10
RI -250 D A Y T wg Qre,, 1 250 Watt FM Transmitter Incorporating the NEW NEW inn ZINGAPE Ilk MOW Phase Shift Modulation
RI -250
D A Y T wg Qre,,
1 250 Watt FM Transmitter Incorporating the NEW NEW
inn ZINGAPE Ilk
MOW Phase Shift Modulation
YOU CAiV GET ALL OF THESE
IMPORTANT FEATURES ONLY FROM RAYTHEON
i Simplified circuit design through the Cascade system gives stability and efficiency to Raytheon FM.
2 Direct crystal control, independent of modulation, gives positive and automatic control of the mean carrier frequency. No complicated electronic or mechanical fre- quency stabilizers are used. A single high quality crystal does the job.
3 An inherently lower noise level is achieved by Cascade Phase Shift Modulation which adds the phase shift of six simple stages. No single complicated stage. nor a large number of multipliers with resulting high noise level. is necessary.
4 Very low harmonic distortion-less than 1.0(, from 50 to 15000 CPS with 100 KC frequency deviation.
5 A built-in tuning meter, conveniently located on front panel. allows the operator to check any circuit. instantly. ichile on the air.
f Conservatively operated circuits assure a high fidelity signal without program in- terruption over long periods of continued operation. Normal life of all tubes and com- ponents is materially increased : annoying replacements are reduced to a minimum.
7 No expensive special tubes. The modulator unit uses only inexpensive receiver type tubes of proven reliability.
8 Simple, very fast tuning. Periodic circuit tune-ups, although rarely necessary. are easily accomplished in two or three minutes, and the transmitter requires virtually no adjustment between service periods. All circuits are stable-require no critical adjust- ments. No external measuring instruments are ever needed.
9 Lasting economy. A Raytheon transmitter not only costs less to purchase but through increased operating efficiency you continue to save daily on power costs. Advanced en- gineering design plus modern styling guarantee your satisfaction for many years.
1C Easy to service. Excellent mechanical layout. vertical type chassis and full height front and rear doors make servicing both fast and easy.
i 1 Unit construction. There is no obsolescence to Raytheon FM Transmitters. This 250 watt transmitter is also the basic exciter unit for all higher powers and later you can add an amplifier to give any increased power you desire. All units are perfectly matched in size, styling and colors.
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EASY TO SERVICE
Open front view, above, shows complete metering and simplicity of controls. Each fuse has an indicator light and all are located on the front panel to give instant accessibility.
The full height rear view shown at right emphasizes the clean vertical chassis type of construction used throughout. The final amplifier, driver and tripler stages are located at the top in a completely shielded compartment. Each of these stages is further shielded fron the others, giving complete stability and mini- mum radiation external to the cabinet. This lack of external radiation is of great importance when low level audio equipment is to be operated in close prox- iuity to the transmitter.
Below these stages is a panel carrying additional multiplier circuits. The control panel with its switches, push -buttons and indicating lights is located at a con- venient height for ease of operation.
I nmediately below the control panel is the Cascade Phase Shift Modulator chassis which is described in
detail on pages 6 and 7. The power supplies for the modulator and low level RF stages are just below the modulator.
On the floor of the cabinet are located the compo- nents for high voltage power supplies, power input cir- cuits, etc. A line voltage regulator mounted on the left side wall supplies regulated AC for the modulator power supply.
All units are readily accessible for maintenance and servicing. All components have been carefully selected to assure excellence in performance and long trouble - free service.
The 'iev above shows the linear thpz tank circuits used on all sty¡es operating above 30 MC. Though more expen- sive :hem the conventional "coil ar-d condenser" type the use cf linear tanks as- sures srdiility, reliability, high e ffi.dertcy, high spurious signal dtmuation and ease of adjusiuent.
Power components are lo- .ated or. he bottom of the
.mabinet w=th a wiring duct for interconnection of the RF -25) neh units of higher dower.
Schematic Diagram of the RAYTHEON
250 Watt FM Transmitter
RF -250
Simplicity and straightfor,,,lr l circuit design are evident from the RF -25o schematic. The modulator and low frequency multiplier stages are shown within the white area.
Phase shift modulation permits direct crys- tal control of the carrier frequency and the Cascade system makes possible direct multi- plication (972 times overall) from the crystal to the final carrier frequency. The modulator chassis contains the crystal oscillator, a regu- lar and standby crystal in temperature con- trolled ovens, the six cascade phase shift stages and amplifiers and multipliers which provide output at about 1.2 MC from crystal frequen- cies centering around 100 KC.
