Collins_618T-2_in_C-141_aircraft.pdf

HF COMMUNICATION GENERAL Two High Frequency (HF) radio systems are used to transmit and receive voice communications. They are used primarily for long range communica- tions outside the range of other systems. Long range communication is pos- sible since this system is not limited by line-of-sight characteristics. This system not only provides the conventional Amplitude Modulation (.AJ.1) method of vce communications but also the more recent Sing! Sideband (SSB) method. This too enhances the longer range capability of the system. Am.CRAFT INSTALLATION Each system consists of a Model618T-2 transceiver, a lodel 180 R-4A an- tenna coupler, a 309A-1A coupler accessory unit, a 452A-1A lightning ar- restor relay unit, a Model 714E-2A Control Panel, and one rod type fixed antenna shared by both systems. -=-e transcei:ers are in tl:e cer:ter avionics equipment rack. Tt.:= contrcl panels are Jn the ce:.:er console; t.1e m:pler accessory un . ; a=e 1r: :e ver:- cal stabilizer base; and the lightning arrestor relay unit and an:1na coupers are in the horizontal stabilizer bullet. The antenna is mounted in the forward bullet fairing of the empennage. Primary power necessary for system operation is 25-volt D-C power and 3- phase AC, 2 08 volts, phasa-to-phase. Four circuit breakers for each system are on the avionics circuit breaker panel. System No. l supplied from the A-C avionics and main D-C avionics No. 1 busses. No. 2 HF is supplied in a:1 identical manner from the No. 2 busses. Keying inte:dock between the two HF systems is provided by relays in the cou- pler accessory units and the lig!J.tning_arrestor relay_ unit. The interlock circuits prevent both transceivers from being keyed simultaneously, and pro- vide the necessary switching to enable one antenna to be used by both systems. VOL. VI 4-1 . (. : - MAIN -5A DC AVIONICS BUS No.1 -07A MAIN DC AVIONICS BUS No.2 COUPLER No.1 CABIN PRESSURE uu () u 100) (J . ILU z n._ AVIONICS CIRCUIT BREAKER PANEl. (.t.LL Hf CIRCUIT BREAKERS) . : TRANSCEIVERS LOCATED ON RIGHT SIDE CENTER AVIONICS RACK. TRANSCEIVERS INSTALLATION fLIGHT STATION CENTER CONSOLE LIGHTNING ARIUSTOR No.1 AND No.2 - SYSTEM OPERATION (only one system is discussed) The function, or mode of operation is selected by a four-position rotary switch. In the USB position, the system operates using SSB with carrier suppression. Only the Upper Sideband (USB) is transmitted. The next position is alsonUSB. LSB need not be labeled on the control panel since this mode is disabled in the aircr wiring. In AM operation, the carrier and upper sideband are transmitted. This is also SSB operation, but the carrier is also transmitted. When the switch is in the OFF position, the equipment is deenergized. The frequency selected may be read directly from the control panel. Frequency is determined by dialing the four frequency knobs until the de- sired operating frequency appears in the panel window. The antenna tuning system automatically tunes the an-:- tenna for each transmitter frequency in order to obtain maximum power transfer from the transmitter to the antenna. In order to transmit and re- ceive from an HF system, the desired system must be selected on t.l:e interphone control panel rotary selector. If reception only is desired, the HF mixer switch should be MHz 1 KHz _10KHz OR ODE SEECTOR StECTOR 100KHz SE:ECTOR C 0 N T R 0 L P A L turned on. The volume level of the HF audio should be controlled by the inter- pnone olume controls only. The transm.itter can be keyed by pressing tne rnicrophone button on the headset cord or on the control wheel. Audio signals will be supplied from the micro- phone to the transmitter. RF energy from the transmitter is applied to the tuning system and antenna. Sidetone audio will be heard in the headphones during transmission. When the microphone button is released, the antenna is connected to the rece1ver input. Received signals are amplified, detected, and then routed to the interphone system. Best received signal to noise ratio is adjusted by the RF sensitivity control. VOL. VI . 4-3 When each new frequency is selected and the transmitter is first keyed, the antenna tuning system will tune. The tuning cycle is indicated to the operator CONTROL . PANEL MIC FREQUENCY SELECTION AND CONTROL TRANSCEIVER HEADSET KEY AUDIO (PTT) INTERPHONE CONTROL PANEL I MIC BUTTON MIC AUDIO SYSTEM 8 L 0 C K ANTENNA TUNING SYSTEM COUPLER AND ACCESSORY UNIT HEADPHONES D I A G R A M by a 1KHz tone in the headphones. The average tuning time of the tuning system is five seconds. When tuning is complete, the tone will cease. Keying the transmitter the second time will provide full power transmission. This operation is known as Radio Silence meaning that the transmitter is not on the air for tuning purposes until the transmitter is first keyed. This reduces needless tr:l;:s- missions that cause interference when new receive frequencies are selected. I! the system fails to tune in 75 seconds, a thermal cutcut will disable the t:.ming cycle and prev-:mt transmission. After allowing coolin6 ti.:-:e, a new .:reqt.:enc? must be selected and the :ic buttTn. p!essed i:J. ,Jrcer :o i.:i:i.:.:e tt.? :i:.:; ,:,_2 again. During receive operation, the antenna is connected directly to an an:plifier in tb.e accessory unit, bypassing the tuning circuits in :he cotpler used :!tUi:g .:.!li; and transmit operations. The amplifier output is supplied to the receiver. 