There were nine new questions in this chapter, including questions on FT8, WSPR, mixers, intermodulation, and waterfall displays…..Dan
Carriers and modulation: AM; FM, single and double sideband, modulation envelope, overmodulation
Modulation is the process by which we convey information over a radio link, and amplitude modulation (AM) is perhaps the most basic type of modulation. When we amplitude modulate a signal, what we’re doing is varying the amplitude of what we call the RF carrier signal in proportion to the amplitude of an audio signal. This, in turn, varies the instantaneous power level of the signal.
The modulation envelope is the smooth curve that connects the peaks of the modulated signal. When we demodulate an AM signal, what we get is a waveform that looks like the modulation envelope. The figure below shows an amplitude modulated signal and the modulation envelope.
An amplitude-modulated carrier.
QUESTION: What type of modulation varies the instantaneous power level of the RF signal? (G8A05)
ANSWER: Amplitude modulation
QUESTION: What is the modulation envelope of an AM signal? (G8A11)
ANSWER: The waveform created by connecting the peak values of the modulated signal
Single sideband, or SSB, is a type of amplitude modulation. A conventional AM signal has three components, the carrier and two sidebands. Since the carrier signal carries no information and the information in both sidebands is the same, someone figured out that by eliminating the carrier and one of the sidebands you could still transfer the same amount of information.
One of the advantage of SSB voice communications is that it has a narrower bandwidth than AM, FM, or phase modulation. An AM signal requires a bandwidth of 6 kHz or more, and an FM signal can require up to 15 kHz, but a single sideband signal takes up a bandwidth of only 3 kHz.
QUESTION: Which of the following phone emissions uses the narrowest bandwidth? (G8A07)
ANSWER: Single sideband.
You must be careful when setting the audio level used to modulate an AM or SSB phone signal. If you set the level too high, the signal will be over-modulated, and this may cause your signal to be distorted due to “flat-topping.” It can also cause your signal to take up more bandwidth than is allowed.
QUESTION: What is meant by the term “flat-topping,” when referring to a single sideband phone transmission? (G8A10)
ANSWER: Signal distortion caused by excessive drive
QUESTION: Which of the following is an effect of overmodulation? (G8A08)
ANSWER: Excessive bandwidth.
Another type of modulation that we use in amateur radio is frequency modulation, or FM. In an FM signal, the carrier frequency changes proportionally to the instantaneous amplitude of the modulating signal. Most VHF and UHF repeaters use frequency modulation.
QUESTION: What is the name of the process that changes the instantaneous frequency of an RF wave to convey information? (G8A03)
ANSWER: Frequency modulation
Another way to produce a frequency-modulated signal is to use phase modulation (PM). As the name implies, to produce a phase modulated signal, the modulator changes the phase angle of the signal. The modulator used to produce a PM signal is a reactance modulator. Changes in reactance cause a phase shift in the output signal.
QUESTION: What is the name of the process that changes the phase angle of an RF signal to convey information? (G8A02)
ANSWER: Phase modulation
QUESTION: What emission is produced by a reactance modulator connected to a transmitter RF amplifier stage? (G8A04)
ANSWER: Phase modulation
Some modes, such as radio teletype (RTTY) use a modulation method called frequency-shift keying, or FSK. An RTTY signal is a digital signal consisting of “marks” and “spaces.” A mark is sent at one frequency, and the space at another. The RTTY signal, therefore, alternates between one and the other.
QUESTION: How is an FSK signal generated? (G8A01)
ANSWER: By changing an oscillator’s frequency directly with a digital control signal
Another way to send digital information is by using phase-shift keying (PSK). When you send a PSK signal, you’re changing the phase of the modulation, not the frequency of the modulated signal. In a binary phase-shift keying, or BPSK, signal you shift the phase 180 degrees to send a one or a zero.
Some phase-shift keying methods, however, let you send more than one bit at a time. Quadrature phase-shift keying, or QPSK, for example, allows you to send two bits at a time. It does this by allowing more phase-shift options than BPSK. A BPSK signal will only shift 180 degrees, either plus or minus. A QPSK signal, however can shift 45, 135, 225, or 315 degrees. Using QPSK allows you to send data faster than you can with BPSK, but the cost is a slightly higher bandwidth.
