Signals and Emissions Flashcards
Carriers and modulation: AM, FM, and single sideband; modulation envelope; digital modulation; overmodulation; link budgets and link margins
How is direct binary FSK modulation generated?
A. By keying an FM transmitter with a sub-audible tone
B. By changing an oscillator’s frequency directly with a digital control signal
C. By using a transceiver’s computer data interface protocol to change frequencies
D. By reconfiguring the CW keying input to act as a tone generator
B. By changing an oscillator’s frequency directly with a digital control signal
Hint: FSK (Frequency Shift Keying) = digital control
FSK is a method of rapidly changing the carrier frequency of a transmitter between two different frequencies. These two carrier frequencies are often referred to as the Mark frequency and the Space frequency, or simply Mark and Space. FSK is commonly used for RTTY (radioteletype) as well as ASCII-based transmissions like packet. Since there are only two frequencies to generate, a digital control signal is used. The terms digital, binary, ON-OFF, and two-state are all basically equivalent. This two-state control signal can be used to shift the frequency of an oscillator back and forth between a Mark frequency and a Space frequency. After appropriate amplification, the carrier signal generated by the oscillator can leave the transmitter to energize an antenna.
Quick and dirty mnemonic: “F”requency “S”ignal “K”ontrol.
Silly hint; the correct answer is the only one that has the word “direct” in the answer.
What is the name of the process that changes the phase angle of an RF signal to convey information?
A. Phase convolution
B. Phase modulation
C. Phase transformation
D. Phase inversion
B. Phase modulation
Phase modulation is the name of the process that changes the phase angle of an RF wave to convey information. Phase modulation changes the signal by modifying the instantaneous phase angle of the carrier. PM is often used for data transmission methods such as PSK (phase-shift keying).
What is the name of the process that changes the instantaneous frequency of an RF wave to convey information?
A. Frequency convolution
B. Frequency transformation
C. Frequency conversion
D. Frequency modulation
D. Frequency modulation
Hint: modulate CONVEYs & controls
Frequency modulation (FM) is the process which changes the frequency of an RF wave to convey information. The instantaneous frequency of the carrier is altered to convey the information, while the amplitude remains constant. FM is used for frequency-shift keying, radar in addition to radio voice transmissions.
What emission is produced by a reactance modulator connected to a transmitter RF amplifier stage?
A. Multiplex modulation
B. Phase modulation
C. Amplitude modulation
D. Pulse modulation
B. Phase modulation
Hint: PHASE MODULATION is produced by REACTANCE
Phase modulation is the emission produced by a reactance modulator connected to an RF power amplifier. The reactance modulator changes the instantaneous phase angle of the carrier, generating phase modulated signals.
Silly hint: The child REACTED because they were going through a PHASE.
What type of modulation varies the instantaneous power level of the RF signal?
A. Power modulation
B. Phase modulation
C. Frequency modulation
D. Amplitude modulation
D. Amplitude modulation
Hint: “Amplitude” is the power level of RF.
(D). Amplitude modulation (AM) is the type of modulation that varies the instantaneous power level of the RF signal. The power or signal strength of the wave is measured as the wave amplitude (may also be called the envelope).
Which of the following is characteristic of QPSK31?
A. It is sideband sensitive
B. Its encoding provides error correction
C. Its bandwidth is approximately the same as BPSK31
D. All these choices are correct
D. All these choices are correct
Hint: Quick Provide Sideband = ALL
QPSK31 stands for Quadrature Phase Shift Keying, 31-baud
From http://www.podxs070.com/frequently-asked-questions/introduction-to-psk, here’s an explanation of QPSK31: So what’s different about QPSK31?
The QPSK31 format transmits two bits per phase state, and transmits one of four phase states each time. (If you think about it, there are four possible combinations of two bits, and each of these combinations is assigned a unique phase.) This allows twice as many bits to be sent in the same period of time (and bandwidth) as with BPSK. Martinez coupled this with a rate-1/2, constraint length 5 convolutional code to improve the power efficiency of the modulation (defined as the signal-to-noise ratio required to deliver a specified bit-error-rate, or BER).
This adds a step in the transmission process - generation of the coded bitstream - after the insertion of fill bits. It also adds a corresponding step in the receive process. After the bits are recovered from the phase transition, a Viterbi decoder removes the redundancy inserted by the code, corrects bit errors (up to a point) and recovers the original bit stream for translation of the Varicode.
