shaft held stat ionary. Thermocouples were placed on the case of each cell. An external source of 3-phase a-c power was supplied to the rectifier assembly. T h e load was the main alternator field, which is the load in the application. These tests showed t h a t the rectifier assembly could continuously supply the necessary field power (50 amperes a t 30 volts) for ra ted generator ou tpu t of 40 kva with 10 cubic feet per minute cooling air a t 30 C flowing through the assembly. These results indicated t h a t the system could be satisfactorily cooled under all the required al t i tude cooling conditions.

MACHINE OPERATION

Tests were run on the generator in the al t i tude chamber and the operation was satisfactory a t sea level with 120 C cooling air and at 65,000 feet with — 55 C cooling air. A 500-hour life test was then run with a cooling air temperature of 120 C and the operation a t the finish of the test

was almost identical wi th t h e operat ion a t the s tar t .

I t is a requirement for aircraft genera­tors to deliver 150% overload for 5 minutes and 2 0 0 % load for 5 seconds over the al t i tude t empera tu re range given pre­viously. Under these generator load conditions t he d-c field power required from the rectifier assembly is 70 amperes a t 40 vol ts and 82 amperes a t 50 vol ts respectively for the brushless 40-kva a-c generator. These overloads have been applied to one 40-kva un i t over 50 t imes , thus far with no de t r imenta l effects. There was an addi t ional requi rement for th is generator t h a t i t deliver a min imum of 3 0 0 % ra ted current on a 3-phase sym­metrical short circuit a t t h e generator terminals. Dur ing th is short circuit, the rectifier assembly m u s t supply 120 amperes a t 72 vol ts . A t least 50 short-circuit tes ts have been run on the genera­tor wi thout a single failure.

T h e results of t he tes ts on t he brushless

air-cooled 40 k v a were satisfactory t o t h e point t h a t designs immediately proceeded

on other rat ings. A t present, brushless 30- and 60-kva generators are being tes ted and several other ra t ings are in the design stages.

A complete brushless, air-cooled 6,000-rpm aircraft 40-kva a-c generator is shown in Fig. 9. This generator is approxi­mate ly 9 3 A inches in diameter and 12 inches long.

References

1. T H E F U S E D SILICON R E C T I F I E R , Herbert W. Henkels. AIEE Transactions, vol. 75, pt. I, 1956 (Jan. 1957 section), pp. 733-46. 2. A N O I L - C O O L E D A-C G E N E R A T O R FOR H I G H -S P E E D H I G H - A L T I T U D E AIRCRAFT, Η. J. Braun, W. J. Shilling. Ibid., vol. 74, pt. II, 1955 (Jan. 1956 section), pp. 456-60. 3. A B R U S H L E S S A I R - C O O L E D A I R C R A F T A-C G E N E R A T O R , R. E. Smith. Ibid., vol. 76, pt. II, Sept. 1957, pp. 189-92. 4. E N V I R O N M E N T A L T E S T I N G , AERONAUTICAL A N D A S S O C I A T E D E Q U I P M E N T . Military specification MIL-E-5272A, U. S. Bureau of Aeronautics, Washington, D. C , Nov. 16, 1953.

A Modified Microwave System for Color Television Transmission

F. F. McCLATCHIE A S S O C I A T E M E M B E R A I E E

TH E author ' s company has a continuing operational requirement for approxi­

mately 30 portable microwave radio relay systems in the Southern California area These relay systems are used to extend the Bell System broad-band network to temporary service locations in the area, to bridge existing facilities in the event of transmission failure, or to provide semi­permanent studio to t ransmit ter links.

These portable microwave radio relay systems may be briefly described as fre­quency modula ted (FM) , broad-band t ransmit ters and receivers operable in the 6- to 7-kilomegacycle band a t one-watt radio-frequency power output . T h e in­termediate-frequency amplifiers are 20-megacycles (mc) wide and are centered a t 130 mc. T h e receiver noise power is approximately — 85 dbm (decibels referred to 1 milliwatt) which corresponds to a noise figure of 16 db.

T h e principal subscriber demand re­sulting in the use of these portable micro­wave radio relay systems has been for t he transmission of monochrome television signals. While the transmission require­

ments specified for this signal are most exacting it has been possible to mee t t he demand on a rout ine basis. Our require­ment is, given a service request , t o expect t h a t any sys tem in stock can be assigned for service wi thout excessive prior line-up t ime being expended.

