Amateur Extra License Class Chapter 8 Radio Modes and Equipment

Digital Protocols and Modes Symbol Rate, Data Rate, and Bandwidth Data speeds Air link. Speed that data is transmitted over the air. Data stream. Speed that data is transferred between modem & PC. Data throughput. Overall data transfer speed.

Digital Protocols and Modes Symbol Rate, Data Rate, and Bandwidth Data rate = Bits per second (bps). Symbol rate = Symbols per second (baud). Data rate may or may not equal symbol rate. RTTY or 1200 baud packet Data Rate = Symbol Rate. bps = baud 9600 baud packet Data Rate = 2 x Symbol Rate. Bps = 2x baud

Digital Protocols and Modes Symbol Rate, Data Rate, and Bandwidth Required bandwidth. BW = B x K B = Symbol rate in bauds. K = Factor relating to shape of keying envelope.

E2D02 -- What is the definition of baud? The number of data symbols transmitted per second The number of characters transmitted per second The number of characters transmitted per minute The number of words transmitted per minute

E8A15 -- What would the waveform of a stream of digital data bits look like on a conventional oscilloscope? A series of sine waves with evenly spaced gaps A series of pulses with varying patterns A running display of alpha-numeric characters None of the above; this type of signal cannot be seen on a conventional oscilloscope

Digital Protocols and Modes Protocols and Codes Protocol . Set of rules controlling the exchange of digital data. Protocol does not specify method of modulation. e.g. – Packet uses SSB on HF & FM on VHF.

Digital Protocols and Modes Protocols and Codes Code. Method of changing information to digital data. Elements  Individual symbols that make up the code. Code does not specify how data is transmitted. e.g. – Morse code can be sent by radio, flashing light, or sound.

Digital Protocols and Modes Protocols and Codes Most codes use the same number of elements (bits) in each character. Baudot & ASCII are examples. Some codes have a variable number of elements (bits) per character. This is called Varicode. Morse & PSK31 are examples. Morse uses different length elements.

Digital Protocols and Modes Protocols and Codes Morse. 2 Symbols. ❶ = Signal on ⓪ = Signal off. 5 Elements. Dit = ❶. Dah = ❶ ❶ ❶. Inter-element space = ⓪. Inter-character space = ⓪⓪⓪. Inter-word space = ⓪⓪⓪⓪⓪⓪⓪.

Digital Protocols and Modes Protocols and Codes Morse and varicode. PSK31. 2 elements. 0. 1. Number of elements per character varies from 1 to 10. Requires less bandwidth. Two 0’s in a row  Space between characters.

Digital Protocols and Modes Protocols and Codes Baudot. a.k.a. – International Telegraph Alphabet Nr 2 (ITA2) Elements Mark. Space. Characters. Combinations of 5 elements each. Each element = 1 data bit.

Digital Protocols and Modes Protocols and Codes Baudot. 5 bits per character. Maximum of 32 (25) characters. Special characters LTRS & FIGS (shift codes) switch between 2 sets of characters. Maximum of 60 different characters can be represented. Upper-case letters only. Start & stop bits frame each character.

Digital Protocols and Modes Protocols and Codes ASCII. 2 Elements 0. 1. Each element = 1 data bit. 7 or 8 bits per character. 8th bit can be a parity bit. Parity bit used to detect some types of transmission errors. Or the 8th bit could be an additional data bit

Digital Protocols and Modes Protocols and Codes ASCII. Maximum of 128 (27) characters. Both upper & lower case letters can be encoded. 256 (28) maximum characters (if 8 data bits). Start bit at beginning of each character. Stop bit(s) at end of each character. 1, 1.5, or 2 stop bits can be used.

E2E09 -- Which of the following HF digital modes uses variable-length coding for bandwidth efficiency? RTTY PACTOR MT63 PSK31

E8C01 -- Which one of the following digital codes consists of elements having unequal length? ASCII AX.25 Baudot Morse code

E8C02 -- What are some of the differences between the Baudot digital code and ASCII? Baudot uses four data bits per character, ASCII uses seven or eight; Baudot uses one character as a shift code, ASCII has no shift code Baudot uses five data bits per character, ASCII uses seven or eight; Baudot uses two characters as shift codes, ASCII has no shift code Baudot uses six data bits per character, ASCII uses seven or eight; Baudot has no shift code, ASCII uses two characters as shift codes Baudot uses seven data bits per character, ASCII uses eight; Baudot has no shift code, ASCII uses two characters as shift codes

E8C03 -- What is one advantage of using the ASCII code for data communications? It includes built-in error-correction features It contains fewer information bits per character than any other code It is possible to transmit both upper and lower case text It uses one character as a shift code to send numeric and special characters

E8C12 -- What is the advantage of including a parity bit with an ASCII character stream? Faster transmission rate The signal can overpower interfering signals Foreign language characters can be sent Some types of errors can be detected

Digital Protocols and Modes Digital Modes A digital mode consists of a protocol plus a modulation method. Can be used to transmit voice, video, or data. Different FCC emission designators for each type of information. Digital signals can be regenerated several times without error.

