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1、WAR DEPARTMENT TECHNICAL MANUAL FREQUENCY METER SETS SCR-211-A,B,C,D,E,F,J,K,L,M,N,O,P,Q,R,T AA,AC,AE,AF,AG,AH,AJ,AK,AL WAR DEPARTMENT 20 JUL Y 1944 , I THIS SUPPLEMENT WILL .,.IN IN EFFECT ONLY UNTIL THlNFORMA TION IS PUBLISHED IN AN OFFICIAL WAR DEPARTMENT PUBLICATION. . SUPPLEMENT 2 157 157 TL-IO
2、I20 Fiure I. FreqllellY ,Ueter BC-22I-(UIc fj, Frequency .lfder BC-:2;1I-(OOdfrl cabinet. VII Figwe J. Freqiltllo .ldeIJ BC-:!.! .1, BC ;.! C, I1l1(i BC ;:;1 D, LUlitlul jJililti II)-u(li. TL -10122. , . . q Fi,/c;. F,!,) .lctrry UC-:!:!I-B, Be -:!:1/-1., BC-:!:!I-N, BC-:!:!l (2., Be 221-.1:1, BC-2:
3、!I-AE, alld BC-22I-:1G, control jillnif o),-illi. IX x SIIIlW. CGlIPS. U. S. ARMY FREllUi.CY METER BC-221F 125 HI 21),600 IULOCYC.ES SU1%t 1111 _ ORIlER NI.I. ZUNIIlj,;41 Ml ,/(I /i(. _.1 .!, IX _I (), 1:(; _)l I. nr: _./ I? 1(, _I r. IfC _I h:, He ,., .IF, (II,I nc :.:1 .111. (II,dll,! 1;:! I J- ,t
4、o Xl xn . Figure /1. Fre1jlellcy .Hetel BC l AA, wlltlul punt! Iily-uut. XIII q A This manual supersedes TM 11-300, 19 December 1941; TM II-300F, 26 February 1942; TM 11-300J, 30 June 1942; TM II-300L, 11 May 1942; TM 11-300Q, 17 March 1943; TM II-300T, 15 May 1943; TM 11-300AA, 28 June 1943; TM 11-
5、300AC, 15 June 1943; TM II-300AE, 7 August 1943; TM 11-300AF,2 November 1943; TM 11-300AG, 9 December 1943 and 14 January 1944; TM 11-300AH, 20 December 1943; TM 11-300AJ, 26 Nooember 1943; TM 11-300AL, 10 March 1944; and section I, Training Circular No.4, War Department, 1943. CHAPTER 1 GENERAL INF
6、ORMATION AND INSTRUCTIONS Section I. DESCRIPTION circuits. 1. Introduction a. This manual is intended to give a detailed description of the operation and use of Fre quency Meter Set SCR-211- ( by the use of the 2d, 4th, and 5th harmonics of the high-frequency range, any frequency between 4,000 and 2
7、0,000 kc can be obtained. The crystal oscillator has a fundamental frequency of 1,000 kc. Its fundamental and harmonic frequencies are used to check the calibration of the variable frequency oscillator at a large number of points. d. The various models of Frequency Meter BC-22 1-(S-D post. e. A care
8、ful study of these panel lay-outs will indicate that there are certain controls which are the same in all models of Frequency Meter BC-221-( in figure 4, it is marked OFF-CRYSTAL OPERATE-CHECK; in figure 8, OFF-WARM UP - CRYSTAL - OPERATE -MODULA TE CHECK; in figure 6, HET. OSC-XT AL CHECK-XTAL ONLY
9、; and in figure 7, MOD. Cause OSC.-HET. OSC-XTAL CHK-XTAL ONLY. The use of these controls will be described in detail in the chapter on each particular model. 3. Frequency Setting Error Frequency Meter Set SCR-2Il- ( 180 cycles at 250 kc; and 180 cycles at 125 kc. 4. Components A complete listing of
10、 the various components normally furnished with each model of Fre quency Meter Set SCR-2Il- (;u.rc; 1U. . . BA-23 WIRING HARNESS Spetion II. INSTALLATION AND OPERATION 7. Initial Procedure lnpack FleqllLn( retcr Set SCR-!o 1-(;FRAI. Frcqllcllcy lctcr BC-:!:! J-(R-:j ohtaiIlS all of its oJnating ()lt
11、ag:s frolll bat teries. These are not ordillarily illclllded !Jen TL-IOI24 Figure 1 I. Battery COllljJartme11t, wooden cabinet, showing Jibel connecting board and A battery leads. 6 - shipment is made, since batteries may deterio rate and cause damage to the interior of the equipment. Before the ins