QUESTION: Which of the following is characteristic of QPSK31? (G8A06)
ANSWER: The bandwidth is slightly higher than BPSK31
One of the most popular new digital modes is FT8. FT8 get its name from the fact that it was developed by Frankie and Joe Taylor, K1JT, and uses eight-tone frequency shift keying modulation. To operate FT8, you use the WSJT software, which uses some very sophisticated digital processing to allow it to receive very weak signals with low signal-to-noise ratios.
QUESTION: What type of modulation is used by the FT8 digital mode? (G8A09)
ANSWER: 8-tone frequency shift keying
QUESTION: Which of the following narrow-band digital modes can receive signals with very low signal-to-noise ratios? (G8A12)
ANSWER: FT8
Frequency mixing, multiplication, bandwidths of various modes, deviation; duty cycle
One of the most important circuits found in amateur radio equipment is the mixer. A mixer takes two input signals and outputs the sum and difference of the two input signals. This process is called heterodyning.
QUESTION: What is another term for the mixing of two RF signals? (G8B03)
ANSWER: Heterodyning
QUESTION: What combination of a mixer’s Local Oscillator (LO) and RF input frequencies is found in the output? (G8B11)
ANSWER: The sum and difference
In a superheterodyne receiver, the amplified signal from the antenna is combined with the signal from a circuit called the local oscillator to produce a signal at an intermediate frequency, or IF. When you tune the radio, you’re actually changing the frequency of the local oscillator.
To see how this might work, consider this simple example. Say you want to receive a signal on 14.250 MHz, and the IF of your receiver is 455 kHz. You would then tune the local oscillator to 13.795 MHz because 14.250 – 13.795 MHz = 455 kHz. To tune the receiver to 14.240 MHz, you’d set the local oscillator to 13.785 MHz.
QUESTION: Which mixer input is varied or tuned to convert signals of different frequencies to an intermediate frequency (IF)? (G8B01)
ANSWER: Local oscillator
One problem with simple superheterodyne receivers is that two different frequencies will actually produce a 455 kHz mixer output because the mixer outputs both the sum and the difference of the two input frequencies. Say that you tune your local oscillator to 13.800 MHz to receive a 14.255 MHz signal. If your receiver does not have good input filtering, then a signal at 13.345 MHz will also produce a 455 kHz mixer output and you could hear signals on both frequencies simultaneously. We call this type of behavior an image response.
QUESTION: If a receiver mixes a 13.800 MHz VFO with a 14.255 MHz received signal to produce a 455 kHz intermediate frequency (IF) signal, what type of interference will a 13.345 MHz signal produce in the receiver? (G8B02)
ANSWER: Image response
Another thing that can happen is that the mixer does not operate in a linear fashion. When this happens, the two signals at the input of the mixer can produce outputs at random frequencies. This is called intermodulation, or simply “intermod.” This can happen in any non-linear circuit or connection.
QUESTION: What process combines two signals in a non-linear circuit or connection to produce unwanted spurious outputs? (G8B12)
ANSWER: Intermodulation
FM transmitters use multipliers to produce a VHF signal. They often generate an FM signal in the 12 MHz range and then multiply that signal by 12 to output a signal in the 2 m band (144- 148 MHz).
QUESTION: What is the stage in a VHF FM transmitter that generates a harmonic of a lower frequency signal to reach the desired operating frequency? (G8B04)
ANSWER: Multiplier
When modulating the 12 MHz signal, you must use a proportionally smaller deviation if you plan to multiply that signal to the 2 m band. If you are going to multiply the 12 MHz signal 12 times, then the deviation must be 1/12 of the deviation when transmitting in the 2 m band.
QUESTION: What is the frequency deviation for a 12.21 MHz reactance modulated oscillator in a 5 kHz deviation, 146.52 MHz FM phone transmitter? (G8B07)
ANSWER: 416.7 Hz
5 kHz x 1/12 = 416.7 kHz
FM phone is often used on the VHF and UHF bands, but not below 29.5 MHz because it requires a fair amount of bandwidth. The equation for calculating the bandwidth of an FM signal is:
BW=2(Δf+fm)
where Δf = deviation, BW = total bandwidth (for 98% power), and fm = modulating frequency.