Digital Protocols and Modes Digital Modes CW. Actually an AM emission (A1A). Speed usually expressed in words per minute (wpm). Use standard word PARIS. PARIS contains 50 elements. 50 elements in 60 seconds = 0.83 baud. Baud = wpm / 1.2 Typical shape factor (K) for CW is 4.8. BW = (wpm / 1.2) x 4.8 = wpm x 4.

Digital Protocols and Modes Digital Modes CW. Keying envelope  Shape of the leading edge and the trailing edge of each element. Changing keying envelope changes K and consequently changes bandwidth. The slower the rise & fall times of the signal, the narrower the bandwidth.

Digital Protocols and Modes Digital Modes CW. Keying envelope & resulting bandwidth when the rise & fall times are 2 ms.

Digital Protocols and Modes Digital Modes CW. Keying envelope & resulting bandwidth when the rise & fall times are 8 ms.

Digital Protocols and Modes Digital Modes FSK/AFSK. FSK = shifting frequency of oscillator (F1B or F1D). AFSK = modulating SSB transmitter with frequency-shifted tones (J1B or J1D). AFSK with properly adjusted SSB transmitter is not distinguishable from FSK. BW = (K x Shift) + B. Typical value for K is 1.2. BW = (1.2 x 170) + 45.45 ≈ 250 Hz. Selective fading.

Digital Protocols and Modes Digital Modes PSK31. G3PLX developed PSK31 for keyboard-to-keyboard communications. PSK = phase-shift keying. 31 = data rate (31.25 baud). Uses a variable-length code (Varicode). Most common characters have shortest code. Uses 00 as separator between characters. Bandwidth ≈ 37.5 Hz. Narrowest of all HF digital modes, including CW. Special sinusoidal shaping of characters minimizes bandwidth.

Digital Protocols and Modes Digital Modes HF Packet. Uses AX.25 protocol (same as VHF packet). Limited to 300 baud. Mostly FSK at 300 baud. VHF packet uses AFSK at 1200 baud. Not well suited for HF propagation conditions. Needs good conditions with minimal fading. Higher data rate than RTTY, AMTOR, or PSK31 when conditions are good.

Digital Protocols and Modes Digital Modes PACTOR (J2D). PACTOR-I developed by DL6MAA & DK4FV. Overcome shortcomings of AMTOR & HF packet. Works well in weak-signal & high-noise conditions. PACTOR-II & PACTOR-III used today. Automatic repeat request (ARQ) used to eliminate errors. Adjusts speed (“trains”) to match conditions. 5 kbps data rates possible. Used to transfer binary files.

Digital Protocols and Modes Digital Modes Winlink. Not really a mode but a system of modes, protocols, & Internet services to provide e-mail & file transfer services. One of the more poplar applications of PACTOR. Does NOT support direct keyboard-to-keyboard operation.

Digital Protocols and Modes Digital Modes Multitone Protocols. MFSK16. Uses 16 tones to modulate signal. Bandwidth ≈ 316 Hz. Data rate ≈ 63 bps. Includes error correction. MT63. Uses 64 tones to modulate signal. Bandwidth ≈ 1 kHz. Includes extensive error correction.

Digital Protocols and Modes Digital Modes WSJT Protocol. Developed by K1JT for weak-signal VHF/UHF work. Family of 5 digital protocols. FSK441 for meteor scatter. JT65 for moonbounce (EME). Will copy without error signals below the noise level! JT65-HF developed for HF operations. JT6M for 6m meteor scatter. EME for monitoring your own signals bounced off the moon. CW for 15 wpm EME QSO’s.

Digital Protocols and Modes Digital Modes Transmitting digital mode signals. Transmitted signal quality EXTREMELY important! Do NOT overdrive transmitter audio! After setting ALC & microphone gain, perform on-air test. 2nd receiver. Nearby station.