With t he adven t of color television, and the adoption of t h e Nat iona l Television Systems Commit tee (NTSC) s t andard color television signal, t h e transmission requirements became such t h a t no existing

Paper 5 7 - 9 1 8 , recommended by the AIEE Tele­vision and Aural Broadcasting Systems Committee and approved by the AIEE Technical Operations Department for presentation at the AIEE Pacific General Meeting, Pasco, Wash., August 28-30, 1957. Manuscript submitted May 8, 1957; made available for printing June 19, 1957. F. F . M C C L A T C H I B is with the Pacific Telephone and Telegraph Company, Los Angeles, Calif. The author wishes to thank J. Η. Clark and R . Η. Coe for their contributions to the system redesign and for their aid and counsel throughout this proj­ect. The author also wishes to acknowledge the help and contributions of the members of the field and operating forces of the Plant Department, Southern California Area of the Pacific Telephone and Telegraph Company. Personnel of this de­partment aided in the editing of the instruction manual and in providing some basic system objec­tives.

portable microwave radio relay un i t could be expected to t ransmi t the N T S C signal wi thout excessive main tenance and line-up t ime being expended to mee t routine service requirements . T h e color qual i ty of t h e N T S C signal is controlled by two transmission parameters which have not been markedly significant to the proper transmission of monochrome tele­vision pictures. These two parameters are specified as differential phase and differential gain. 1 In t he part icular case of N T S C color signal transmission these parameters specify wha t t he variat ion of the phase and ampl i tude of a 3.579545-mc subcarrier is when i t is superimposed on a larger ampl i tude low-frequency signal.

Anticipat ing t h a t we would be faced with an increasing number of requests to car ry N T S C color television signals, an investigation was ini t iated to determine t h e feasibility of improving our p lan t ei ther b y t he purchase of new equipment or by the modification of existing equip­ment . Our purpose was to reduce t he cost of main tenance b y improving t h e plant to a point where N T S C color signal transmission demands could be me t in t he same rout ine m a n n e r as we had previously me t t h e transmission re­quirements for monochrome television services.

Studies made on existing sys tem types revealed t h a t all sys tem types available for investigation were deficient when con­sidered wi th reference to t he differential phase and differential gain aspects of the

6 2 6 McClatchie—A Modified Microwave System for Color Television NOVEMBER 1 9 5 7

Fig. 1. Cascade i-f amplifier showing band-pass network, bridge T band rejection neutralizingcircuit, and crystal test point

system transmission requirements. Vari­ous technical expedients were attemptedto minimize these deficiencies. One ofthese expedients was the introduction ofphase equalizers for the intermediate-fre­quency amplifier to produce a substan­tially constant delay through the centerportion of the intermediate frequencypass band. Another expedient was to re­duce the frequency deviation introducedby low-frequency components thereby de­creasing over-all deviation without intro­ducing excessive high-frequency noise. Itwas necessary in most cases investigatedto simultaneously employ both tech­niques. The conclusion was that the ex­pedients necessary to up-grade existingsystem types to meet our requirementswere unsatisfactory, both from a trans­mission quality viewpoint as well asfrom the economic viewpoint of mainte­nance personnel tirne required toachieve satisfactory results.

The principal suppliers of equip­ment of the type being consideredwere questioned relative to their plans forproducing a unit which would be satis­factory for our purposes. Encouragingresponses from a technical viewpoint werereceived to the effect that the transmissionrequirements would be met by new sys­tems in initial design stages but no pre­cise information was available on the im­portant factors of when the systems wouldbe available, how much the units wouldcost, how the technical requirementswould be met, or what the maintenanceaspects of the systems would be. Wetherefore investigated upgrading ourexisting plant to meet our requirements.Preliminary investigation indicated thatfor an approximate investment amount­ing to one fourth of the initial costs ofthe relay systems we could improve ourexisting plant to meet all of our transmis­sion, plant cost, maintenance cost, andavailability time requirements. A de­tailed study was therefore initiated on ourexisting plant.

The modification of our existing systemswas considered with respect to the follow­ing points:

1. System transmission characteristicswithout pre-emphasis or i-f (intermediate­frequency) phase equalizers

(a) The over-all system gain frequencycharacteristic to be essentially flat from5 cycles to 8 me,(b) The over-all system differentialgain to be less than 1 db.(c) The over-all system differentialphase to be less than 3 degrees.(d) The receiver noise power to be lessthan -85 dbm. Equivalent to a noisefigure of 16.db.(e) The 60-cycle residual interference

AGe

B+

to be minimized.(f) The system to be well shielded fromr-f (radio-frequency) interference.(g) The base-band noise spectrum to beessen tially triangular and free fromspurious tones.

2. System operational characteristics(a) The receiver local oscillator to becontrolled to seek a frequency within1/2 mc of the frequency which producesthe appropriate i-f resultant.(b) There shall exist no built-in elec­trical or other hazards for the operator.(c) A minimum of operator time to beexpended on system equalization.(d) A complete set of written opera­tional instructions to be provided withthe completed unit.

3. System maintenance characteristics(a) All system maintenance alignmentsto be accessible without removing sub­component covers.(b) All major system subcomponentsto be interchangeable by virtue ofunitized plug in construction.(c) Sufficient integral test points anddetectors to permit simplified precisionalignment of the system.(d) A complete set of written mainte­nance instructions to be supplied withthe completed unit.ee) Due care to be given to the selec­tion and placement of parts to mini­mize electrical hazards during mainte­nance operations.

System TransmissionCharacteristics

The microwave radio relay systemshave been generally described in the in­troduction.! The transmitter unit wasacceptable for NTSC color signal trans­mission but the receiver unit did not meetthe differential phase and gain objectives.Reference sources and independent in­vestigation indicated that the greater

AGe

portion of the distortion was introducedin the i-f section of the receiver. Initialefforts were directed toward improve­ments in this area.

