Transmitters and Modulation

Transmitters and Modulation VE1FA 1915 1929 1952 1990s

A transmitter does the following: 1. Generate an RF carrier from DC power 2. Increases the strength of the carrier 3. Superimposes information on the carrier

CW Transmitter Master Oscillator Buffer Power Amplifier Antenna Power Supply Telegraph Key

Master Oscillator Makes radio frequency (RF) energy from DC. Oscillator: amplifier with positive feedback Designed to minimize unintended frequency variations (electrically and mechanically stable). Drift due to: heat, vibration, aging, DC power fluctuations, load changes, etc. Oscillator is often run at a fraction of the transmit frequency.

CW Transmitter Master Oscillator Buffer Power Amplifier Antenna Power Supply Telegrap h Key

Driver /Buffer Amplifier Buffer : isolates master oscillator from its load, the power amplifier (PA). Prevents load variations from affecting master oscillator frequency. Driver : increases RF power

Power Amplifier Amplifies RF from the buffer stage to desired output power. Often has bandpass filter, and matches output Z to 50 ohms. Keying can be of osc, buf, or power amps: often osc is keyed.

Power supply : Converts AC => appropriate DC voltage + current for transmitter (Tx). Modern 100W HF transceiver: 13.8VDC @ 21A Typical 2-5W 2m H/T: 7.6-15 VDC @ 0.5-1.0 A Typical 100W tube transceiver: 6.3 V @ 8A 250 VDC @200 ma 800 VDC @250 ma -65 VDC @ 5 ma

Telegraphy (CW) Key used to turn RF carrier on and off by grounding amp Morse dits are one time unit long; dahs are 3 time units long. Simplest transmitter type

Advantages of CW Narrow signal => many signals fit in a narrow band. => narrow Rx bandwidth => good S/N ratio! CW punches through under poor conditions. No accent to hinder comms. CW is its own language, with many abbreviations and Q signals.

Sending CW Straight key - still used by many. Mechanical bug - makes Dits, but Dahs still made manually. Paddles + electronic keyer- automatically make Dits and Dahs. Keyboard + computer- generate code and key the transmitter. Also can read CW and display it on the screen. Leave at least 150-500 Hz separation between your frequency and a contact (QSO) in progress.

Straight Key Paddles (need keyer) Bug

Modulation: Tx carrier (RF energy) can carry info by: 1. --off/on (CW) 2. amplitude modulation (AM + SSB/DSB) 3. frequency modulation (FM) 4. phase modulation (PM) 5. frequency shift keying (FSK), radioteletype (RTTY) and digital modes, like AMTOR, PACTOR, packet, PSK-31, etc.

AM Transmitter Variable Frequency Oscillator Buffer Frequency Multiplier Speech Amplifier Modulator Power Amplifier Microphone Antenna

AM Transmitter Variable Frequency Oscillator Buffer Frequency Multiplier x2, x3, x4, x5, etc Speech Amplifier Modulator Power Amplifier Microphone Antenna

AM Transmitter Variable Frequency Oscillator Buffer Frequency Multiplier Speech Amplifier Modulator Power Amplifier Microphone Antenna

Audio Signal Modulation Envelope modulator (a type of mixer)

Amplitude Modulation (AM) Instantaneous power in the radio wave varies in time with a modulating signal. Amplitude of the carrier is made to vary in accordance with the modulating audio. Power from high level modulator adds to Tx output power

Peak Envelope Power (PEP) The power delivered to the antenna transmission line during one RF cycle, at the crest of the modulation envelope. PEP

Modulation Percentage (as seen on oscilloscope) 0% modulation (just carrier) 100% modulation (with pure audio tone)

Modulation Percentage 30% (undermodulated) overmodulated (low level) overmodulated (high level)

AM Signal Quality Overmodulation => distortion => audio peaks will cut off the carrier => bad audio and spurious frequencies Ensure Mic. Gain and Automatic Level Control (ALC) set correctly. And/Or: monitor signal with oscilloscope More audio not necessarily better!

Mixing 2 Signals Together The 4 outputs are: 1000 Hz(audio) 3.8 MHz 3.799 MHz 3.801 MHz Amplitude Frequency 3.800 3.799 3.801

Sidebands 3 3 6 3

Let s Suppress the Carrier Puts all power into the two sidebands: => Double Sideband, Suppressed Carrier (DSB) Each DSB sideband carries same info. So DSB takes up twice the bandwidth required

Upper Sideband (USB). Single Sideband Lower Sideband (LSB). Advantages: If 100W in AM sig (25W /sideband) all put into one sideband, 4x the communicating power! Can use narrower Rx bandpass => better S/N ratio. No carrier heterodynes! Transmitter is putting out full power only on voice peaks.