One interesting note is that while the BPSK31 signal is phase-symmetric, QPSK31 is not. This means that if you are using BPSK, it doesn’t matter if you are using USB or LSB - the signal is the same. If you are using QPSK, both stations must be using the same sideband, or one must use the “invert” function in their software.
Which of the following phone emissions uses the narrowest bandwidth?
A. Single sideband
B. Vestigial sideband
C. Phase modulation
D. Frequency modulation
A. Single sideband
For each part of the signal that is suppressed, the bandwidth will be reduced. By supressing the carrier and filtering the unwanted sideband, the typical bandwidth of a single-sideband signal is reduced to only about 3 kHz.
Which of the following is an effect of overmodulation?
A. Insufficient audio
B. Insufficient bandwidth
C. Frequency drift
D. Excessive bandwidth
D. Excessive bandwidth
Overmodulating the signal causes increased distortion or deviation of the waveform. This increases the formation of spurious emissions with distortion outside the normal bandwidth.
What type of modulation is used by FT8?
A. 8-tone frequency shift keying
B. Vestigial sideband
C. Amplitude compressed AM
D. 8-bit direct sequence spread spectrum
A. 8-tone frequency shift keying
Hint: FT8 means Frequency Tone 8. Bingo!
Silly Hint: It’s the only answer with 8 syllables.
FT8 uses 8 tone FSK (frequency shift keying) to encode time-sequenced digital data transmissions.
From http://physics.princeton.edu/pulsar/K1JT/Release_Notes_1.8.0.txt
Brief Description of the FT8 Protocol:
WSJT-X Version 1.8.0 includes a new mode called FT8, developed by K9AN and K1JT. The mode name “FT8” stands for “Franke and Taylor, 8-FSK modulation”. FT8 uses 15-second T/R sequences and provides 50% or better decoding probability down to -20 dB on an AWGN channel. An auto-sequencing facility includes an option to respond automatically to the first decoded reply to your CQ. FT8 QSOs are 4 times faster than those made with JT65 or JT9. FT8 is an excellent mode for HF DXing and for situations like multi-hop E_s on 6 meters, where deep QSB may make fast and reliable completion of QSOs desirable.
What is meant by the term “flat-topping,” when referring to an amplitude-modulated phone signal?
A. Signal distortion caused by insufficient collector current
B. The transmitter’s automatic level control (ALC) is properly adjusted
C. Signal distortion caused by excessive drive or speech levels
D. The transmitter’s carrier is properly suppressed
C. Signal distortion caused by excessive drive or speech levels
Flat-topping (or clipping) is where overmodulation causes a distortion of the waveform in which the amplitude appears flattened on the oscilloscope, because the maximum levels of output voltage or current are exceeded.
For more info see Wikipedia: Flat-topping or Clipping
(Hint: You will DRIVE faster when on FLAT ground.) {OR} You drive on a flattop slang for a road)
What is the modulation envelope of an AM signal?
A. The waveform created by connecting the peak values of the modulated signal
B. The carrier frequency that contains the signal
C. Spurious signals that envelop nearby frequencies
D. The bandwidth of the modulated signal
A. The waveform created by connecting the peak values of the modulated signal
Hint: Think of an envelope’s four corners, these would be the peak values.
In AM, the frequency is stable and only the amplitude modulates (Amplitude Modulation). When this happens, the carrier signal’s peak value will change during modulation. In FM, the signal’s peak value remains constant.
What is QPSK modulation?
A. Modulation using quasi-parallel to serial conversion to reduce bandwidth
B. Modulation using quadra-pole sideband keying to generate spread spectrum signals
C. Modulation using Fast Fourier Transforms to generate frequencies at the first, second, third, and fourth harmonics of the carrier frequency to improve noise immunity
D. Modulation in which digital data is transmitted using 0-, 90-, 180- and 270-degrees phase shift to represent pairs of bits
D. Modulation in which digital data is transmitted using 0-, 90-, 180- and 270-degrees phase shift to represent pairs of bits
QPSK (Quadrature Phase Shift Keying) modulation is a digital modulation technique used in communication systems to transmit data over radio waves or other transmission media.
In QPSK modulation, the information is encoded into four different phase states of the carrier signal. These phase states are 0 degrees, 90 degrees, 180 degrees, and 270 degrees. Each phase state represents a specific combination of two bits of data.