QUESTION: What is the total bandwidth of an FM phone transmission having 5 kHz deviation and 3 kHz modulating frequency? (G8B06)
ANSWER: 16 kHz
“Digital modes,” such as PACTOR-III, are getting more popular on the HF bands. The bandwidth of a PACTOR-III is about the same as an SSB signal, or 2300 Hz.
QUESTION: What is the approximate bandwidth of a PACTOR-III signal at maximum data rate? (G8B05)
ANSWER: 2300 Hz
In general, digital modes have much higher duty cycles than the traditional HF modes, such as CW or SSB. It is important to know this because most transmitters used in amateur radio are not designed to be operated at 100% duty cycle. That is to say that they may be capable of outputting 100 W, but not continuously for an extended time.
QUESTION: Why is it important to know the duty cycle of the mode you are using when transmitting? (G8B08)
ANSWER: Some modes have high duty cycles which could exceed the transmitter’s average power rating
It’s also important to know the symbol rate of the digital mode that you’re operating. The reason for this is that digital modes that transfer data at a high rate need more bandwidth than modes that send data at a slower rate.
QUESTION: What is the relationship between transmitted symbol rate and bandwidth? (G8B10)
ANSWER: Higher symbol rates require higher bandwidth
Noise can be a problem when operating digital modes because noise can cause errors, and many of these digital modes do not have error correction. One way to minimize the effects of noise is to use filters to filter out the noise and to adjust your receiver’s bandwidth so that it only passes the signal that you’re interested in. This is also true for modes other than digital modes, such as CW or SSB.
QUESTION: Why is it good to match receiver bandwidth to the bandwidth of the operating mode? (G8B09)
ANSWER: It results in the best signal to noise ratio
Digital emission modes
Digital modes have become very popular in amateur radio. To use the digital modes, you connect a computer to your radio, and a computer program then encodes and decodes the digital signal.
PSK 31 is one of the more popular digital mode on the HF bands. It’s called PSK31 because the transmitted symbol rate is approximately 31 baud. That’s pretty slow, but it is about as fast as a person can type. In addition, its bandwidth is approximately 31 Hz.
QUESTION: What does the number 31 represent in “PSK31”? (G8C09)
ANSWER: The approximate transmitted symbol rate
PSK31 can be a very efficient mode. One reason for this is that it encodes characters with a type of code called Varicode. It is called Varicode because the number of data bits sent in a single PSK31 character varies. More frequently used characters use fewer bits, while less frequently used characters, such as upper case letters, use more bits.
QUESTION: Which type of code is used for sending characters in a PSK31 signal? (G8C12)
ANSWER: Varicode
QUESTION: Which of the following statements is true about PSK31? (G8C08)
ANSWER: Upper case letters use longer Varicode symnbols and thus slow down transmission
Another popular digital mode is radio teletype, or RTTY. In the past, we used clunky, mechanical teletype machines to send and receive RTTY, but now most amateurs use a computer to operate this mode.
RTTY uses the Baudot code, which is really a pretty ancient code. It only uses five bits per character, meaning that to send letters, numbers, and punctuation, you have to shift between two different sets of characters.
QUESTION: Which of the following describes Baudot code? (G8C04)
ANSWER: A 5-bit code with additional start and stop bits
RTTY uses frequency-shift keying to send the ones and zeroes. One frequency is interpreted as a one, or “mark.” The other frequency is interpreted as a zero or “space.”
QUESTION: How are the two separate frequencies of a Frequency Shift Keyed (FSK) signal identified? (G8C11)
ANSWER: Mark and space
PSK31 and RTTY are “general purpose” modes. That is to say that they are used by amateurs for ragchewing, contesting, DXing, and other activities. One digital mode that is more specialized in nature is Weak Signal Propagation Reporter, or WSPR for short. WSPR is design to find potential propagation paths with low-power transmissions. Transmissions carry a station’s callsign, Maidenhead grid locator, and transmitter power in dBm. The program used to receive WSPR signals can decode signals with signal-to-noise ratios as low as −28 dB. Stations with internet access can automatically upload their reception reports to a central database called WSPRnet, which displays the paths over which signals are being received.