E2D01 -- Which of the following digital modes is especially designed for use for meteor scatter signals? WSPR FSK441 Hellschreiber APRS

E2D03 -- Which of the following digital modes is especially useful for EME communications? FSK441 PACTOR III Olivia JT65

E2D09 -- Under clear communications conditions, which of these digital communications modes has the fastest data throughput? AMTOR 170-Hz shift, 45 baud RTTY PSK31 300-baud packet

E2D12 -- How does JT65 improve EME communications? It can decode signals many dB below the noise floor using FEC It controls the receiver to track Doppler shift It supplies signals to guide the antenna to track the Moon All of these choices are correct

E2E01 -- Which type of modulation is common for data emissions below 30 MHz? DTMF tones modulating an FM signal FSK Pulse modulation Spread spectrum

E2E04 -- What is indicated when one of the ellipses in an FSK crossed-ellipse display suddenly disappears? Selective fading has occurred One of the signal filters has saturated The receiver has drifted 5 kHz from the desired receive frequency The mark and space signal have been inverted

E2E06 -- What is the most common data rate used for HF packet communications? 48 baud 110 baud 300 baud 1200 baud

E2E07 -- What is the typical bandwidth of a properly modulated MFSK16 signal? 31 Hz 316 Hz 550 Hz 2.16 kHz

E2E08 -- Which of the following HF digital modes can be used to transfer binary files? Hellschreiber PACTOR RTTY AMTOR

E2E10 -- Which of these digital communications modes has the narrowest bandwidth? MFSK16 170-Hz shift, 45 baud RTTY PSK31 300-baud packet

E2E11 -- What is the difference between direct FSK and audio FSK? Direct FSK applies the data signal to the transmitter VFO Audio FSK has a superior frequency response Direct FSK uses a DC-coupled data connection Audio FSK can be performed anywhere in the transmit chain

E2E12 -- Which type of digital communication does not support keyboard-to-keyboard operation? Winlink RTTY PSK31 MFSK

E4A09 -- Which of the following describes a good method for measuring the intermodulation distortion of your own PSK signal? Transmit into a dummy load, receive the signal on a second receiver, and feed the audio into the sound card of a computer running an appropriate PSK program Multiply the ALC level on the transmitter during a normal transmission by the average power output Use an RF voltmeter coupled to the transmitter output using appropriate isolation to prevent damage to the meter All of these choices are correct

E8A12 (D) What type of information can be conveyed using digital waveforms? Human speech Video signals Data All of these choices are correct

E8A13 -- What is an advantage of using digital signals instead of analog signals to convey the same information? Less complex circuitry is required for digital signal generation and detection Digital signals always occupy a narrower bandwidth Digital signals can be regenerated multiple times without error All of these choices are correct

E8C04 -- What technique is used to minimize the bandwidth requirements of a PSK31 signal? Zero-sum character encoding Reed-Solomon character encoding Use of sinusoidal data pulses Use of trapezoidal data pulses

E8C05 -- What is the necessary bandwidth of a 13-WPM international Morse code transmission? Approximately 13 Hz Approximately 26 Hz Approximately 52 Hz Approximately 104 Hz

E8C06 -- What is the necessary bandwidth of a 170-hertz shift, 300-baud ASCII transmission? 0.1 Hz 0.3 kHz 0.5 kHz 1.0 kHz

E8C07 -- What is the necessary bandwidth of a 4800-Hz frequency shift, 9600-baud ASCII FM transmission? 15.36 kHz 9.6 kHz 4.8 kHz 5.76 kHz

E8C13 -- What is one advantage of using JT-65 coding? Uses only a 65 Hz bandwidth The ability to decode signals which have a very low signal to noise ratio Easily copied by ear if necessary Permits fast-scan TV transmissions over narrow bandwidth

Digital Protocols and Modes Spread Spectrum Techniques Spreading signal out over a wide bandwidth has the following advantages: Spread spectrum signal sounds like low-level broadband noise to a conventional receiver. Strong on-frequency conventional signals are ignored by a spread-spectrum receiver. By using different spreading algorithms, several different signals can share the same band of frequencies without interfering with each other.

Digital Protocols and Modes Spread Spectrum Techniques Spread-spectrum communications technology was first described on paper by an actress and a musician! In 1941 Hollywood actress Hedy Lamarr and pianist George Antheil described a secure radio link to control torpedos. They received U.S. Patent #2.292.387. The technology was not taken seriously at that time by the U.S. Army and was forgotten until the 1980s, when it became active. Since then the technology has become increasingly popular for applications that involve radio links in hostile environments.