To transmit the data, the carrier signal’s phase is shifted to match the corresponding phase state of the data. By changing the phase of the carrier signal, the receiver can detect and decode the transmitted information.
QPSK modulation is widely used in various communication systems, including satellite communications, wireless networks, and digital television broadcasting. It is known for its efficiency in transmitting data and its ability to provide higher data rates compared to simpler modulation schemes.
Memory visualization: Think of a QPSK signal like an analog clock; 0, 90, 180, and 270 degrees would be 12, 3, 6, and 9 respectively. There are 4 characters in “QPSK” and 4 phases that the modulation is encoded into, matching the 4 “quarter hour” positions on a clock.
What is a link budget?
A. The financial costs associated with operating a radio link
B. The sum of antenna gains minus system losses
C. The sum of transmit power and antenna gains minus system losses as seen at the receiver
D. The difference between transmit power and receiver sensitivity
C. The sum of transmit power and antenna gains minus system losses as seen at the receiver
A “link” budget can be seen as a series (like “links” of a chain) of events that take place from a transmitter output to the receiver effecting power of signal. Therefore, to calculate the power at receiver we need to know what the power output is from transmitter, plus any gain from antenna such as Dipole 2.15dB, yagi etc, and then deduct any losses, such as distance signal travels, feed line at receiver, mismatched impedance etc, to come up with the final actual power of signal at receiver.
Silly hint: Both the “link budget” and “link margin” questions require math to be performed “at the receiver”, only the correct answers have all three words.
What is link margin?
A. The opposite of fade margin
B. The difference between received power level and minimum required signal level at the input to the receiver
C. Transmit power minus receiver sensitivity
D. Receiver sensitivity plus 3 dB
B. The difference between received power level and minimum required signal level at the input to the receiver
A receiver requires a minimum signal strength to operate effectively to enable signals to be heard (depending on manufacturers specifications). The difference in the minimum value needed by receiver, and the actual power level received is known as the “link” margin.
Hint: Margin = difference
Silly hint: Both the “link budget” and “link margin” questions require math to be performed “at the receiver”, only the correct answers have all three words.
Which mixer input is varied or tuned to convert signals of different frequencies to an intermediate frequency (IF)?
A. Image frequency
B. Local oscillator
C. RF input
D. Beat frequency oscillator
B. Local oscillator
Hint: the local oscillator is the TUNER (VFO) on your receiver
In the front end of a receiver, the incoming signal is at a fixed frequency. The only thing we have control over is the LO frequency. As this local oscillator is varied, a different incoming RF signal is mixed with it and presented to the IF. This facilitates “tuning” the radio to different RF frequencies.
What is the term for interference from a signal at twice the IF frequency from the desired signal?
A. Quadrature response
B. Image response
C. Mixer interference
D. Intermediate interference
B. Image response
In Software Defined Radios (SDRs), “image response” refers to interference that occurs when there is an unwanted signal present at a frequency that is double the Intermediate Frequency (IF) of the desired signal. The IF is a lower frequency to which the received radio signal is converted before further processing in an SDR.
The mixing process in an SDR can sometimes unintentionally create these interference signals at the double IF frequency. Image response interference can degrade the quality and clarity of the desired signal, making it harder to receive and interpret the intended communication.
Silly hint: I for IF, I for image. Also, since this is an SDR thing, you can remember images being transmitted by software.
What is another term for the mixing of two RF signals?
A. Heterodyning
B. Synthesizing
C. Frequency inversion
D. Phase inversion
A. Heterodyning
The heterodyning system uses a local HF oscillator, a mixer, and detector to modulate the carrier signal producing upper and lower sidebands.
Hint: Hetero means different or other like 2 different signals.
What is the stage in a VHF FM transmitter that generates a harmonic of a lower frequency signal to reach the desired operating frequency?
A. Mixer
B. Reactance modulator
C. Balanced converter
D. Multiplier
D. Multiplier
Hint: A harmonic has MULTIPLE sounds
The frequency multiplier is the stage in a VHF FM transmitter that generates a harmonic of a lower frequency signal to reach the desired operating frequency. The frequency multiplier produces signals at harmonic multiples (double, triple, etc) of the modulated signal to bring the frequency to the desired output level.
Which intermodulation products are closest to the original signal frequencies?
A. Second harmonics
B. Even-order
C. Odd-order
D. Intercept point
C. Odd-order
If the original frequencies are near each other, then the difference frequency is relatively close to DC (0). So the odd frequencies like 2f1 - f2 can be rewritten as f1 + (f1-f2), which is relatively close to f1.