QUESTION: Which digital mode is used as a low-power beacon for assessing HF propagation? (G8C02)
ANSWER: WSPR
Packet radio is a form of digital switching technology used by several different digital modes, including APRS and PACTOR. In packet radio, data transmitted from one station to another are bundled up in packets. In addition to the actual data, a packet contains a header, which tells computers along the way how to handle and route the packet.
QUESTION: What part of a packet radio frame contains the routing and handling information? (G8C03)
ANSWER: Header
PACTOR is a particularly robust mode that uses packet radio technology to extend Internet functionality to remote users via amateur radio. The reason amateurs use PACTOR is that it can detect data errors and request that packets be resent. There is a limit, however. If the radio link is poor, and too many retransmissions are requested, the two stations will simply stop trying.
QUESTION: In the PACTOR protocol, what is meant by an NAK response to a transmitted packet? (G8C05)
ANSWER: The receiver is requesting the packet be retransmitted
QUESTION: What action results from a failure to exchange information due to excessive transmission attempts when using PACTOR or WINMOR? (G8C06)
ANSWER: The connection is dropped
Another method for controlling errors when sending and receiving data packets is called Automatic Repeat Request, or ARQ.
QUESTION: How does the receiving station respond to an ARQ data mode packet containing errors? (G8C07)
ANSWER: Requests the packet be retransmitted
Some digital modes, such as FreeDV use what’s called forward error correction, which allows the receiving station to not only detect errors in a data packet, but also to correct them. It does this by sending redundant data. The obvious disadvantage of this approach is that it is slower than other data modes because redundant data needs to be sent.
QUESTION: How does forward error correction (FEC) allow the receiver to correct errors in received data packets? (G8C10)
ANSWER: By transmitting redundant information with the data
To operate most digital modes, you use software that supports that particular mode. For example, to operate PSK31, you might use a program called fldigi. To operate FT8, you use a program called WSJT. One thing that these programs have in common is that they display the signals they are receiving in what’s called a waterfall display.
The waterfall display is a time record of the intensity of signals over a range of frequencies. Waterfall displays are really pretty cool. You get a quick indication of the activity on a band and can quickly tune in stations that you want to contact. Waterfall displays can also give you an indication of the quality of the signals that you’re receiving. You can, for example, quickly see if a station is overmodulating.
QUESTION: Which of the following describes a waterfall display? (G8C14)
ANSWER: Frequency is horizontal, signal strength is intensity, time is vertical
QUESTION: What is indicated on a waterfall display by one or more vertical lines on either side of a digital signal? (G8C13)
ANSWER: Overmodulation
On some bands, amateurs share frequencies with other services. For example, some 2.4 GHz WiFi frequencies actually fall within the amateur radio band. As a result, amateurs are able to use commercial WiFi equipment, but are not encumbered by some of the restrictions on that equipment. For example, if you really wanted to, you could operate with high power and directional antennas to allow you to set up some very wide area networks. In fact, some amateur radio operators do this.
QUESTION: On what band do amateurs share channels with the unlicensed Wi-Fi service? (G8C01)
ANSWER: 2.4 GHz
Dave New, N8SBE says
“RTTY uses the Baudot code, which is is really a pretty ancient code” –> “RTTY uses the Baudot code, which is really a pretty ancient code”
“meaning that to letters, numbers, and punctuation” –> “meaning that to send and receive letters, numbers, and punctuation”
“A 5-bit code with additional start and stop bits” (the last word “bits” needs to be bold-faced).
“ANSWER: Mark and space” (need a blank line after this answer)
“One digital that is more specialized” –> “One digital mode that is more specialized”
“signal-to-noise rations as low as −28 dB” –> “signal-to-noise ratios as low as −28 dB”
“Waterfall displays can also give you an indication of the quality ofo the signals” –> “Waterfall displays can also give you an indication of the quality of the signals”
Dan KB6NU says
Yipes. This chapter really was typo city. Thanks for catching all those.