Digital Protocols and Modes Spread Spectrum Techniques Two main types of spread spectrum transmissions are used: Frequency hopping. Direct sequence.

Digital Protocols and Modes Spread Spectrum Techniques Frequency hopping. Transmit frequency is rapidly changed to one of several pre-determined frequencies in a pre-determined sequence. Method invented by Hedy Lamarr and George Antheil.

Digital Protocols and Modes Spread Spectrum Techniques Direct sequence. Phase of transmitted signal is shifted by a very fast pseudo-random binary bit stream.

E2C08 -- Why are received spread-spectrum signals resistant to interference? Signals not using the spectrum-spreading algorithm are suppressed in the receiver The high power used by a spread-spectrum transmitter keeps its signal from being easily overpowered The receiver is always equipped with a digital blanker circuit If interference is detected by the receiver it will signal the transmitter to change frequencies

E2C09 -- How does the spread-spectrum technique of frequency hopping work? If interference is detected by the receiver it will signal the transmitter to change frequencies If interference is detected by the receiver it will signal the transmitter to wait until the frequency is clear A pseudo-random binary bit stream is used to shift the phase of an RF carrier very rapidly in a particular sequence The frequency of the transmitted signal is changed very rapidly according to a particular sequence also used by the receiving station

E8C08 -- What term describes a wide-bandwidth communications system in which the transmitted carrier frequency varies according to some predetermined sequence? Amplitude compandored single sideband AMTOR Time-domain frequency modulation Spread-spectrum communication

E8C09 -- Which of these techniques causes a digital signal to appear as wide-band noise to a conventional receiver? Spread-spectrum Independent sideband Regenerative detection Exponential addition

E8C10 -- What 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? Frequency hopping Direct sequence Time-domain frequency modulation Frequency compandored spread-spectrum

E8C11 -- What spread-spectrum communications technique uses a high speed binary bit stream to shift the phase of an RF carrier? Frequency hopping Direct sequence Binary phase-shift keying Phase compandored spread-spectrum

Digital Protocols and Modes Error Detection and Correction. Error detection Determining when an error has occurred. ASCII parity bit. Detects errors in a single bit. Checksum. Cyclic redundancy check (CRC).

Digital Protocols and Modes Error Detection and Correction. Error correction. Action to correct the error. Automatic repeat request (ARQ). If an error is detected, a request for the data to be repeated is sent back to the sending station. Forward error correction (FEC). Transmit extra data to help identify & correct errors.

E2E02 -- What do the letters FEC mean as they relate to digital operation? Forward Error Correction First Error Correction Fatal Error Correction Final Error Correction

E2E03 -- How is Forward Error Correction implemented? By the receiving station repeating each block of three data characters By transmitting a special algorithm to the receiving station along with the data characters By transmitting extra data that may be used to detect and correct transmission errors By varying the frequency shift of the transmitted signal according to a predefined algorithm

E2E05 -- How does ARQ accomplish error correction? Special binary codes provide automatic correction Special polynomial codes provide automatic correction If errors are detected, redundant data is substituted If errors are detected, a retransmission is requested

Amateur Television Amateur Television (ATV) Many amateurs enjoy sending video or pictures over the air. Often used for public service or emergency operations. Two different types: Fast-scan television (ATV). Slow-scan television (SSTV).

Amateur Television Fast-Scan Television AM television. Most common type. Closely resembles analog broadcast TV. Vestigial sideband. Only a portion of one sideband is transmitted. Reduces bandwidth with simple detector circuitry. Bandwidth = 4MHz to 6 MHz. 420 MHz band or above. ATV repeaters.

Amateur Television Fast-Scan Television Video. North American stations normally use NTSC standard. 525 lines per frame, interlaced. 262.5 lines per field. 60 fields per second. 30 frames per second. Sound subcarrier at 4.5 MHz.

Amateur Television Fast-Scan Television.

Amateur Television Fast-Scan Television.

Amateur Television Fast-Scan Television Video. Video data plus synchronization pulses called “baseband video” or “composite video”. Vertical sync pulses mark beginning of new field. 2 vertical sync pulses per frame. Horizontal sync pulses mark beginning of each scan line. Video is blanked during sync pulses. Chroma burst is a short pulse of 3.5789 MHz signal to keep chroma oscillator synchronized.

Amateur Television Fast-Scan Television Video. Levels measured in IRE units. White = 100 Black = 7.5

Amateur Television Fast-Scan Television Audio NTSC. FM audio sub-carrier 4.5 MHz above video carrier. Audio may be transmitted separately. Different band (2m FM). FM modulate the video carrier.