Silly hint: Original -> Odd-Order or O O O it’s Magic, you know!
What is the total bandwidth of an FM phone transmission having 5 kHz deviation and 3 kHz modulating frequency?
A. 3 kHz
B. 5 kHz
C. 8 kHz
D. 16 kHz
D. 16 kHz
(fdev+fmod)×2=FM Bandwidth
To calculate the total bandwidth of an FM phone transmission, use Carson’s bandwidth rule by adding together the frequency deviation and the modulating frequency then multiplying the sum by 2:
BW=(fdev+fmod)×2
Where:
BW is the total FM phone bandwidth
fdev is the frequency deviation
fmod is the modulating frequency
So for this question:
fdev=5kHz
fmod=3kHz
Therefore:
BW=(5kHz+3kHz)×2
BW=8kHz×2
BW=16kHz
Memory aid: 5×3=15 and 16 is the closest
What is the frequency deviation for a 12.21 MHz reactance modulated oscillator in a 5 kHz deviation, 146.52 MHz FM phone transmitter?
A. 101.75 Hz
B. 416.7 Hz
C. 5 kHz
D. 60 kHz
B. 416.7 Hz
416.7 Hz is the frequency deviation for a 12.21 MHz reactance modulated oscillator in a 5-kHz deviation, 146.52 MHz FM phone transmitter.
To calculate the frequency deviation, first calculate the multiplication factor of the FM transmitter:
Multiplication Factor = (Transmitter Frequency) / (HF Oscillator Frequency)
= 146.52 MHz / 12.21 MHz = 12
Next, divide the transmitter deviation by the multiplication factor:
Frequency deviation = (Transmitter Deviation) / (Multiplication Factor)
=5 kHz / 12 = 5000 Hz / 12 = 416.7 Hz
Silly Hint: 146 think 416, and 146.52, so 5 + 2 = 7, and 416.7.
Alt: The standard FM deviation allowed is 5 KHz so, 5 ÷ 12 = 0.416 KHz, or 416 Hz
Alt: 5/150 simplifies to 1/30. 1/30th of 12 is 0.4
Why is it important to know the duty cycle of the mode you are using when transmitting?
A. To aid in tuning your transmitter
B. Some modes have high duty cycles that could exceed the transmitter’s average power rating
C. To allow time for the other station to break in during a transmission
D. To prevent overmodulation
B. Some modes have high duty cycles that could exceed the transmitter’s average power rating
Data modes vary in the percentage of time that they are actually transmitting at full power versus the amount of “off time” between signals. This is referred to as the duty cycle. As an example, the intermittant dots and dashes of CW mean that the transmitter is actually only operating at full power for somewhere around 40 to 50% of the time. Some of the RTTY data modes on the other hand, can actually run at full power for close to 100% of the transmission time. Because these modes have such high duty cycles, it may be necessary to reduce the power used so that the transmitter’s average power rating is not exceeded.
SILLY HINT: Do your “duty” “duty”
Why is it good to match receiver bandwidth to the bandwidth of the operating mode?
A. It is required by FCC rules
B. It minimizes power consumption in the receiver
C. It improves impedance matching of the antenna
D. It results in the best signal-to-noise ratio
D. It results in the best signal-to-noise ratio
Matching bandwidths reduces the amount of noise outside the desired frequency range.
Doing so means less energy is lost in filtering, resulting in a better signal-to-noise ratio.
What is the relationship between transmitted symbol rate and bandwidth?
A. Symbol rate and bandwidth are not related
B. Higher symbol rates require wider bandwidth
C. Lower symbol rates require wider bandwidth
D. Bandwidth is half the symbol rate
B. Higher symbol rates require wider bandwidth
The relationship between transmitted symbol rate and bandwidth is that higher symbol rates require higher amounts of bandwidth.
As the symbol rate for a data transmisson increases (baud rate), the amount of bandwidth required to send that signal must also increase in order to maintain a low signal-to-noise ratio.
Hint: More information requires more space.
What combination of a mixer’s Local Oscillator (LO) and RF input frequencies is found in the output?