4-4 VOL. VI . ,_/ SPECIFICATIONS COLLINS 618T-2 CHARACTERISTIC Altitude range Power requirements Frequency range Frequency channels Frequency stability Time required to change channels Transmit Characteristics. RF power output RF output impedance Audio input impedance Audio-frequency response Distortion VOL. VI . SPECIFICATION Pressure equivalent of 30, 000 feet with externally supplied cooling air. 115 volts (line to neutral), 3-phase, 400 Hz. 1000 watts 115 volts, 400Hz. single phase. 160 watts, 27.5 volts, DC, 120watts 2. 000 to 29.999 MHz 28,000. 0. 8 part per million per month. 8 seconds average (independent of external antenna tuner). ., SSB: 400 watts pep. -i db. AM: 125 watts carrier 1 db. CW: 52 ohms. 80 ohms ujaianced anc.: 600 0h.n:s balanced . .5 db peak-to-valley ratio from 300 to 3000Hz. SSB: Third-order distortion products down at least 30 db. AM: Less than 20 percent at 80 percent modulation with 1000 Hz. 4-5 SPECIFICATIONS (Continued) COLLINS 618T-2 CHARACTERISTIC Receive Characteristics. Sensitivity Selectivity A.GC characteristic IF and image rejection Audio output power Audio distortion . :.1C:J.o-!reque:1cy reson.se Image rejection 4-6 ., SPEC !FICA TION SSB: 1 microvolt for 10 db S+N/N ratio. AM: 3 microvolts modulated 30 percent 1000 Hz for a 6 db S+N/N ratio. SSB: 2. 85KHz, 6 db dmvn. 6. 0 KHz, 60 db down. AM: 5.5 KHz, 6 db down. 14.0 KHz, minimum, 60 db down. Maximum variation of audio putput is 6 db for input signals from 10 to 100,000 microvolts. No overload below 1-volt signal input. 80 db, minimum. 100 milliwatts into 300 ohm load. i.ess than 10 ;-.ercent wit lvJO-n:i.cro- volt input, modulated 8 0 perce:1t a.t 1000Hz . 5 c!.j :.-!o-J.!le:,: !;.L:io ZOl:l 3GO tv 3000 Hz. 60 db mini.r.nun below desired frequency relative to 5 microvolt input. VOL. VI MODULE FUNCTION - Al Frequency divider A2 RF oscillator A3 IF translator A4 Kilohertz-frequency stabilizer AS Low-voltage power supply A6 Electronic control amplifier A7 3 phase AC high-voltage power supply -:-:-A9 AM/ audio amplifier AlO Megahertz-frequency stabilizer All Power amplifier Al2 RF translator Al2Al Autopositioner (submodule) Al2A2 I Variable-f!equency oscillator (VFO I submodule) . .- VOL VI 4-7 . . BLOCK DIAGRAM THEORY OF OPERATION SINGLE SIDEBAND CONCEPT The need for SSB communication systems has arisen because commercial and military services need the long-range propagation characteristics obtainable in the h frequency band. Since HF spectrum space is limited, the best possible use of available frequencies requires communication systems with minimum band- width. An AM signaliJJ composed of three parts: an RF carrier frequency, upper sideband, and lower sideband. All of the audio (Voice) information is contained in each sideband. Each sideband merely duplicates the information of the other. The carrier contains no information. If one sideband and the carrier are eliminated, a SSB siinal results. The SSB signal is half the bandwidth of an AM signal. A SSB signal therefore requires only half the spectrum space for transmission of infor- mation. The principal advantages of SSB are high energy carrier elimination and narrow frequency bandwidth. A SSB transmitter provides full rated power output in one sideband while an AM transmitter provides one-fourth of ita rated power in each of two sidebands. A SSB transmitter with much less power than an AM transmitter will give equal performance. For the advantages of SSB communication to be fully realized, attention must be given to frequency stability, filter selectivity, and low distortion linear power amplification. The maximum frequency error which can be tolerated in an SSB system used for voice communication is approximately 100 Hertz (Hz). Noticeable distortion occurs at 50 Hz or greater. Another consideration in airborne SSB equipment is Doppler shift, due to relative motion between transmitter and receiver. At an operating frequency of 20 MHz when transmitting from a modern jet aircraft to ground, the frequency shift is approximately 2 0 Hz. This represents approximately one-half the frequency error at whic!l noticeable distortion occurs, placing :dditional re- strictions on the maximum allowable frequency error of the SSB equipmer.t. To enable SSB equipment to meet frequency and stability requirements, it is necessary to use a stable