It should be noted that great care has been taken in all amplifier and multiplier stages to reduce to a minimum the harmonic and spu- rious signals which would otherwise be pres- ent. Band pass, overcoupled, double tuned cir- cuits are used when required for good attenu- ation and adequate band width. Higher fre- quency multipliers are of the single tuned type and above 30 MC all tank circuits have linear type elements to provide high "Q" and further reduction of unwanted signals.
By using pentode and tetrode RF multiplier and amplifier tubes a minimum of stages are required to give increased efficiency and re- liability. The RF output from the final ampli- fier is terminated in an end seal fitting for attaching standard 51 ohm coaxial line. A gas inlet is provided where a pressurized line is
used. Control circuits provide simple and positive
protection of all operations. On those circuits subject to possible overload, such as the final amplifier power supply, circuit breaker pro- tection is used to provide instant reoperation after the fault has cleared. Fuses with indi- cator lights protect other components from abnormal faults. All control circuits are ar- ranged for interconnection with higher power units providing unified control.
Adequate metering allows instant check of all circuit operations for rapid maintenance and service checks. Rapid tube change is pos- sible through quick -opening doors wherever shielding is employed.
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The rear of the modulator shows the audio in- put and RF output receptacles and the intercon- necting terminal strip.
What is this entirely new
CASCADE SYSTEM of Phase Shift Modulation?
Basically the Cascade Phase Shift system consists of a
crystal oscillator operating at about 100 KC whose RF voltage is phase modulated by six simple phase shift networks. The six phase shift networks are in cascade for RF voltages but the audio voltages controlling theta are in parallel. Thus the phase shift produced by each section is additive so that approximately six times the phase shift of a single section is obtained without mul- tiplication of the carrier frequency. The output of the last phase shift section is then amplified and multi- plied by a factor of 12 within the modulator unit. i The overall multiplication is 972 from crystal frequency to carrier frequency).
The basic constant impedance phase shift network
PHASE SHIFT MODULATION
The front view of the modulator unit shows the simplicity of parts and layout. A single tuning meter makes possible a
rapid check or tune-up of all circuits. A running time meter provides a record of all "on the air" hours. Dual crystal sockets, with instantaneous changeover, are provided.
JXL
2
Constant Impedance Network
is shown above. When XL is made equal to 2XC any variation in R2 will have no effect upon the magnitude of the impedance across terminals 1 and 2. The phase of the output voltage will shift, however, between the limits of plus 90° and minus 90° as R2 is either open circuited or short circuited. With R2 open, current will How only through XL and must be inductive with the voltage leading the current by 90°. With R2 short cir- cuited and XL = 2XC, the current through XC will be twice that through XL and the net current will be capa- citive with the voltage lagging by 90°. With R_ at inter- mediate values the voltage vector will he at sonie inter- mediate angle. By replacing R2 with the cathode to ground resistance of a vacuum tube we have a means of controlling the phase shift with audio frequency oltages. If six of these circuits with appropriate am-
plifier tubes are placed in cascade, and the grids of the audio controlled resistance tubes are paralleled, a six stage Cascade Phase Shifter results.
Six stages are required because the largest phase shift obtainable, with low distortion, in any single phase shifter is about 25° or about 13 cycles frequency change
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BRINGS YOU IMPORTANT NEW ADVANTAGES ELIMINATES MAJOR DISADVANTAGES
for an audio frequency of 30 cycles. To produce the required 75 KC deviation we multiply 13 by 6 which gives 78 cycles shift produced by the six phase shift sections and again multiply by the overall multiplica- tion 972) which follows. This gives 75.8 KC deviation.
The constant impedance type network is required to minimize amplitude modulation which. if present. in- creases the distortion.
The complete circuit is shown in white on the sche- matic diagram.
What are the advantages of the
CASCADE CIRCUIT?
The Cascade Phase Shift system has many exclusive and outstanding advantages. Direct crystal control is a basic requirement for any broadcast transmitter. When this can he accomplished without an excessively high order of frequency multiplication. without the use of special purpose tubes and yet retain simplicity of circuit and components, this requirement becomes a must. All comparison oscillators. discriminators. motor controls, frequency dividers and frequency in- dicators are immediately eliminated. The output fre- quency depends only on the crystal!