Amateur Television Fast-Scan Television Components of an analog TV signal.

Amateur Television Fast-Scan Television FM Television. Better image quality for strong signals. Worse weak-signal performance. Does not provide immunity from fading. Bandwidth ranges from 17 MHz to 21 MHz. Due to extremely wide bandwidth, only used on: 1.2 GHz (23cm). 2.4 GHz (13cm). 10.25 GHz (3cm).

E2B01 -- How many times per second is a new frame transmitted in a fast-scan (NTSC) television system? 30 60 90 120

E2B02 -- How many horizontal lines make up a fast-scan (NTSC) television frame? 30 60 525 1080

E2B03 -- How is an interlaced scanning pattern generated in a fast-scan (NTSC) television system? By scanning two fields simultaneously By scanning each field from bottom to top By scanning lines from left to right in one field and right to left in the next By scanning odd numbered lines in one field and even numbered ones in the next

E2B04 -- What is blanking in a video signal? Synchronization of the horizontal and vertical sync pulses Turning off the scanning beam while it is traveling from right to left or from bottom to top Turning off the scanning beam at the conclusion of a transmission Transmitting a black and white test pattern

E2B05 -- Which of the following is an advantage of using vestigial sideband for standard fast- scan TV transmissions? The vestigial sideband carries the audio information The vestigial sideband contains chroma information Vestigial sideband reduces bandwidth while allowing for simple video detector circuitry Vestigial sideband provides high frequency emphasis to sharpen the picture

E2B06 -- What is vestigial sideband modulation? Amplitude modulation in which one complete sideband and a portion of the other are transmitted A type of modulation in which one sideband is inverted Narrow-band FM transmission achieved by filtering one sideband from the audio before frequency modulating the carrier Spread spectrum modulation achieved by applying FM modulation following single sideband amplitude modulation

E2B07 -- What is the name of the signal component that carries color information in NTSC video? Luminance Chroma Hue Spectral Intensity

E2B08 -- Which of the following is a common method of transmitting accompanying audio with amateur fast-scan television? Frequency-modulated sub-carrier A separate VHF or UHF audio link Frequency modulation of the video carrier All of these choices are correct

E2B16 -- Which of the following is the video standard used by North American Fast Scan ATV stations? PAL DRM Scottie NTSC

E2B18 -- On which of the following frequencies is one likely to find FM ATV transmissions? 14.230 MHz 29.6 MHz 52.525 MHz 1255 MHz

Amateur Television Slow-Scan Television

Amateur Television Slow-Scan Television Astronaut Gordon Cooper SSTV broadcast from Faith 7 Astronaut Neil Armstrong SSTV broadcast from Apollo 11

Amateur Television Slow-Scan Television Still images. Any frequency where phone transmissions are allowed. Bandwidth must not exceed normal voice transmission. 14.230 MHz. 100% duty cycle. Vertical Interval Signaling (VIS). Cod transmitted to identify mode being used.

Amateur Television Slow-Scan Television Varying tone frequency gives image brightness. Black = lowest frequency. White = highest frequency. Specific tone frequencies used for horizontal and vertical sync pulses. Frequencies below black frequency.

Amateur Television Slow-Scan Television Black and White. 120 lines per frame. Non-interlaced. 8 seconds per frame. Bandwidth ≈ 2 kHz.

Amateur Television Slow-Scan Television Black and White. Black frequency = 1500 Hz. White frequency = 2300 Hz. Sync pulses = 1200 Hz. Horizontal sync pulse = 5 ms. Vertical sync pulse = 30 ms.

Amateur Television Slow-Scan Television Color. Different encoding formats. Vertical interval signaling (VIS) code identifies format. 128 or 256 lines per frame. Non-interlaced. 120 or 240 lines per frame also used, but less common. 12 seconds to more than 4 minutes per frame. Bandwidth ≈ 3 kHz.

Amateur Television Slow-Scan Television Digital. Digital Radio Mondiale (DRM). Used by shortwave broadcasters for high quality audio. Bandwidth > 4 kHz. Amateurs adapted DRM protocol for image transmission. Bandwidth ≈ 3 kHz. No additional hardware required.