master oscillator, by which other variable frequency oscillators are stabilized. This is accomplhed by comparing their output frequency with a frequency derived from the master oscillator. In SSB transceivers, the signal bandwidth must be narrow to reject interference and the unwanted sideband. The filter used, therefore, must have a vary narrow and flat bandpass. These filter requirements are met by mechanical filters which operate at a frequency cf 500 KHz. These filters provide a. much higher Quality (Q) than is possible in LC circuits. Because the SSB signal is a translated audio spectrum, it must be amplified linearly like an audio signal in order to prevent excessive distortion. In addition, linear amplification essentially eliminates the generation of harmonics, thereby 4-8 VOL. VI preventing adjacent channel interference. Class C RF amplifiers, like those used to amplify AM signals, therefore, cannot be used in SSB transmissions. The RF amplifiers and drivers are usually pentode vacuum tube stages operating class A. The SSB signal is generated in the HF communication system by a filter-type SSB generator, consisting of a balanced modulator and a very selective bandpass filter. Th output amplitude of the balanced modulator depends on the audio input amplitude. When there is no audio input, the balanced modulator has no output. 501KHz USB 499f(Hz LSB USB 501KHz A s UDIO I GN.AL_j I!IALANC0 I 1KHz l MODUL.ATO,_ j IF SIGNAL 500KHz MECHANICAL FILTER 501KHz USB PLUS 499KHz LSB MNV 1 500-503KHz I!I.ANDPASS - CARR.I R REINSERT Using a 500 KHz signal as the carrier frequency and a 1000 Hz audio tone as inputs, the output of the balanced modulator consists of the upper and lower sidebands, one on each side of 500 KHz,just as in an AM modulator. Unlike an AM modulator output however, the balanced modulator contains no appreciable amount of the 500 KHz carrier component. Thus, the carrier has been suppressed. The double-sideband, suppressed-carrier signal (501 KHz and 499 KI-!z) from the balanced modulator is fed to an upper sideband (USB) mechanical filter. The bandwidth of the filter is 3KHz; wide enough to pass only the modlating spectrum. Therefore, only the upper sideband will be passed (501 KHz). Note that the SSB signal is a sine wave, constant in amplitude, when a single-:one audio signal is used for modulation. This SSB signal is displaced from its original carrier frequency by an amount equal to the frequency of the modulating audio signal. This modulated signal is heterodyned in several mixers until the selected trans- mitter frequency has been developed. To recover the audio signal at the receiver, the SSB signal must be mixed wi a carrier frequency which is generated at the receiver. The mixer-that performs this demodulation is called a product detector. In the example given, combining 501KHz with 500 KHz (carrier signal) in the product detector produces a difference frequency of 1KHz, which is the .audio signal. VOL. VI . 4-9 Remember that in the transmitter, the carrier is suppressed and the sideband is transmitted. In the receiver, the sideband is received and the carrier reinserted. TRANSCEIVER GENERAL The transceiver is the major unit of the system. The unit is composed of 11 plug- in modules, including an interchangeable internal high voltage power supply. The Model 6l8T-2 uses a 3-phase, A-C high voltage power supply. MODULES 1 he functions of the modules are summarized below. MODULE Al. The frequency divide module transforms a 100KHz signal from the RF oscillator module to a 10 KHz pulse and a 1KHz spectrum which is centered at 550KHz. These outputs are used for VFO frequency stabilization in the kilohertz frequency stabilizer module. A 1KHz cal tone is also provided. T R A N S C I V R MOD1:LE A2. The radio frequency oscillator module outains the .:-.m.z .il:? os- cillator, which is the stable master oscillator of t:e :ystem. This :)SCl.i.ator is used to produce three output signals: a 500 KHz sig:u:.l .: ;:.he :F t:-2-:J.s:a:o: for comr:1unicatio:1s, a 500 KHz signal to :l:e :1.egahe!tz c,:-.!.: ,1 frequer: s :.:t:Jiz.er to be usee i:l stajiliz.:l;:ic:J. of the HF a:cl :7.5 I!-iz :;:a:;:-s, :1nC: _ i:v ,:-:- s :nal to the frequenc: cliv ide module. MODl: LE A3. The IF translator receives the mic:-otJhone audio frorr. A.:1; aucio Jdule and a 50 IC.!z signal froo the R? cscillator !l:odule and bene:-ates a 500 KHz SSE or A.:r1 signal in tae transml ;:node. L1.;.:; :.ortLe ae; co:t.:.i:3 SS3 :r amplifiers and a product detector which are used in the SSE receive mode. MODuLE A4. In the KHz frequency stabilizer, the VFO Irequency is phase- locked in 1 KHz steps with the RF oscillator reference frequency f!om t.b.e fre- quency divide module. This action of tre KHz frequency stabilizer provides a D-C tuning voltage for tuning the voltage-sensitive capacitors in the VFO tuned circuits. MODULE AS. The low voltag