I-F Amplifier

The direct approach toward eliminat­ing distortion in the i-f system would beto eliminate the i-f section itself and con­struct a tuned r-f receiver rather than asuperheterodyne receiver. Suitable com­ponents such as traveling wave-tubeamplifiers, wave-guide preselectors anddiscriminators are available to implementthis solution. While this approach wasfeasible it was rejected because of argu­ments in favor of a certain solution possi­ble using conventional techniques.

While no claim to originality is madewith respect to the design reasoning usedon this i-f amplifier it is felt that it issufficiently unique to bear detailed ex­planation.

The basic requirements for the i-famplifier were stated. These are:

1. Accept and pass a discrete set of fre­quencies (120 to 140 me) with a minimum ofphase, gain, or signal/noise degradation.2. Reject all frequencies outside of this set.3. Raise the level of the desired set to apoint where the information content can beconveniently extracted, consideration beinggiven to match the amplifier input and out­put to preceding and succeeding elements.

These requirements were transformed tothe simplest physical system possible.This was a band-pass filter with inherentgain.

The interelectrode capacities of thevacuum tubes were utilized as filter sec­tion components. Investigation of thetransfer characteristics of various simplefilter sections indicated that a constantk, pi section, band-pass filter adjusted to

NOVElVIBER 1957 McClatchie-A Modified Microwave System for Color Television 627

necessary for impedance transformation)networks. As various stages of the ampli­fier provide varying proportions of theover-all i-f gain under field operating con­ditions (path lengths vary from less than1 to 100 miles) the various stage transfercharacteristics assume different degrees ofsignificance with reference to the over-alli-f amplifier transfer characteristic. Ifall stages are of like type, no change isapparent in the over-all transfer char­acteristic due to relative signal level. Itwas possible for the system to meet thefollowing test because of this feature ofthe i-f amplifier.

The complete microwave system wasaligned without external equalization topass an NTSC color signal with 1/2 dbdifferential gain and two degrees differ­ential phase distortion. All i-f amplifiertubes were replaced with a random selec­tion of new i-f amplifier tubes. Therewas no perceptable change in the NTSCcolor signal transmission characteristicsafter the tube replacement. This featurehas not been characteristic of othersystems. In fact certain systems mustbe returned to the factory for realignmentafter three tube replacements.

The i-f amplifier must operate over adynamic range of input levels of up to 50db without incurring video level changesor excessive changes in the differentialphase and gain parameters. Levelchanges of this magnitude would causeseveral i-f amplifier stages to overload onstrong signals and cause changes in thetransfer characteristic. These levelchanges cannot be compensated for witha manual i-f gain control because fadingmight occur at unexpected intervals. AnAGC has been added to correct for theselevel changes. The gain of the i-f ampli­fier is set to maintain a constant level intothe limiter circuit. This also serves toimprove the action of the limiter circuitby restricting the dynamic range it mustbe able to handle. The AGe circuitderives its signal level indication from agermanium rectifier connected to theinput of the fourth main i-f stage; seeFig. 2. This rectified positive voltage isfiltered and applied to the grid of the AGeamplifier. This tube is biased to cut offby an augmented cathode bias and theplate voltage is derived from a voltagedivider that allows 100-volt positive toappear at the plate when the tube is cutoff. The plate of the AGe amplifier isconnected to the grids of all the grounded­grid stages of the i-f amplifier throughsuitable r-f isolation filters. When the

Automatic Gain Control (AGC)

"'--_~ CARRIER LEVELMETER

B+

/­/

(

\

"

after considerable aging of tubes. Inpractice, the transconductance of all thei-f tubes may be reduced 30% without im­pairing the limiter function. Tube lifehas been extended by operating the plateat 35% of the maximum rated dissipationand by providing heat conducting tubeshields.

The interstage coupling network isshown in Fig. 1. The characteristicimpedance at the input (plate) sideof the coupling network is 1,500 ohms.The impedance of the series resonantcoupling section is 75 ohms, The im­pedance at the output (grid) side ofthe network is 450 ohms. The inputof the network is adjusted to 1,500 ohmsto provide the highest gain, consistentwith stability, that the tubes can deliver.In order that the series resonant elementsbe realizable values of inductance andcapacitance, the impedance of this sec­tion must be kept low. The 75 ohmvalue was chosen as a compromise betweenease of impedance matching and realizabil­ity of component values. The output sideof the network was made as high as pos­sible, consistent with transit time loadingby the vacuum tube in order to sustain theleast voltage loss through the network.The voltage reduction due to impedancetransformation in the network is 5.2 db,other losses associated with the networkare about 3 db, therefore the gain seen atthe plate of the tube must be about 28 dbto realize a stage gain of 20 db.

A desirable condition obtains with theiteration of identical (with compromises

BT

--- "J/

AGC METER +O---'---+-'V'V'v--

1ST 2NDSTAGE STAGE -J---I I-------~h------Ir--- ~~

3RD STAGE I I.F. ~MPLIFIER 4TH STAGE Ir ~ p

----+-___ IIII

-------L----H---------L--B+

Fig. 2. Automatic gain control circuit

AGe AMPLIFIER

r---'vvv-- B+

approach a Butterworth performancecurve would be the most expeditiouscompromise between phase linearity,and adjacent channel rejection. An in­vestigation was instituted to obtain thehighest gain per stage that was con­sistent with stability, using availablelong-life tubes.