A. The ratio
B. The average
C. The sum and difference
D. The arithmetic product
C. The sum and difference
Hint: SUM of both gives you DIFFERENCE ( -10 + 5)
A mixer combines two input frequencies to produce an output signal containing both frequencies. For example, a Local Oscillator signal of frequency A is combined with an RF signal of frequency B. When mixed, the output signal will contain a signal of frequencies (A + B) and abs(A - B).
Mixers have many applications, such as in superheterodyne receivers, which convert very high frequencies down to lower frequencies that are more easily handled by the radio. In that case, where you want a lower frequency, the A+B signal will be filtered out.
Sometimes you may want to produce a signal with a higher frequency; in that case you can use a mixer to generate A+B and A-B signals, and simply filter out the A-B signal.
What process combines two signals in a non-linear circuit to produce unwanted spurious outputs?
A. Intermodulation
B. Heterodyning
C. Detection
D. Rolloff
A. Intermodulation
Many situations can result in intermodulation: two signals mixing in the preamp as they are received, two transmitters combining their outputs into a single antenna, two sources of RF near a non-linear junction (the rusty bolt syndrome), or other situations in which two (or more) sources of RF are present in the same area. The result is that the non-linear junction produces the same mixing effect as a mixer, and creates a third (or fourth) signal which is then heard by a receiver.
Hint: Unwanted / Inter(upted)modulation
Which of the following is an odd-order intermodulation product of frequencies F1 and F2?
A. 5F1-3F2
B. 3F1-F2
C. 2F1-F2
D. All these choices are correct
C. 2F1-F2
For even order intermodulation products, the absolute values of the coefficients to the frequencies will add up to an even number–so in the case of 5F1-3F2 the order is 5+3=8, which is even, and for 3F1-1F2 the order is 3+1=4, which is also even. The order for 2F1-1F2 is 2+1=3, so it is an odd-order IM product.
Silly Hint: It’s odd that it’s the even one.
On what band do amateurs share channels with the unlicensed Wi-Fi service?
A. 432 MHz
B. 902 MHz
C. 2.4 GHz
D. 10.7 GHz
C. 2.4 GHz
Which digital mode is used as a low-power beacon for assessing HF propagation?
A. WSPR
B. MFSK16
C. PSK31
D. SSB-SC
A. WSPR
Silly Hint: a low power signal is just a whisper (WSPR)
WSPR (often pronounced as ‘whisper’) is an acronym for Weak Signal Propagation Reporter and was designed by Joe Taylor (also creator of JT-65 and FT-8) to test propagation of HF frequency radio waves by sending low-power signals and logging decoded signals from other locations to WSPRnet. Stations do not respond to decoded signals, thus it acts as a one-way beacon.
The incorrect options, Olivia, PSK31, and SSB-SC, are all two-way communication modes.
What part of a packet radio frame contains the routing and handling information?
A. Directory
B. Preamble
C. Header
D. Trailer
C. Header
When forming a packet for transmission the information is organized into a specific order so that the message segments may be properly reassembled and decoded after all parts are received.
Each data packet contain several fields, including the Header, which contains routing and handling information.
The packet also includes an address field indicating where the message is going; a control field, indicating the type of data; a frame number which indicates the section order; the data field; and the frame check or cyclic redundancy check field.
Which of the following describes Baudot code?
A. A 7-bit code with start, stop, and parity bits
B. A code using error detection and correction
C. A 5-bit code with additional start and stop bits
D. A code using SELCAL and LISTEN
C. A 5-bit code with additional start and stop bits
The Baudot code is a character set used with some radioteletype (RTTY) transmissions. This code assigns each letter of the alphabet with a 5 bit code, with stop and start bits for each character.
Hint: the word “Baudot” has 5 letters plus an additional letter (6 total)
In an ARQ mode, what is meant by a NAK response to a transmitted packet?
A. Request retransmission of the packet
B. Packet was received without error
C. Receiving station connected and ready for transmissions
D. Entire file received correctly
A. Request retransmission of the packet
NAK means Not Acknowledged - that’s what the receiving station sends when it detects an error in the transmitted packet. That NAK is a request to send the packet again, a retransmission of the packet.
What action results from a failure to exchange information due to excessive transmission attempts when using an ARQ mode?
A. The checksum overflows
B. The connection is dropped
C. Packets will be routed incorrectly
D. Encoding reverts to the default character set
B. The connection is dropped
In Automatic Repeat Request (ARQ) mode, each transmission is acknowledged as “received without error” or a “request for retransmission”. After an excessive number of attempts to receive a packet, the receiving or transmitting station will drop the connection.
