sine wave signal on an analog oscilloscope. (E8D01) The relationship between the peak-to-peak voltage and the peak voltage amplitude of a symmetrical waveform is 2:1. (E8D02) Peak voltage is a valuable input-amplitude parameter for evaluating the signal-handling capability of a Class A amplifier.(E8D03)

For sinusoidal voltages, the peak voltage is 1.414 times the RMS voltage, and the peak-to-peak voltage is 2.828 times the RMS voltage. The peak voltage of a sinusoidal waveform would be 48 volts if an RMS-reading voltmeter reads 34 volts. (E8D12) If an RMS-reading AC voltmeter reads 65 volts on a sinusoidal waveform, the peak-to-peak voltage is 184 volts. (E8D05) 

120V AC is a typical value for the RMS voltage at a standard U.S. household electrical power outlet. (E8D15) 170 volts is a typical value for the peak voltage at a standard U.S. household electrical outlet. (E8D13) 340 volts is a typical value for the peak-to-peak voltage at a standard U.S. household electrical outlet. (E8D14) 120V AC is the RMS value of a 340-volt peak-to-peak pure sine wave. (E8D16)

The peak envelope power of a radio signal is equal to V2peak/2 x 1/R. Consequently, the PEP output of a transmitter that develops a peak voltage of 30 volts into a 50-ohm load is 9 watts. (E8D04)

Vpeak = 30 V, V2peak = 900 V2

PEP = 900 V2 / 2 x 50 = 9 W.

The average power of a radio signal is equal to V2RMS/R. The average power dissipated by a 50-ohm resistive load during one complete RF cycle having a peak voltage of 35 volts is 12.2 watts. (E8D11)

V2RMS = 35 V / 1.414 = 24.75V

V2RMS = 612 V2

Pavg = 612 V2 / 50 = 12.2 W.

Radio amateurs most often specify the output power of a single-sideband transmitter as peak envelope power and use a peak-reading wattmeter.  The advantage of using a peak-reading wattmeter to monitor the output of a SSB phone transmitter is that it gives a more accurate display of the PEP output when modulation is present. (E8D06) A peak-reading wattmeter should be used to monitor the output signal of a voice-modulated single-sideband transmitter to ensure you do not exceed the maximum allowable power. (E8D10)

Extra Class question of the day: Transmission line characteristics

The physical length of a coaxial cable transmission line shorter than its electrical length because electrical signals move more slowly in a coaxial cable than in air. (E9F03) The term we use to quantify the difference in how fast a wave travels in air versus how fast it travels in a feedline is velocity factor.

The velocity factor of a transmission line is the velocity of the wave in the transmission line divided by the velocity of light in a vacuum. (E9F01) Put another way, velocity factor is the term for the ratio of the actual speed at which a signal travels through a transmission line to the speed of light in a vacuum. (E9F08) The dielectric materials used in the line determines the velocity factor of a transmission line. (E9F02)

The typical velocity factor for a coaxial cable with solid polyethylene dielectric is 0.66. (E9F04) That makes the approximate physical length of a solid polyethylene dielectric coaxial transmission line that is electrically one-quarter wavelength long at 14.1 MHz about 3.5 meters. (E9F05)The approximate physical length of a solid polyethylene dielectric coaxial transmission line that is electrically one-quarter wavelength long at 7.2 MHz is 6.9 meters. (E9F09)

The velocity factor of air-insulated, parallel conductor transmission lines is a lot closer to 1 than the velocity factor for coaxial cable. The approximate physical length of an air-insulated, parallel conductor transmission line that is electrically one-half wavelength long at 14.10 MHz is 10 meters. (E9F06)

While having a higher velocity factor is not really such a big advantage, open-wire or ladder line feedlines do have other advantages. For example, ladder line has lower loss than small-diameter coaxial cable such as RG-58 at 50 MHz. (E9F07)

Sometimes we use various lengths of coax to match an antenna system or to filter out frequencies. A 1/8-wavelength transmission line presents an inductive reactance to a generator when the line is shorted at the far end. (E9F10) A 1/8-wavelength transmission line presents a capacitive reactance to a generator when the line is open at the far end.

A 1/4-wavelength transmission line presents a very low impedance to a generator when the line is open at the far end. (E9F12) A 1/4-wavelength transmission line presents a very high impedance to a generator when the line is shorted at the far end. (E9F13)

A 1/2-wavelength transmission line presents a very low impedance to a generator when the line is shorted at the far end. (E9F14) A 1/2-wavelength transmission line presents a very high impedance to a generator when the line is open at the far end. (E9F15)

Extra Class question of the day: More on coordinate systems

Admittance is the inverse of impedance. So, in polar coordinates, the impedance of a circuit that has an admittance of 7.09 millisiemens at 45 degrees is 141 ohms at an angle of -45 degrees. (E5C16) You calculate it this way:

|Z| = 1/7.09×10-3 = 141 ohms

The angle is the mirror image about the x axis:

θ = 0 – -45 degrees = 45 degrees

Let’s look at another example. In rectangular coordinates, the impedance of a circuit that has an admittance of 5 millisiemens at -30 degrees is 173 +j100 ohms. (E5C17)

|Z| = 1/5×10-3 = 200 ohms

θ = 0 – -30 degrees = 30 degrees

R = |Z| × cos 30 degrees = 200 × .866 = 173 ohms

X (the reactance part of the impedance) = |Z| × sin 30 degrees = 200 × .5 = +j100

Figure E5-2

Now, let’s take a look at some actual circuits.