The Western Electric type 5842/417Aminiature triode was chosen because ofits high transconductance (28,000 'micro­mhos), low" noise (gold-plated grid), andlong life (10,000 hours). Experimentalcascode amplifier stages were built whichresulted in gains up to 24 db per stage at20-mc band pass. In order to insurestability, the gain was reduced to 18 dbper stage at 20-mc band pass.

Plate-to-grid feedback was reduced byincorporating the plate-to-grid capaci­tance of the vacuum tube into a band re­jection, bridged T, filter network; see Fig.1. This network improves the gain andstability figure over that possible with themore conventional parallel resonant neu­tralizing method. Simple resonant neu­tralization cannot be effective over the 20­me pass-band range because, if the Q ishigh enough to isolate the grid from theplate, the band width of neutralization istoo narrow.

High-gain per stage permitted theconstruction of an i-f amplifier with atotal gain of 124 db requiring only sevenband-pass interstage coupling networksand one transformer coupling network.High over-all gain insures that the limiterwill be in full operation on noise alone

628 McClatchie-A Modified Microwave System for Color Television NOVEMBER 1957

+ 65V +200V

Fig . 3 . L imi te r circuit

signal voltage rises above a certain mini­m u m value, t he AGC control tube is caused to conduct in proportion t o t h e signal intensity. This causes t h e plate voltage of t he AGC tube and t h e grid voltage of t he i-f amplifiers to become less positive and thereby reducing the gain of t he amplifiers. In practice t h e voltage a t t he grids of t he i-f amplifiers will va ry from 100 vol ts a t zero signal to about 40-volt a t maximum signal.

Limiter

T h e function of a limiter in an i-f sys­t em used to receive F M signals is to es tab­lish a constant r-f level input to t he dis­criminator regardless of carrier level or frequency. To function properly on N T S C color as well as voice carrier and pulse signals a limiter mus t meet these conditions :

1 . Limit all frequencies within the i-f pass band to a single voltage level.

2. Suppress amplitude modulation present on the carrier a t all video frequencies. 3. Present a very low driving-point imped­ance for the discriminator.

4. Have sufficient output voltage to drive the discriminator rectifiers for linear opera­tion.

T h e usual vacuum tube limiters were deficient in one or more of t h e points no ted above. T h e requirement of A M (ampli tude-modulat ion) suppression a t t he color bu r s t frequency (3.58 mc) was part icularly severe and can be m e t only wi th circuits of inherent ly shor t t ime con­s tant .

Biased diode limiters can meet the short t ime constant requirement . New de­velopments in germanium diode tech­nology made possible a diode-type limiter of superior characteristics. Diodes wi th a forward conductance of 300 milliampere (ma) a t 1 volt (best available previously were 30 m a a t 1 volt) were used as bo th series and shun t limiters. See Fig. 3 for schematic of t h e circuit used.

A forward bias cur ren t flows in t h e series limiter, allowing low ins tantaneous i-f currents to flow to t h e shun t limiter wi th l i t t le a t tenua t ion . When t h e in­s tantaneous value of t he i-f current ex­ceeds the bias current , t he impedance of t he series diode rises proport ionately wi th

t he i-f driving voltage tending to main­ta in a cons tan t i-f current . I n this way t h e series l imiter acts as a variable loss device in which t h e signal cur ren t limiting level is set b y t h e value of t h e bias cur­rent . A bias cur ren t of abou t 3 m a is used in this a r rangement .

T h e shun t limiter is a voltage operated device as opposed to t h e current operated series limiter. T h e shun t limiter is re­verse biased b y a fixed low-impedance voltage source (batteries) . This circuit is high impedance t o low values of in­s tan taneous i-f voltage. When t h e i-f voltage exceeds t he bias voltage t h e diodes switch to a low-impedance condition and hold t h e peak i-f voltage to a value close to t he bias voltage.

T h e shun t and series limiters operate together so when t h e i-f signal exceeds a predetermined value a t t h e limiter inpu t t h e series limiter switches to a high im­pedance s ta te while t he shun t limiter switches to a low impedance condition. This results in a very " h a r d " limiter ac­tion. Since t h e diodes switch on positive and negat ive port ions of each i-f signal t h e t ime cons tant is easily low enough t o suppress any video frequency A M . T h e limiter contains no frequency-sensitive elements t h a t are selective within t h e i-f band pass, therefore all frequencies will be l imited a t t h e same voltage. T h e shun t l imiter presents a low driving point impedance and delivers a 3-volt peak-to-peak i-f signal to t h e discriminator.