Which of the following narrow-band digital modes can receive signals with very low signal-to-noise ratios?
A. MSK144
B. FT8
C. AMTOR
D. MFSK32
B. FT8
Hint: “FT8” is the narrowest answer.
FT8 uses an 8-frequency shift keying format. Tones are spaced at 6.25 Hz, and an FT8 signal occupies just 50 Hz.
MSK144 is designed for meteor scatter, would need to worry about low signal-to-noise ratios, but is a wideband protocol designed to move data quickly, given the short windows in which meteor scatter propagation functions. It is generally used on VHF or higher frequencies, given the width.
AMTOR & MFSK32 are not good protocols for very low signal-to-noise ratio HF transmission.
Which of the following statements is true about PSK31?
A. Upper case letters are sent with more power
B. Upper case letters use longer Varicode bit sequences and thus slow down transmission
C. Error correction is used to ensure accurate message reception
D. Higher power is needed as compared to RTTY for similar error rates
B. Upper case letters use longer Varicode bit sequences and thus slow down transmission
Hint: Remember PSK31 uses Varicode for bauds (31 bauds)
Upper case letters use longer Varicode signals and thus slow down transmission
PSK31 uses Varicode.
The idea behind varicode is that symbols often used for transmission like the character “e” is given a smaller length of data, whereas obscure characters like “^” are assigned longer length codes to encode them.
Upper case letters are given longer codes compared to their lower case companions since lower case letters are used much more often than upper case letters.
Note: This is the same concept as International Morse Code - more common letters like “e” (dit) are shorter than less common letters like “h” (dit-dit-dit-dit).
Which is true of mesh network microwave nodes?
A. Having more nodes increases signal strengths
B. If one node fails, a packet may still reach its target station via an alternate node
C. Links between two nodes in a network may have different frequencies and bandwidths
D. More nodes reduce overall microwave out of band interference
B. If one node fails, a packet may still reach its target station via an alternate node
A mesh network is made up of a collection of connected nodes. Each node may be connected to one or more other nodes.
There are usually several paths a packet can take through nodes to reach its target station. If a node on a path fails, the packet might be routed to its target station through an alternate node.
Hint: You microwave a packet of popcorn.
How does forward error correction (FEC) allow the receiver to correct data errors?
A. By controlling transmitter output power for optimum signal strength
B. By using the Varicode character set
C. By transmitting redundant information with the data
D. By using a parity bit with each character
C. By transmitting redundant information with the data
Hint: FEC forwards (TRANSMITS) redundant data
(C). Forward error correction allows the receiver to correct errors in received data packets by transmiting redundant information with the data.
This type of correction used in packet data transmissions is useful especially over “noisy” bands or in case of interference which may corrupt the signal.
Hint: Look for the word “Data” in the answer.
How are the two separate frequencies of a Frequency Shift Keyed (FSK) signal identified?
A. Dot and dash
B. On and off
C. High and low
D. Mark and space
D. Mark and space
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave.[1] The simplest FSK is binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s) information.[2] With this scheme, the “1” is called the mark frequency and the “0” is called the space frequency.
Hint: FSK is “identified” with a “mark”.
Which type of code is used for sending characters in a PSK31 signal?
A. Varicode
B. Viterbi
C. Volumetric
D. Binary
A. Varicode
Hint: since 31 is a baud CODE, remember VARICODE
Hint 2: Both the question and the answer have the word “code” in them.
Varicode is used sending characters in a PSK31 signal.
Unlike coding systems such as Baudot (5 bits per symbol) and ASCII (7 bits per symbol) which use a fixed length sequence to represent characters, PSK31 uses a variable-length encoding (1-10 bits) which assigns more commonly used characters to shorter symbols.
Variable-length encoding is generally more time efficient, at least when using the English language, as it requires fewer total symbols to be transmitted for a given message. Improved efficiency is important because of PSK31’s very low data rate of 31 symbols per second.
The name chosen for the encoding scheme is Varicode which can be remembered as a shortened form of “Variable Length Coding”
Some other digital modes have adopted varicode including PSK63, PSK128 (both essentially faster version of PSK31) and QPSK31, QPSK128, etc. which transmit two bits per symbol instead of one. The FreeDV digital voice mode uses Varicode to efficiently encode a text subchannel in its datastream.
What is indicated on a waterfall display by one or more vertical lines on either side of a data mode or RTTY signal?