On Figure E5-2, the point that best represents the impedance of a series circuit consisting of a 400 ohm resistor and a 38 picofarad capacitor at 14 MHz is Point 4. (E5C19) Right off the bat, we know that the only choices are really Points 2, 4, and 6 because the resistance is 400 ohms. Next, we calculate the capacitive reactance:

XC = 1/2πfC = 1/(2 × 3.14 × 14×106 × 38×10-12) ≈ 300 ohms

Because the reactance is capacitive, it’s plotted as a negative value.

On Figure E5-2, the point that best represents the impedance of a series circuit consisting of a 300 ohm resistor and an 18 microhenry inductor at 3.505 MHz is Point 3. (E5C20) The resistance is 300 ohms and the reactance is:

XL = 2πfL = 2 × 3.14 × 3.505×106 × 18×10-6) ≈ 400 ohms

And, since the reactance is inductive, it’s plotted as a postive value.

On Figure E5-2, the point that best represents the impedance of a series circuit consisting of a 300 ohm resistor and a 19 picofarad capacitor at 21.200 MHz is Point 1. (E5C21) The resistance is 300 ohms, and the reactance is:

XC = 1/2πfC = 1/(2 × 3.14 × 21.2×106 × 19×10-12) ≈ 400 ohms

Because the reactance is capacitive, it’s plotted as a negative value.

On Figure E5-2, the point that best represents the impedance of a series circuit consisting of a 300-ohm resistor, a 0.64-microhenry inductor and an 85-picofarad capacitor at 24.900 MHz is Point 8. (E5C23) This problem is a little tougher because it has both capacitive and inductive reactance.

XC = 1/2πfC = 1/(2 × 3.14 × 29.4×106 × 85×10-12) ≈ 63.7 ohms

XL = 2πfL = 2 × 3.14 × 29.4×106 × 0.64×10-6) ≈ 118.2 ohms

X = XL – XC = 118.2 – 63.7 = 55.5 ohms

Because the net reactance is inductive, it is plotted as a positive value, and because the resistance is 300 ohms, the answer is Point 8.

Extra Class question of the day: Filter types and applications

Different types of filters have different characteristics. For example, a Chebyshev filter is a filter type described as having ripple in the passband and a sharp cutoff. (E7C05)On the other hand, the distinguishing features of an elliptical filter are extremely sharp cutoff with one or more notches in the stop band. (E7C06)

Filters have both amplitude and phase-response characteristics. In some applications, both are important. Digital modes, for example, are most affected by non-linear phase response in a receiver IF filter. (E7C14)

The Chebyshev filter was named for Pafnuty Chebyshev, whose mathematical work led to the development of these filters. Sometimes filters are named for their circuit topoology. Pi is the common name for a filter network which is equivalent to two L networks connected back-to-back with the inductors in series and the capacitors in shunt at the input and output. (E7C11) When you look at the circuit diagram for a filter of this type, you’ll see that it looks like the Greek letter pi.

Often, you’ll choose a filter type for a particular application. For example, to attenuate an interfering carrier signal while receiving an SSB transmission, you would use a notch filter. (E7C07)

Today, many of these filters are implemented using digital signal processing. The kind of digital signal processing audio filter might be used to remove unwanted noise from a received SSB signal is an adaptive filter. (E7C08) The type of digital signal processing filter might be used to generate an SSB signal is a Hilbert-transform filter. (E7C09)

Some filters are used almost exclusively in a particular application. A cavity filter, for example, would be the best choice for use in a 2 meter repeater duplexer. (E7C10)

Extra Class question of the day: Amateur Satellite Service

The amateur satellite service is a radio communications service using amateur radio stations on satellites. (E1D02) In the amateur satellite service, the satellites are called space stations and are remotely controlled by telecommands.

Only 40m, 20m, 17m, 15m, 12m and 10m are the amateur service HF bands have frequencies authorized to space stations. (E1D07) 2 meters is the only VHF amateur service band that has frequencies available for space stations. (E1D08) 70 cm, 23 cm, 13 cm are the amateur service UHF bands that have frequencies available for a space station. (E1D09)

One special provision that a space station must incorporate in order to comply with space station requirements is that the space station must be capable of terminating transmissions by telecommand when directed by the FCC. (E1D06) A telecommand station in the amateur satellite service is an amateur station that transmits communications to initiate, modify or terminate functions of a space station. (E1D03)

An amateur station eligible to be a telecommand stations is any amateur station so designated by the space station licensee, subject to the privileges of the class of operator license held by the control operator. (E1D10) All classes of licensees are authorized to be the control operator of a space station. (E1D05)

Another important concept in the amateur satellite service is the Earth station. An Earth station in the amateur satellite service is an amateur station within 50 km of the Earth’s surface intended for communications with amateur stations by means of objects in space. (E1D04) Any amateur station, subject to the privileges of the class of operator license held by the control operator is eligible to operate as an Earth station. (E1D11)

To obtain information about the operation of the space station itself, many space stations send telemetry. Telemetry is defined as one-way transmission of measurements at a distance from the measuring instrument. (E1D01)