T h e noise power a t t h e inpu t of t he receiver is —85 d b m when using the IN23C mixer crystal . Ful l l imiting re­quires + 18 d b m o u t p u t from t h e i-f amplifier, therefore, a min imum gain of 103 db is required to obtain full l imiting on noise alone. Since t h e over-all i-f amplifier gain is 124 db, a margin of 11 db

NOVEMBER 1 9 5 7 McClatchie—A Modified Microwave System for Color Television 6 2 9

Fig . 5 . V i d e o ampl i f ier ou tpu t circuit

is established. This margin allows a 3 0 % reduction of t ransconductance of all i-f tubes before the system fails to limit on noise alone.

Discriminator

A good discriminator should meet the following conditions when intended for color television and carrier telephone usage.

1. Frequency—amplitude characteristic must be linear over the entire i-f pass band.

2 . Frequency—phase characteristic must be linear over the entire i-f pass band. 3. Conditions 1 and 2 must remain stable with respect to aging, temperature, power-supply variations, and mechanical shock and vibration. 4 . Video output must be sufficient so that excessive video gain is not required.

The first condition specified can be me t by making t h e bandwidth of t h e dis­criminator wider t h a n the band pass of the i-f amplifier and utilizing a circuit t h a t is inherently linear. T h e phase characterist ic can also be made linear over t he i-f pass band by the same con­siderations. Condition 3 requires stable components. This precludes the use of vacuum tubes in t he circuit associated with t he discriminator. Condition 4 can be me t by supplying the discriminator with a relatively high level of i-f voltage.

An investigation was made into the var­ious discriminators now in common use. No discriminator of conventional type me t our requirements so, by a process similar to t h a t taken in designing the inter­mediate frequency amplifier gain stages, there was evolved a readily adjustable discriminator with an extremely high degree of linearity. J . H . Clark is cover­ing the complete description of this dis­criminator in a pa ten t application. This discriminator is essentially two reactive networks derived by the Fosters React­ance theorem.

T h e actual circuit used may be found

in Fig. 4(A) and t h e reactance plot for each network is to be found in Fig. 4(B). T h e resistance Rl and R2 are made equal to t h e driving point impedance of each network. This bridge circuit consisting of Rl, R2 and t h e two networks is a special case in which the driving point impedance remains essentially resistive and constant a t all frequencies, therefore the load on the limiter is no t varied wi th frequency. By properly adjusting t he location of t he poles and zeros, t h e recti­fied ou tpu t voltage of t h e two networks in combination is linear wi th frequency within t h e limits of t he i-f pass band . T h e rectifiers used in conjunction wi th the discriminator are matched germa­nium diodes t h a t exhibit high rectification efficiency and are stable with respect to tempera ture . T h e network itself is con­s t ructed of glass-invar piston capacitors with a very low tempera tu re coefficient and are mechanically stable. T h e in­ductors are wound on subminia ture low-loss coil forms. T h e coils are space wound and t h e t u r n s held in place wi th low-loss coil dope. T h e Q of t h e coils a t the operat ing frequency exceeds 200. T h e load presented to t h e networks by the diode rectifiers m a y be compensated for b y t he ad jus tment of t he values of Rl and R2 and t h u s main ta in a balanced bridge circuit in spite of t h e load pro­duced by the rectifiers. Wi th this circuit it is possible to obtain a l inearity be t t e r than ± 1 . 0 % . Since t he band pass of the discriminator exceeds t he band pass of the i-f by 5 0 % it is no t necessary to ad­jus t the discriminator each t ime the i-f band pass is adjusted.

T h e bridge-type discriminator easily meets conditions 1 and 2. Tes t s under actual operat ing conditions and in t he laboratory indicate t h a t condition 3 is me t adequate ly with respect to aging, t empera ture and mechanical shock and vibration. T h e effects of power supply variations cannot enter into t he operat ion of this circuit. T h e video o u t p u t of t he

discriminator is abou t 0.5 volts peak to peak for t h e full 20-mc i-f band-pass sweep. A ca thode follower is connected to t h e discriminator ou tpu t . I t ' s func­tion is an impedance t ransformer to drive a shor t 93-ohm cable between t h e i-f chassis and t h e video-amplifier chassis.

I t is desirable to opera te t h e micro­wave sys tem wi th t h e local oscillator above t h e carrier frequency in the upper end of t h e band and wi th t h e local oscilla­tor below t h e carrier in t h e lower end of t he band . This mode of operat ion mini­mizes image interference problems. Transforming from one mode of operat ion to the o ther inverts t he video signal. T h e crystals used in t h e discriminator a re of plug-in type and can be reversed. Re­versing the crystals compensates for re­versing t h e mode of operat ion.

Automatic Frequency Control (AFC)

I t has been common practice in micro­wave video transmission systems to lock the receiver A F C to t h e sync pulse from the video signal. This practice resulted in d-c restorat ion of t h e signal and aided in t he correction of distortion in t h e low frequency response of t he system. Wi th the advent of color transmission, video signal pre-emphasis became more common When pre-emphasis is used, t he low fre­quencies are a t t e n u a t e d with a conse­quent over-shoot or spike being produced a t t he sync pulse. Since t h e sync pulse no longer has a flat bo t t om for t h e A F C t o operate upon, this type of A F C tends to create more trouble t h a n i t cures. Tes t s on several types of microwave systems in our P l a n t have indicated t h a t t he average type of A F C is superior to t h e sync pulse reference type when t ransmi t t ing a pre-emphasized signal. For this reason an average-type frequency control is used. Opt imum color operation often depends on precise adjus tment of i-f operat ing fre­quency through adjus tment of t h e A F C , therefore i t should be easily adjustable.