E2B09 -- What hardware, other than a receiver with SSB capability and a suitable computer, is needed to decode SSTV using Digital Radio Mondiale (DRM)? A special IF converter A special front end limiter A special notch filter to remove synchronization pulses No other hardware is needed

E2B10 -- Which of the following is an acceptable bandwidth for Digital Radio Mondiale (DRM) based voice or SSTV digital transmissions made on the HF amateur bands? 3 KHz 10 KHz 15 KHz 20 KHz

E2B11 -- What is the function of the Vertical Interval Signaling (VIS) code transmitted as part of an SSTV transmission? To lock the color burst oscillator in color SSTV images To identify the SSTV mode being used To provide vertical synchronization To identify the call sign of the station transmitting

E2B12 -- How are analog SSTV images typically transmitted on the HF bands? Video is converted to equivalent Baudot representation Video is converted to equivalent ASCII representation Varying tone frequencies representing the video are transmitted using PSK Varying tone frequencies representing the video are transmitted using single sideband

E2B13 -- How many lines are commonly used in each frame on an amateur slow-scan color television picture? 30 to 60 60 or 100 128 or 256 180 or 360

E2B14 -- What aspect of an amateur slow-scan television signal encodes the brightness of the picture? Tone frequency Tone amplitude Sync amplitude Sync frequency

E2B15 -- What signals SSTV receiving equipment to begin a new picture line? Specific tone frequencies Elapsed time Specific tone amplitudes A two-tone signal

E2B17 -- What is the approximate bandwidth of a slow-scan TV signal? 600 Hz 3 kHz 2 MHz 6 MHz

E2B19 -- What special operating frequency restrictions are imposed on slow scan TV transmissions? None; they are allowed on all amateur frequencies They are restricted to 7.245 MHz, 14.245 MHz, 21.345, MHz, and 28.945 MHz They are restricted to phone band segments and their bandwidth can be no greater than that of a voice signal of the same modulation type They are not permitted above 54 MHz

Break

Receiver Performance Good receiver performance is essential to successful amateur radio communications. “If you can’t hear ‘em, you can’t work ‘em!” The topics we will cover in this section will allow you to intelligently compare receivers based on published specifications and test results.

Receiver Performance Sensitivity and Noise Receiver sensitivity is a measure of how weak a signal a receiver can receive. a.k.a. – Minimum discernible signal (MDS). a.k.a. – Noise floor. Determined by noise figure and bandwidth of receiver.

Receiver Performance Sensitivity and Noise Minimum discernible signal (MDS). Expressed in dBm or μV. 0 dBm = 1 mW into 50Ω load (≈223mV). Theoretical minimum = -174 dBm/Hz. Noise power at the input of an ideal receiver with a bandwidth of 1 Hz at room temperature. -174 dBm ≈ 4 x 10-9 mW ( 4 billionth of a mW).

Receiver Performance Sensitivity and Noise Minimum discernible signal (MDS). At HF frequencies with an antenna attached, MDS is determined by atmospheric noise. At VHF frequencies & up, MDS is determined by noise generated inside the front end of the receiver.

Receiver Performance Sensitivity and Noise Minimum discernible signal (MDS). Calculating MDS. MDS = 10 x log(fBW) – 174. Example: What is the MDS of a 400 Hz bandwidth receiver with a noise floor of -174dB/Hz? 10 x log(400) = 26. MDS = 26 – 174 = -148 dB.

Receiver Performance Sensitivity and Noise Noise figure. The noise figure of a receiver is the difference in dB between the noise output of the receiver with no antenna connected and that of an ideal receiver with the same gain & bandwidth. NF = (Internal Noise) / (Theoretical MDS). “Figure of merit” of a receiver. Typically a “good” VHF or UHF preamplifier has a NF ≈ 2dB. Actual noise floor = (Theoretical MDS) + NF.

Receiver Performance Sensitivity and Noise Signal-to-noise ratio (SNR). SNR = (Input signal power) / (Noise power).

E4C04 -- What is the definition of the noise figure of a receiver? The ratio of atmospheric noise to phase noise The noise bandwidth in Hertz compared to the theoretical bandwidth of a resistive network The ratio of thermal noise to atmospheric noise The ratio in dB of the noise generated by the receiver compared to the theoretical minimum noise

E4C05 -- What does a value of -174 dBm/Hz represent with regard to the noise floor of a receiver? The minimum detectable signal as a function of receive frequency The theoretical noise at the input of a perfect receiver at room temperature The noise figure of a 1 Hz bandwidth receiver The galactic noise contribution to minimum detectable signal

E4C06 -- A CW receiver with the AGC off has an equivalent input noise power density of -174 dBm/Hz. What would be the level of an unmodulated carrier input to this receiver that would yield an audio output SNR of 0 dB in a 400 Hz noise bandwidth? 174 dBm -164 dBm -155 dBm -148 dBm