Circuit simplicity permits the incorporation of a
simple metering circuit which makes possible com- plete tune-up of the unit without the use of any ex- ternal laboratory measuring equipment. This arrange- ment also gives very adequate maintenance and serv- ice tests which permit rapid isolation of possible faults.
Tuning is exceptionally easy; band pass circuits have push button operated damping resistors which permit simple "peaking" adjustments rather than compli- cated "sweep oscillator" or other means of tuning. Ade- quate metering is provided on all circuits beyond the modulator and all essential circuits are metered with large, easily read rectangular meters.
Linear tank circuits used above 30 MC provide high
efficiency, high "Q," simple adjustment and excellent stability. Final amplifier efficiencies of from 75 to 80 per cent are easily obtained.
The output power control provided by a variable screen voltage on the output stage gives a complete power range from 50 to 250 or more watts without change in output efficiency. Filament voltage control is provided and a single transformer type automatic voltage regulator supplies primary power for the modulator power supply. No DC filament supplies are required and no DC voltage regulator tubes or circuits are needed.
Control circuits provide all needed starting. stop- ping and interlocking operations and are coordinated with higher power units for simplified control when additional power amplifiers are added.
Simplicity of tube and circuit complement makes the power consumption of this transmitter remarkably low. Line voltage variations are readily accommodated.
The mechanical construction is such that in spite of ample size and sturdy construction the unit can easily be transported through standard doors. and in pas- senger elevators where building top location is desired. Additional high power amplifiers may be added at any time forming a unified design of modern pleasing ap- pearance.
The actual performance of this transmitter is con- clusive proof of the sound design and advanced en- gineering incorporated throughout. Performance is
superior to FCC and RMA requirements in all respects and simplicity and stability assure continued high quality performance over long periods with minimum maintenance and adjustment problems.
Inside view of complet. modulator unit with back cover removed. The extreme simplicit' of the Cascade Phase Shift circuit is well illustrated.
FOR TM fsR d1ACAS1f INgUSt .Y
SPECIFICATIONS
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1)1,13-40!1-111{0
RAYTHEON 250 WATT FM TRANSMITTER
RF -250 Output Frequency '1ny specified frequency between 88 and 108 MC Type of Emission A-3 Power Output 50-25(1 watts R. F. Output Impedance 35-70 ohms Carrier Frequency Stability ±1000 cycles
pe of Oscillator Direct Crystal Control I' of Modulation Cascade Phase Shift
Modulation Capability +100 KC Audio Input impedance 600 ohms 150/600 on special order Average Program Level +5 VIT ±2 VU 100% Modulation Level +12 VU ±2 VU Audio Frequency Response ....50-15,000 cycles ±1 db front 1000 cycle reference Pre -emphasis Standard 75 micro second with removal switch
Audio Frequency Distortion. 50-15,000 cycles 1 0% 50- 100 cycles 1.0%
100- 7,500 cycles 1 0% 7,500-15,000 cycles 1 0%
\I \aise Nevel 65 db below 75 KG swing \ \I Noise Level 50 db below 100% amplitude modulation
Power Line Requirements: Voltage 208/230 three wire Frequency Phase Regulation 5% maximum Power Consumption 1400 watts approx. Power Factor
50/60 cyel -
87),
Mechanical Specifications Size, with trim height 84", width 36". depth 28" Size, without trim height 84", width 30", depth 28"
1200 lbs. Weight
TUBE COMPLEMENT Modulator Unit: R. F. Amplifiers:
Crystal Oscillator 1 6SJ7 1st Quadrupler 1 6AC7 Phase Shifters 6 6SJ7 1st Tripler 1 6AC7 Resistance Tubes 3 6SN7 2nd Tripler 1 6AG7 Multipliers 3 6SJ7 3rd Tripler 1 829B
Amplifier 1 6SJ7 Amplifier 1 829B Audio Amplifiers 2 6SN7 Output Stage 2 4-125A Detector -Amplifiers 1 6SL7 300 Volt Supplies 2 5U4G
600 Volt Supply 2 866A 2000 Volt Supply 2 866A Output Power Indicator 1 6AL5
Complete FCC filing data will be supplied upon request.
In order to incorporate constantly the finest in engineering, design and components in our Broadcast Equipment, Raytheon reserves the
right to make engineering changes at any time.
RAYTHEON MANUFACTURING COMPANY Broadcast Equipment Division
7517 North Clark Street, Chicago 26, Illinois Printed in U.S. A.