SSB Transmitter RF Oscillator Balanced Modulator Sideband Filter Microphone Speech Amplifier Mixer Variable Frequency Oscillator Antenna Linear Amplifier

Balanced Modulator Fed with RF carrier and audio modulation Passes 2 AM sidebands while cancelling original RF carrier. (DSB) If 500 khz RF carrier is mixed with 2 khz audio the output is 498 khz (LSB) + 502 khz (USB). Power 498 502 Frequency =>

Sharp (very selective) frequency filters Collins mechanical filter Inrad crystal filter

Notes on SSB LSB used on 160, 80 and 40m. USB used on all other bands. Amateurs specify carrier frequency when giving their SSB operating frequency, even though no signal there! Amateur SSB: started in late 1950s. 1970s: SSB was standard voice mode on HF

Frequency Modulation (FM) Carrier frequency varied in accordance with the amplitude of the audio modulation. Amount by which the carrier frequency is raised or lowered from its normal (unmodulated) value is called the deviation. Rate at which carrier deviates is proportional to the frequency of the modulating audio.

FM Transmitter Microphone Speech Amplifier Modulator Master Oscillator Antenna Power Amplifier Frequency Multiplier

Audio signal React. Modulator/MO

FM Modulator Often a reactance modulator : the master oscillator frequency-determining circuit is altered by the audio (via a varactor diode) Frequency variation is usually small, but increased by the frequency multiplication factor. Master oscillator is generally operated at a fraction (say 1/12 th ) of the desired frequency.

FM Transmitter Microphone Speech Amplifier Modulator Master Oscillator Antenna Power Amplifier Frequency Multiplier

FM Advantages -- very quiet (interference and static resistant) -- very good audio quality Disadvantages -- occupies lots of space: 15kHz ham, 50kHz com -- poor S/N on weak signals Notes: -- never used below 29 MHz -- commonest mode in VHF -- invented in early 30s by EH Armstrong

Phase Modulation Carrier phase instead of frequency is changed by audio modulation. Phase Modulation is also generated in a reactance modulator/master osc. The block diagram same as for FM.

Signal Bandwidth All modulated RF signals occupy a finite space in the E-M spectrum called their bandwidth. E-M spectrum: a valuable, finite resource. Amateur bandwidths (narrow to wide): CW 100 Hz wide 150 500 Hz spacing RTTY 250 Hz wide 250 500 Hz spacing SSB 2.7 khz wide 3 khz spacing AM 5.0 khz wide 6 khz spacing FM 15 khz wide 15 khz spacing See Industry Canada RBR-4!

High Power

High power amplifier 5-100 W usual on HF, VHF, UHF Canadian amateurs max: 750 W (CW carrier output) Advanced licensees only US: 1500 W UK 400 W Linear amplifier: works with CW, SSB. AM, FM, RTTY at reduced output Max. of 8db signal strength improvement (over 100W) Less effective than good directional antenna

Radioteletype (RTTY) Transmits a tone-modulated frequency-shifted carrier (FSK). 5-bit Baudot code + upper/lower (2x32 characters) Carrier rests on the mark frequency (2125 Hz), and is shifted 170 Hz (amateur) to the space (2295 Hz) frequency. Mechanical TTY machines replaced by computer and modern transceiver. Use LSB for RTTY, regardless of the band.

Model 19 Teletype Set -very complex electromechanics -60, 75 wpm

RTTY Transmission Now: computer soundcard connected to the microphone input of the radio + RTTY software. Two tones, corresponding to mark and space, are sent to the mic input when the radio is keyed. This is called Audio Frequency Shift Keying (AFSK or FSK) Messages sent at 60 wpm (45.4 baud)

Packet Radio Computer to computer mode, particularly on 2m. Data bundled into packets. A Terminal Node Controller (TNC) is used to interface radio and computer. VHF Packet : 1200 baud (now some 9600 baud) HF Packet : 300 baud. Uses 8-bit ASCII code (American Standard Code for Information Interchange).

TNC and switch/connector box

Automatic Packet Reporting System (APRS) System for real time digital communications in the local area. Now the primary use of Packet Radio. Displays position, weather info, announcements etc. Retransmitted using digipeaters and the Internet. Maps integral part of the system.

Phase Shift Keying 31 (PSK 31) The most popular HF digital mode at present. Variable length text code with a narrow bandwidth phase-shift keying (PSK) signal using DSP* techniques. Uses a simple interface between the radio and computer sound card. Excellent low power/weak signal capabilities. *DSP = digital signal processing

Slow Scan TV (SSTV) Commercial TV: 6 MHz bandwidth SSTV : same bandwidth as an SSB voice signal (2.7 khz). 8 seconds/frame for the fastest mode, and up to 72 seconds/frame for more detailed, color pictures.

Fast Scan TV Called ATV (Amateur TV). Same NTSC format as regular (non-hd) TV. Bandwidth requirements limit it to 70cm (430MHz) and higher. ATV Repeaters can be found in the 903 MHz and 1.2 GHz bands. Video signal is AM, while audio is FM.

End of transmitters + modulation

Baudot Code

Sound Card Modes PSK31 WSJT RTTY Hellschriber MT63 Throb MFSK16 Etc. Etc. Etc.