E4C07 -- What does the MDS of a receiver represent? The meter display sensitivity The minimum discernible signal The multiplex distortion stability The maximum detectable spectrum

E4C08 -- How might lowering the noise figure affect receiver performance? It would reduce the signal to noise ratio It would improve weak signal sensitivity It would reduce bandwidth It would increase bandwidth

E4C15 -- What is the primary source of noise that can be heard from an HF receiver with an antenna connected? Detector noise Induction motor noise Receiver front-end noise Atmospheric noise

E6E05 -- Which of the following noise figure values is typical of a low-noise UHF preamplifier? 2 dB -10 dB 44 dBm -20 dBm

Receiver Performance Image Response 2 different frequencies, when mixed with the local oscillator frequency will result in a signal at the IF frequency. Image frequency is the desired frequency plus or minus twice the IF frequency

Receiver Performance Image Response

Receiver Performance Image Response Images can only be reduced by improving the selectivity in the receiver front-end, BEFORE the 1st mixer. Pre-selector.

Receiver Performance Image Response Image reduction made easier when the image frequency is as far as possible from the desired frequency. Use as high an IF frequency as possible. Use a local oscillator frequency above the desired frequency. Superheterodyne.

Receiver Performance Selectivity The ability to select the desired signal & reject all others. Determined by receiver’s ENTIRE filter chain. Filters at RF frequency. Filters at IF frequency. Filters at AF frequency.

Receiver Performance Selectivity Receiver filters. Band pass filter (pre-selector). At input to RF pre-amp. Reduces interference from strong out-of-band signals. Reduces interference from image response.

Receiver Performance Selectivity Receiver filters. Roofing filter. Normally located at the input of the 1st IF amplifier, right after the 1st mixer. Typically VHF (70 MHz is common). Sharp crystal filter wider than bandwidth of widest signal to be received. Reduces IMD from strong signals outside of the filter passband.

Receiver Performance Selectivity Receiver filters. IF filters. In final IF stage. Crystal or mechanical resonator. Selectable for different operating modes. 2.4 kHz to 3.0 kHz for SSB. 500 Hz or less for CW. 300 Hz to 500 Hz for RTTY or most digital modes. Typically use soundcard software with 3.0 kHz filter. Being replaced by DSP filters.

Receiver Performance Selectivity Receiver filters. AF filters. Primarily external DSP filters. Can be narrower than IF filters. Adaptive filters can reduce noise, add notches, etc.

E4C02 -- Which of the following portions of a receiver can be effective in eliminating image signal interference? A front-end filter or pre-selector A narrow IF filter A notch filter A properly adjusted product detector

E4C09 -- Which of the following choices is a good reason for selecting a high frequency for the design of the IF in a conventional HF or VHF communications receiver? Fewer components in the receiver Reduced drift Easier for front-end circuitry to eliminate image responses Improved receiver noise figure

E4C10 -- Which of the following is a desirable amount of selectivity for an amateur RTTY HF receiver? 100 Hz 300 Hz 6000 Hz 2400 Hz

E4C11 -- Which of the following is a desirable amount of selectivity for an amateur SSB phone receiver? 1 kHz 2.4 kHz 4.2 kHz 4.8 kHz

E4C12 -- What is an undesirable effect of using too wide a filter bandwidth in the IF section of a receiver? Output-offset overshoot Filter ringing Thermal-noise distortion Undesired signals may be heard

E4C13 -- How does a narrow-band roofing filter affect receiver performance? It improves sensitivity by reducing front end noise It improves intelligibility by using low Q circuitry to reduce ringing It improves dynamic range by attenuating strong signals near the receive frequency All of these choices are correct

E4C14 -- On which of the following frequencies might a signal be transmitting which is generating a spurious image signal in a receiver tuned to 14.300 MHz and which uses a 455 kHz IF frequency? 13.845 MHz 14.755 MHz 14.445 MHz 15.210 MHz

E4D09 -- What is the purpose of the preselector in a communications receiver? To store often-used frequencies To provide a range of AGC time constants To increase rejection of unwanted signals To allow selection of the optimum RF amplifier device

Receiver Performance Dynamic Range Intermodulation (IMD). Caused by non-linear circuits or devices. 3rd order IMD response extremely important. 3rd order subtractive products are near the desired frequency. fIMD3 = 2 x f1 – f2 or fIMD3 = 2 x f2 – f1 Roofing filters help improve IMD. Help eliminate strong in-band signals near desired signal. Will NOT help if interfering signal is within the filter passband. NOT needed for direct conversion (SDR) receivers.