I t is possible to use t he video dis­criminator network as t he A F C discrim­inator network. For this purpose an addit ional set of diodes is connected t o t he video discriminator. T h e o u t p u t of these diodes is connected to a potent iom­eter as indicated in Fig. 4. T h e o u t p u t voltage of the potent iometer is connected to a bridge circuit t h a t controls t he nega­t ive voltage applied to t h e repeller of t h e klystron local oscillator. This sys tem of feed-back local oscillator control operates on t h e klystron to bring t h e voltage out­pu t of t he potent iometer to zero. Since t h e potent iometer can va ry t he sample being taken from each network, t he fre-

630 McClatchie—A Modified Microwave System for Color Television NOVEMBER 1 9 5 7

Fig . 6 . Front v i e w of transmitter con t ro l / transmitter rece iver , receiver c o n t r o l , a n d receiver p o w e r supp ly . Covers off in opera t iona l c o n d i t i o n

F i g . 8 . Same e q u i p m e n t as F ig . 6 . Rear covers r e m o v e d for minor main tenance test ing. Sa fe ty sh ie ld in p l a c e

F ig . 7 . Same as F ig . 6 . Rear v i e w F ig . 9 . Same v i e w as F ig . 6 . Rear covers a n d safety sh ie ld r e m o v e d

for major maintenance testing

quency of t he carrier in the i-f amplifier can be varied by adjusting the potent iom­eter.

This me thod of controlling A F C has a high order or resetability and is very easily adjusted. T h e gain in t he A F C feed-back loop determines the degree of AFC stability achievable. In this circuit, a carrier frequency drift of 10 m c can be held to within 500 kc a t t he i-f amplifier channel.

V i d e o A m p l i f i e r

A video amplifier mus t be capable of raising the voltage level of the ou tpu t of the discriminator to a t least 1.5 volts peak to peak into 75 ohms without intro­ducing differential phase or gain distor­tion. Means mus t be included to set the gain of the amplifier so the peak to peak ou tpu t voltage can be adjusted. T h e usual low-impedance ou tpu t circuits were investigated and found lacking in one or more of the requirements. Basically, any amplifier device where the ou tpu t voltage depends on large percentage of variat ions in the plate current of the vacuum tube is subject to large values of differential phase and gain distortion. T h e differen­tial gain is due to the difference in t rans-conductance of the vacuum tube a t various instantaneous plate currents. The differential phase is caused by the difference in the resistance-capacitance time constant introduced by the ou tpu t capacity and the varying plate resistance of t he tube. A circuit was tried t h a t has been previously used mainly for i ts ability to have bo th a fast rise and a fast fall t ime. This is a combination cathode and anode follower herein referred to as a cathanode follower. T h e cathanode follower as shown in Fig. 5 is characterized by very small changes in plate current causing relatively large changes in ou tpu t voltage.

This is because t he tubes are driven out of phase and as t h e plate resistance of one tube goes u p the plate resistance of the other t ube goes down resulting in an al­most cons tant plate current .

T h e o u t p u t impedance of t he video amplifier is m a d e very low (10 ohms) by application of a high percentage of feed­back around the amplifier. A series re­sistance is connected between the ou tpu t of t h e amplifier and the coaxial ou tpu t connector to m a t c h the characteristic impedance of t h e coaxial cable. T h e effect of nonlinear ou tpu t impedance in the video amplifier is minimized because the greater pa r t of t he ou tpu t impedance of t he amplifier is a resistor. Wi th this design it is possible to obtain mul­tiple ou tpu t s by providing addit ional series resistors connected to t h e other o u t p u t connectors. T h e only effect t h a t the addi t ional ou tpu t s have on each of the o ther ou tpu t s is t h a t t he maximum permissible voltage drive is reduced. As long as t h e o u t p u t voltage of t he ampli­fier is less t h a n the maximum permissible, the connection of addit ional lines does not influence t he o u t p u t voltage on the other lines. On this system, two ou tpu t s and

one high quali ty moni tor point are pro­vided. T h e high qual i ty monitor point has t he same voltage ou tpu t into high impedance t h a t t he other ou tputs have into 75 ohms. T h e ou tpu t impedance of the monitor point is 75 ohms, therefore the length of t he cable connecting it to a high-impedance oscilloscope will not affect the presentat ion unt i l t he length of the cable is such t h a t the transmission characterist ic of the cable itself becomes a significant factor.

Three stages of voltage amplification precede the ou tpu t stage. T h e first stage is no t within the feedback loop and has the function of equalizing t h e loss in the gain frequency characterist ic introduced by the discriminator and coupling coaxial cable. This equalization is no t adjust­able. T h e second and th i rd stages are within the feedback loop. Small changes in t he gain frequency characteristic t h a t may be necessary can be made up by adjus tment of t he feedback capacitor.