12、trument can be oper ated, however, four A Batteries BA-23, and six B Batteries BA-2 must be installed. Select these batteries from fresh stock whenever possible. b. TYPES OF BATTERY INSTALLATION. There are three different types of battery installation used in the various models of Frequency Meter Se
13、t SCR-211- (t1 conLlin e,sentiaY tile same informatioll, ,IlY cxceptions to this general rule arc noted hem. II. FtCTl(). The calihration book is a list o dial sellings and the corresponding freq uen- des produced by the meter at those dial settings. Altogether 1,251 dial.frejuency settings spaced 1
14、00 cycles apart have been determined in the LO freq ueney range, and 2,001 settings spaced 1,000 cycles apart have been deterl11i ned in the HIGH frequency range. Inasmuch as the vari able frequency oscillator circuit of the meter produces more than one frequency at a given setting (funcLl!llelltal
15、and harmonic frequen cies), seyeral of the harmonics prodllced are giyen at each dial setting, The hook also contaillS concise operating instructions for making com InOIl measurements. Olr, Tile allle, li,led ill the C:llihralioll ,clion Jcfcr (lnl 10 tlte lllcln lliJo,c ,el ial nllrnber i, III intl
16、(l on lire Ilv leaf. I he ctlilJlatioll hOK rn!hl he llOteIed. It j, a yailiahle tool. a en important pal I of the e(luipmcnt, and take, a great ,kal of time to dllplicate. li. hDFXIG (fig, q). (1) Each section or the calibratioll book is cross-indexed in the foJ- L TL-IOI2.5 Figure 14. Calibration
17、Book MC-177-( or use the convenient INDEX OF FREQUEN CIES printed at the top of the long pages, b (1) above which enables the operator to open the book to the proper page by the use of the thumb tab desired. (2) After the page has been found, examine the appropriate column of frequencies to locate t
18、he desired frequency. In the left-hand column, on the same line, the correpsonding dial reading will be found. d. FINDING THE FREQUENCY FROM A GIVEN DIAL READING. (1) Open the calibration book to the long center page. From the INDEX TO LOW FREQUENCY DIAL SETTINGS pre ceding the center page, or the I
19、NDEX TO HIGH FREQUENCY DIAL SETTINGS fol lowing the center page, determine the number of the page on which the desired dial reading appears; or use the convenient INDEX OF DIAL SETTINGS printed at the top of the long pages, b (1) above, which enables the opera tor to open the book at the proper page
20、 by the use of the thumb tab. (2) After the page has been found, examine the left-hand column to locate the dial setting nearest the desired value. Move along the same line to the appropriate column of frequencies, and the frequency corresponding to a given dial setting may be read with generally sa
21、tisfactory accuracy. Then greatest accuracy is required, and the dial reading obtained in the measure ment falls between dial readings listed in the book, exact interpolations, as described below, should be made. e. DETERMINING INTERMEDIATE VALUES BE TWEEN LISTED FREQUENCIES. The desired fre. quency
22、 (when the frequency meter is being used to calibrate a radio transmitter) , or the observed dial setting (when the frequency meter is being used to measure an unknown frequency) may often fall between the value listed in the cali bration book. To aid in the calculation of the proper dial setting or
23、 the frequency correspond ing to these intermediate values, the following method (called interpolation) should be used. (1) Given: The observed dial setting is 2754.2. (See fig. 14.) This setting lies between the two successive book values 2756.3 and 2752.7. The corresponding frequencies for these b
