Signals and Emissions Flashcards

Question

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

Answer 1

C. The sum and difference ## Footnote 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.

Answer 2

A. Intermodulation ## Footnote 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

Answer 3

C. 2F1-F2 ## Footnote 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.

Answer 4

C. 2.4 GHz

Answer 5

A. WSPR ## Footnote 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.

Answer 6

C. Header ## Footnote 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.

Answer 7

C. A 5-bit code with additional start and stop bits ## Footnote 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)

Answer 8

A. Request retransmission of the packet ## Footnote 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.

Answer 9

B. The connection is dropped ## Footnote 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.

Answer 10

B. FT8 ## Footnote 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.

Answer 11

B. Upper case letters use longer Varicode bit sequences and thus slow down transmission ## Footnote 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).

Answer 12

B. If one node fails, a packet may still reach its target station via an alternate node ## Footnote 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.

Answer 13

C. By transmitting redundant information with the data ## Footnote 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.

Answer 14

D. Mark and space ## Footnote 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".

Answer 15

A. Varicode ## Footnote 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.

Answer 16

D. Overmodulation ## Footnote 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.

Answer 17

C. Frequency is horizontal, signal strength is intensity, time is vertical ## Footnote 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")

Answer 18

C. The signal-to-noise ratio is equivalent to +3dB in a 2.5 kHz bandwidth ## Footnote 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.

Answer 19

D. DMR, D-STAR, and SystemFusion ## Footnote 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)

Signals and Emissions Flashcards

Carriers and modulation: AM, FM, and single sideband; modulation envelope; digital modulation; overmodulation; link budgets and link margins