Wi th t he lumped equalization removed, this video amplifier can be made flat from 10 cycles to 10 m c with t he frequency response continuing to 14 m c before drop­ping below — 6 db . T h e video amplifier is

F i g . 1 0 . Rece iver uni t t o p v i e w . Safe ty sh ie ld r e m o v e d t o i l lustrate c o m p o n e n t p l a c e ­ment . T h e e lements t o t h e right of center b u l k h e a d present contact hazards. N o contact hazards exist o n left of center b u l k h e a d w h e r e a l l system a d ­

justment points are l o c a t e d

NOVEMBER 1 9 5 7 McClatchie—A Modified Microwave System for Color Television 6 3 1

Fig . 1 1 . Rece iver control unit . Front v i e w

capable of 6 volts peak-to-peak ou tpu t into a single 75-ohm line and 3 volts peak to peak onto 2 lines a t a differential phase and gain of less t h a n 1/4 of a degree and 1/10 of a db. This gives a 3 to 1 margin for feeding 2 lines a t one volt which is t h e normal operating procedure. Sixty-cycle voltages t h a t are introduced into t h e amplifier due to s t ray common ground pa ths can be cancelled ou t by means of a 60-cycle balance control which injects a correctly phased 60-cycle voltage directly into the cathode circuit of t he cathanode follower. By this means the 60-cycle ou tpu t of the receiver can be reduced to ] .5 millivolts rms.

System Operational Characteristics

By operational characteristics is meant the manipulat ive techniques required to install, align, and operate t he over­all relay system. T h e first and guiding principle observed was of course the safety of personnel. This was considered from two aspects. T h e first was, given an operable system, how it could be made safe for all personnel who were required to operate it . T h e second aspect was, given the requirement to do alignment or repair work, how the system could be made safe for people to do this work even though the normal operator protection had been re­moved.

When faced with such an impor tant and complex requirement i t is helpful to use the results of recognized authorit ies when searching for basic principles t o apply. ("Protection of Personnel from Voltages Higher T h a n 150 Volts in Com­municat ion Equipment , Test ing Equip­men t and Telephone Power P lan t s . " Unpublished Specification for Equipment Systems Development Depar tment , Bell Telephone Laboratories, X64587 dated October 26, 1949.) T h e most clear, concise, and general set of principles found

F i g . 1 2 . Rece iver cont ro l uni t w i th i-ί ampl i f ie r r e ­m o v e d f rom main chassis. A p o w e r ex tens ion c o r d per ­

mits opera t ion o i i-ί ampl i f ier in this posi t ion

were these : insulate, isolate, guard, and warn.

I n all cases where an accidental contac t with high potent ia l seemed possible t h e point in question was insulated. High-potential leads were rou ted th rough nor­mally inaccessible locations. * Safety-guard shields which could be removed and replaced simply were devised for all points where hazardous potent ials were accessible. Adequa te warning signs and schematic signs were applied a t all points when i t was felt t h e accident possibili­ties could be reduced. Figs. 6-10 illus­t r a t e t he mechanical a r rangements evolved to implement t h e personnel safety objectives for t h e sys tem. I t should be noted t h a t t h e safety shielding is readily mounted or demounted b y main tenance personnel and i t is felt t h a t th is feature will insure t h a t shielding will be used b y maintenance personnel when i t is feasible to do so during t h e course of ma in tenance operations.

By being careful in t h e design of all of t he active elements of t h e sys tem i t was possible to minimize sys tem equali­zation procedures. Equal iza t ion for a monochrome television signal consists of correctly aligning t h e sys tem according to a simplified main tenance a n d operat ion instruction. Equal izat ion for a N T S C color signal requi rement demands a l i t t le more care in shaping t h e sys tem ampli­fier response curves. An exceptance re­quirement for t h e sys tem is t h a t t h e s imultaneous requi rements of flat band­pass to 8 mc , differential phase below 3 degrees and differential gain below 0.5 db

can be me t in this manner . Once a sys­t em is optimized by these procedures, pre-and de-emphases m a y be applied to t he video portion of t h e circuit fur ther to im­prove t h e system differential phase and differential gain performance.

An a t t e m p t was m a d e to simplify t h e system control ar rangement . Meter ing and operational controls were conven­iently arranged on t h e front panel of t he receiver control un i t and minor field maintenance ad jus tment points located in a readily accessible positions. Con­sidering t h a t a cont inuing t ra in ing re­sponsibility falls on supervisory personnel responsible for t h e operat ion of this equip­m e n t a comprehensive operat ing ins t ruc­tion book was prepared. I t was t h e ob­jective t h a t any person wi th a min imum of knowledge of microwave radio relay equipment could successfully operate t h e system wi th no instruct ion o ther t h a n reading this operators inst ruct ion pam­phlet . I t is felt t h a t t h e operat ion in­struct ions mater ial ly increase t h e use possibilities of t h e sys tem in t h a t any licensed person can, unaided, establish a satisfactory circuit if he b u t follow t h e pr inted directions.