24、ook values are 195.0 and 194.9 kc (fundamental) , respectively. Problem: To find the unlisted frequency corresponding to a dial setting of 2754.2. Solution: The facts are stated numerically in the following form: Dial settings Corresponding frequencies he 195.0 F Diff. 0. 1 kc 1949 II F = Unknown fr
25、equency. (a) 0.1 kc -7-3.6 div. = 0.0277 kc per dial division. (b) 0.0277 kc x 1.5 div. = 0.0415 kc change from lowest calibration frequency to F. (c) 194.9 kc + 0.0415 kc change = 194.9415 kc = F. In the above problem, a change of 3.6 dial divi sions (from 2752.7 to 2756.3) causes a change of 0.1 k
26、c in frequency (from 194.9 to 195.0 kc) . This represents a change of 0.0277 kc per dial division, (a) above. Since there are only 1.5 dial divisions ben,oeen 2752.7 and 2754.2, the dif ference in frequency between these settings will equal 0.0277 kc x 1.5 div, (b) above. This dif ference (0.0415 kc
27、) is then added to the lower known frequency (194.9 kc) at the dial setting of 2752.7, (c) above to give the unknown fre quency F. (2) Given: It is desired to set the frequency meter to a frequency of 191.95 kc. This fre quency lies between the two successive book values 195.0 and 194.9 kc. Problem:
28、 To fmd the dial setting which cor responds to the frequency 194.95 kc. Solution: The facts are stated numericaTIy in the following form: Diff. Dial settillgs Correstwllding frequencies ke 2756 3 195.OO 3.6 D Diff 0.05 kC 94 95 Diff. 0.1 kc 2752.7 1949 D = Unknown dial setting. (a) 3.6 div. -7-0. 1
29、kc = 36 dial divisions per kc. (b) 36 di v. x 0.05 kc = 1.8 dial di visions change from lowest dial reading to D. (c) 275:q + 1.8 = 2754.5, the dial setting D for a frequency of 194.95 kc. In the above problem, changing the dial setting from 2752.7 to 2756.3 (3.6 dial diyisions) causes a change of 0
30、. 1 kc in frequency (from 194.9 kc to 195.0 kc) . This represents a change of 36 dial divisions per kc, (a) above. An increase in fre quency from 194.9 kc to 194.95 kc is a change of 0.05 kc. Since a change of 36 dial divisions causes a change of 1.0 kc in the frequency, an increase of 0.05 kc requi
31、res a change of 36 divo x 0.05 kc equal 1.8 dial divisions, (b) above. This increase (1.8 div.) is then added to the dial reading (2752.7) corresponding to the lm,oer known frequency (194.9 kc) , to obtain the dial setting D corresponding to a frequency of 194.95 kc, (c) above. 12 (3) The methods de
32、monstrated above are ac curate for all frequency columns (harmonics) in the calibration book. 13. Operating Circuits The frequency meter has four principal operat ing parts: a variable frequency oscillator, which is a source of radio waves of adjustable frequency and is controlled by the main tuning
33、 dial; a crystal oscillator circuit, which enables the oper ator to check the frequencies emitted by the variable frequency oscillator circuit; a detector or mixer circuit, which detects the difference of the frequencies of the preceding two oscillator circuits after combining the two frequencies el
34、ectronically; and an audio-amplifying circuit ohich amplifies the difference or beat frequen cies produced in the detector or mixer circuit when impulses from the crystal oscillator, or from an outside source (applied to the ANT post) , encounter impulses from the variable frequency oscillator. 14.