Receiver Performance Dynamic Range Blocking Dynamic Range. As input signal level is increased, a point is reached where output signal no longer increases linearly. Effects of poor blocking dynamic range. Gain compression or blocking. Desensitization. Nearby signal puts receiver into gain compression, reducing the apparent signal strength of desired signal. Cross-modulation.

Receiver Performance Dynamic Range Blocking Dynamic Range. Blocking dynamic range is difference between minimum discernible signal (MDS) & level where 1 dB of gain compression occurs.

Receiver Performance Dynamic Range Intercept Points. Point at which 2 equal strength signals will mix to produce an IMD product of the same strength. Example: If a pair of 40 dBm signals produce a 3rd-order IMD signal with a strength of 40 dBm, then the receiver has a 3rd-order intercept point of 40 dBm.

Receiver Performance Dynamic Range Intercept Points.

Receiver Performance Dynamic Range Intercept Points. IP2 = 2 x PA – PIM IP3 = (3 x PA – PIM3) / 2 The larger IP2 or IP3, the better the receiver linearity. Intermodulation distortion dynamic range measures the ability of a receiver to avoid generating IMD products. IMD DR3 = 0.667 x (IP3 – MDS)

Receiver Performance Dynamic Range

E4D01 -- What is meant by the blocking dynamic range of a receiver? The difference in dB between the noise floor and the level of an incoming signal which will cause 1 dB of gain compression The minimum difference in dB between the levels of two FM signals which will cause one signal to block the other The difference in dB between the noise floor and the third order intercept point The minimum difference in dB between two signals which produce third order intermodulation products greater than the noise floor 

E4D02 -- Which of the following describes two problems caused by poor dynamic range in a communications receiver? Cross-modulation of the desired signal and desensitization from strong adjacent signals Oscillator instability requiring frequent retuning and loss of ability to recover the opposite sideband Cross-modulation of the desired signal and insufficient audio power to operate the speaker Oscillator instability and severe audio distortion of all but the strongest received signals

E4D05 -- What 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? 146.34 MHz and 146.61 MHz 146.88 MHz and 146.34 MHz 146.10 MHz and 147.30 MHz 173.35 MHz and 139.40 MHz

E4D10 -- What does a third-order intercept level of 40 dBm mean with respect to receiver performance? Signals less than 40 dBm will not generate audible third-order intermodulation products The receiver can tolerate signals up to 40 dB above the noise floor without producing third-order intermodulation products A pair of 40 dBm signals will theoretically generate a third-order intermodulation product with the same level as the input signals A pair of 1 mW input signals will produce a third-order intermodulation product which is 40 dB stronger than the input signal

E4D11 -- Why are third-order intermodulation products created within a receiver of particular interest compared to other products? The third-order product of two signals which are in the band of interest is also likely to be within the band The third-order intercept is much higher than other orders Third-order products are an indication of poor image rejection Third-order intermodulation produces three products for every input signal within the band of interest

E4D12 -- What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency? Desensitization Quieting Cross-modulation interference Squelch gain rollback

EE4D13 -- Which of the following can cause receiver desensitization? Audio gain adjusted too low Strong adjacent-channel signals Audio bias adjusted too high Squelch gain misadjusted

E4D14 -- Which of the following is a way to reduce the likelihood of receiver desensitization? Decrease the RF bandwidth of the receiver Raise the receiver IF frequency Increase the receiver front end gain Switch from fast AGC to slow AGC

Receiver Performance Phase Noise. Problem became apparent when receivers got better (lower noise floor). Caused by phase jitter in PLL or DDS oscillator. Increasing noise level as you tune close to a strong signal. Noise can interfere with reception of a weak signal close to the strong one.

E4C01 -- What is an effect of excessive phase noise in the local oscillator section of a receiver? It limits the receiver’s ability to receive strong signals It reduces receiver sensitivity It decreases receiver third-order intermodulation distortion dynamic range It can cause strong signals on nearby frequencies to interfere with reception of weak signals

Receiver Performance Capture Effect. FM receivers behave differently than AM/SSB/CW receivers in the presence of QRM. AM/SSB/CW reception of an S9 signal seriously degraded by an S2 interfering signal. If 2 or more FM signals are on the same frequency, only the strongest one is demodulated. Capture effect. Not really a receiver issue. 3 dB stronger or so.

E4C03 -- What is the term for the blocking of one FM phone signal by another, stronger FM phone signal? Desensitization Cross-modulation interference Capture effect Frequency discrimination

Questions?