A. Long path propagation
B. Backscatter propagation
C. Insufficient modulation
D. Overmodulation
D. Overmodulation
Overmodulation is an often occurring “gotcha” for many new hams intending to work PSK31. The PSK mode requires modulation audio from your computer. However, if the audio volume/amplitude is too high, your radio has no choice but to distort the signal through the use of ALCs (Auto Levelling Circuits). If you’re familiar with audio production, this is a similar process equivalent to Dynamic Range Compression.
The ALC prevents the signal from clipping, but introduces harmonics that manifests itself as copies of the original signal on the left and right of the signal.
SILLY HINT: Waterfalls are vertical. Too much water goes over the side.
The signal near 2050Hz shows this effect. Notice the copies of the signal.
Which of the following describes a waterfall display?
A. Frequency is horizontal, signal strength is vertical, time is intensity
B. Frequency is vertical, signal strength is intensity, time is horizontal
C. Frequency is horizontal, signal strength is intensity, time is vertical
D. Frequency is vertical, signal strength is horizontal, time is intensity
C. Frequency is horizontal, signal strength is intensity, time is vertical
Check out WebSDR to see waterfall displays in action.
Notice that you can move the cursor horizontally along the x-axis in order to select the frequency, and that the display “falls” vertically over time. In other words: the x-axis is frequency, the y-axis is time.
You’ll see a whole bunch of different signals. The CW signals will be very narrow, the data signals will tend to be quite wide, and phone signals somewhere in-between. If the signal is good, the display will show a bright line. A more attenuated line indicates that the signal isn’t as strong. In other words: strength is intensity.
It is often advantageous to monitor the strength and frequency of multiple signals over a short duration of time. A waterfall display accomplishes just this.
If you’ve ever dealt with a spectrum analyzer, spectrogram, or a Fourier transform of a signal, you should notice that the horizontal axis of all the above plots is frequency. A waterfall plot adopts this convention.
(Silly tip: In a waterfall, water FALLS as time goes by).
(Also, try this: A body of water typically runs horizontally, then vertically down a falls with greater intensity).
(Another tip: Frequency Horizontal+Signal Intensity+Time Vertical) = FH+SI+TV = Fuzzy TV)
(Sily tip: A waterfall is vertical. The last word in the correct answer is “vertical”)
What does an FT8 signal report of +3 mean?
A. The signal is 3 times the noise level of an equivalent SSB signal
B. The signal is S3 (weak signals)
C. The signal-to-noise ratio is equivalent to +3dB in a 2.5 kHz bandwidth
D. The signal is 3 dB over S9
C. The signal-to-noise ratio is equivalent to +3dB in a 2.5 kHz bandwidth
FT8 is a digital communications mode which works in high noise situations; it’s not very fast, but it can be used to communicate in situations where the signal strength wouldn’t be good enough to make most other modes possible.
When an FT8 signal report shows a value of +3, it refers to the signal-to-noise ratio (SNR). The SNR is a measure of how much stronger the desired signal is compared to the background noise. In this case, the +3 represents a positive value, indicating that the desired signal is 3 decibels (dB) stronger than the noise level. Since SSB is the mode most commonly used for FT8 the SNR is calculated within the 2.5 kHz bandwidth range.
The SNR measurement helps operators assess the quality of the received signal. A higher positive value like +3 indicates a stronger and clearer signal with less interference from noise. It suggests that the transmission is more likely to be received accurately.
By exchanging signal reports, ham radio operators can gauge the performance of their transmissions and make adjustments if necessary. This information allows them to understand how well their signals are being received and helps ensure effective communication.
So, when an FT8 signal report indicates +3, it means that the desired signal is 3 dB stronger than the noise level within a 2.5 kHz bandwidth. It suggests a good signal quality with a higher chance of successful reception and accurate communication.
Hint: +3 is in both the question and answer.
Which of the following provide digital voice modes?
A. WSPR, MFSK16, and EasyPAL
B. FT8, FT4, and FST4
C. Winlink, PACTOR II, and PACTOR III
D. DMR, D-STAR, and SystemFusion
D. DMR, D-STAR, and SystemFusion
DMR, D-STAR and SystemFusion, although they provide for sending data, also provide for sending digital voice. The other choices do not have voice modes.
Memory aid:
Digital Mobile Radio (DMR)
Digital Smart Technologies for Amateur Radio (D-STAR)