35、Zero Beat Detection Then two frequencies are present in the input of a detector or mixer circuit, the difference be tween the two will appear in the output circuit as a third frequency. This difference is called the beat frequency. If the two original frequen cies are very close to the same value, t
36、he differ ence may be only a few thousand cycles or less. Such a low frequency lies within the audible range, and can be heard in the headset as a defi nite tone. This region of beat frequencies is shown schematically in figure 15. Starting at point (A) on the figure, a very high pitched note can be
37、 heard in the headset. As the two frequencies are brought closer to the same value (decreasing difference) , the tone decreases in pitch down to point (B) where the tone is re placed by a series of rapid clicks. As the process continues still further, the clicks decrease in rapidity until they stop
38、altogether at point (C). This is the point of zero beat (where the two original frequencies are exactly the same, that is, their difference value is zero) . Matching of the nm frequencies, for all practical purposes, is obtained when clicks are heard at rather in frequent intervals. It is extremely
39、difficult to maintain a condition of absolute silence in the headset over prolonged intervals of time. If one of the two original frequencies is varied still further, the rapidity of the clicks increases to point (D). At this point a low pitched tone is heard. Further variation in the same direction
40、 Ii- , )t . causes a gradual increase in pitch until point (E) is reached where the beat note becomes inaudible again. This region of increasing pitch on both sides of the zero beat point is character istic of this procedure. (See par. ISb.) The fre quency-mixing circuit of the frequency meter and i
41、ts associated amplifier are designed to am plify tones of a few cycles per second. Vhen the incoming signal is fairly strong, the clicks ABOVE AUDIBLE RANGE PITCH OF BEAT NOTE REGION OF CLICKS OR NO SOUND to permit the operating temperature inside of the cabinet to become stable. Figure 7 shows that
42、 Frequency Meters BC-221-AJ and BC- 221-AL have a VARM UP position on their pmer s,itches. Figure 8 shows that Frequency Meter BC-22 I-AK has a VARM UP position on its operation switch. Vhen the proper switch is in the V ARM UP position, the filament of the variable frequency oscillator tube only is
43、 POINT OF EXACT ZERO BEAT FREOUENCY DIFFERENCE INCREASING -II- INCREASING TL-7588 Figure 15. Beat frequency diagram, :ero beat region. are sharp and distinct. If the signal is eak, the zero beat condition is evidenced by a slowly changing swish in the headset. 15. Correction in Calibration a. GENERA
44、L. Before making a correction in the calibration of the meter, or using it for measurements, the meter should be turned on and allowed to arm up for 15 to 20 minutes connected to the A battery circuit. This results in a considerable saving of A battery power ,hile permitting the interior of the mete
45、r to warm up to proper operating temperatures. b. CALIBRATION AT CRYSTAL CHECK POINTS. By calibrating and correcting at a crystal check point, the meter is so adjusted that the frequency generated by the variable fre quency oscillator at a giV:ll dial setting is ac- 13 tually the same as the frequen
46、cy listed opposite the dial setting in the calibration book. This is accomplished by matching the output fre quency (or a harmonic) of the variable fre quency oscillator with a known fundamental (or harmonic) frequency of the crystal oscillator. A great many harmonic frequencies generated by the cry
47、stal oscillator circuit have been recorded and assigned definite positions on the main tun ing dial. These are called CRYSTAL CHECK POINTS. Figure 16 shOs a typical tuning curve of the BC-22 1- (T: l :; l:i 1; :l,:r-:i?: -. - TL-IOI26, Figure 16. Frequenc y Meter BC-221-( that is 2,000, 3,000, 4,000
48、 kc, and so on. These harmonics have the same percentage of accuracy as the fundamental and can be used as reference frequencies throughout the range of the meter. 19 b. ApPLICATION OF THE CRYSTAL CIRCUIT. (I) The crystal circuit has two functions. (a) It serves as a standard frequency reference aga
49、inst which to test the variable frequency oscillator circuit. (b) It is a source of 1,000 kc and harmonics of 1,000 kc, to be radiated from the meter an tenna to some external instrument. (This is not the case in the BC-2 2 I-A, BC-22 1-C, or BC- 22 1-D, since no provision is made in. these models for allowing the crystal-oscillator circuit to operate with the variable frequency turned off.) (2) The two functions a
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