System Maintenance Characteristics

W h e n meet ing a continuing demand for qual i ty broad-band transmission facilities on a rout ine basis t h e facet of t h e opera­tion concerned wi th sys tem main tenance assumes significant impor tance . Con­sidering t h a t t h e various un i t s are port­able and t h a t any one of 40 or more

632 McClatchie—A Modified Microwave System for Color Television NOVEMBER 1 9 5 7

• • ι

• • Η

F ig . 1 3 ( l e f t ) . Rece iver contro l unit . Pr inc ipal s u b c o m p o n e n t chassis r e m o v e d . S u b c o m p o n e n t chassis are v i d e o ampl i f ie r , i-f ampl i f ier /

A F C circuit

F i g . 1 4 ( a b o v e ) . M a i n i-f ampl i f ie r , l i m i t e d , a n d discriminator w i th rear cover r e m o v e d , showing parts l a y o u t , interstage s h i e l d i n g , a n d

measures taken t o prevent ent ry o f interfer ing signals

possible operators may be assigned to t ransport , install, operate and clear trouble on any one of t he 30-odd uni ts in p lan t t h e adoption of certain s tandard maintenance objectives is of paramount importance. T h e s tandards were set forth in a comprehensive manua l wri t ten to be readily understood by maintenance personnel . T h e uti l i ty of this manual was checked by a field tr ial and the recom­mendat ions of maintenance personnel incorporated in i ts final version.

T h e complete microwave system con­sists of five principal uni ts as shown in Fig. 6. These uni ts are designated: t rans­mi t te r control unit , t ransmit ter unit , receiver unit , receiver control unit , and receiver power supply unit .

T h e circuitry of t he t ransmit ter , t rans­mi t te r control and receiver power supply uni ts was considered adequate . I t was was necessary however completely to re­design the receiver and receiver control uni ts . T h e subunits of these principal uni t s were designed on demountable sub-chassis assemblies. Wi th subchassis con­struct ion i t is possible t o demount all principal electronic subassemblies for main tenance purposes. This feature ma­terially reduces trouble clearing t ime on t h e relay uni ts in t h a t spare sub­components chassis may be aligned and held for installation in systems which demonst ra te trouble in t he respective components . T h e possibility of separate al ignment of subcomponents material ly reduces system maintenance outage t ime. Figs. 11-13 il lustrate the uti l i ty of plug in subcomponent construction. Fig. 14

shows t he main i-f amplifier, limiter, and discriminator.

Experience in t h e main tenance of sys­tems designed to operate a t video and intermediate frequencies indicated t h a t all such un i t s should be aligned wi th all covers in place in service condition. Consequently, all ad jus tments on t he various subassemblies were located with components positioned for tun ing wi th t h e subuni t installed for operat ion. I t is no t necessary to remove subcomponent covers to perform rout ine main tenance tes ts or adjus tments . This feature en­ables t he main tenance m a n to optimize a sys tem in service condition and effec­tively eliminates t h e heretofore necessary s tep of rechecking after main tenance ad­ju s tmen t have been m a d e and covers re­placed on t h e various act ive elements.

Each stage m u s t be individually aligned so one s tage does no t compensate for nonlineari ty in a preceding stage. To this end, each i-f amplifier stage is equipped wi th a t e s t point . Each tes t point contains i ts own detector circuit, so i t is only necessary to connect an oscillo­scope to t h e t e s t point when making gain and b a n d pass t e s t s ; see Fig. 1. T h e tes t point itself is a Microdot subminia ture coaxial connector. A capacitor is moun ted a t t h e connector to prevent en t ry of interfering signals into an otherwise well-shielded i-f enclosure. Tube failures can be quickly located in t he field by measuring t h e signal voltage a t each stage using an ordinary mul t imeter . I n addi­t ion to band-pass and trouble-location tests , these tes t points m a y also be used in

the measurement of receiver noise figure, conversion loss, and t h e recording of signal levels for engineering studies of microwave pa ths .

Conclusions

I t is possible to extend economically t h e useful life of cer tain obsolescent micro­wave radio relay equipment b y t h e re­design of critical circuitry. N T S C color television signals m a y be t r ansmi t t ed on por table microwave radio relay uni t s in a rout ine manne r if an adequa te system is available and s tandardized sys tem operational and main tenance instruct ions are provided.

T h e i terat ion of identical i-f amplifier coupling networks t ends to reduce t h e variat ion of sys tem differential phase and differential gain wi th t u b e changes and p a t h changes. T h e application of plug-in subcomponent chassis construction techniques mater ial ly reduce system main tenance expense.

Field tes t s of t h e completed sys tem indicate video base b a n d equalization is less t ime consuming and technically superior to i-f amplifier equalization when correcting differential phase and differ­ential gain difficulties.

References

1. D I F F E R E N T I A L P H A S E A N D G A I N M E A S U R E ­M E N T S I N COLOR T E L E V I S I O N SYSTEMS, H. P. Kelly. AIEE Transactions {Electrical Engineering), vol. 7 3 , pt. I, Nov. 1954 , pp. 5 6 5 - 6 9 .

2. O N E A P P R O A C H TO A V I D E O SHF R E L A Y SYSTEM, R . H. Coe, F. F. McClatchie. Ibid., vol. 7 3 , pt. I, Sept. 1954 , pp. 3 2 3 - 2 9 .

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