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Download 442 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC. 12.4
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442 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC. 12.4 Another very important factor in radar receiver design is that of securing a good transient response. Reception of pulses about 1 ~secin length imposes severe requirements on the pass band of the amplifier. At the same time, the amplifier must be capable of amplifying pulses several hundred microseconds in length and yet be fully sensitive to a weak signal immediately following. It must recover immediately from a signal thousands of times stronger than the minimum discernible signal. Frequently special requirements are placed on the receiver. For example, it may be required that it respond to weak pulses even in the presence of a c-w signal of considerable strength. The problem of detecting a weak target echo in the presence of strong sea return is somewhat similar. Special circuits which go under the general title of "anticlutter circuits" are usually required in such cases. The unique requirements outlined in the foregoing three paragraphs have resulted in a receiver that is considerably different from anything previously existing. The design of i-f and video amplifiers to meet these requirements will be discussed in the following sections. 12.4. I-f Ampliiier Design—In an ideal i-f amplifier, all the noise would originate in the generator connected to its input terminals-that is, the crystal that serves as converter in the usual radar receiver. In any practical amplifier there circuit are and additional resistances in the input shot sources due of noise. Thermal flow noiseof from noise to the uneven elec- B+ Fm. 12.4.—Pentodeinput circuit. trons to the plate of the first tube are the chief sources of excess noise, although later stages may contribute slightly. For this reason, care must be taken in the design and tuning of the network coupling the crystal to the first grid as well as in the choice of the operating conditions and type of the first tube. Figure 1264 shows a typical input circuit, with the primary tuned to resonate with the crystal and mixer capacity at the intermediate frequency, and with the secondary tuned to resonate with the input capacity of the tube and socket. The coils are usually fixed-tuned or " slugtuned" 1in order to avoid any extra capacity. The only loading on the circuit is due to the crystal and the input resistance of the tube; thus, there are no additional resistances to contribute thermal noise. Thermal 1The inductanceof a '1slug-tuned" coil is loweredby the effectof eddy currents inducedin a metallicrod (slug)insertedin oneendof thecoil. The degreeof penetration, and thereforethe inductance,can be varied by turningthe slug in a threaded support. SEC.12.4] I-F noise due to the resistance in the coils themselves AMPLIFIER small by using coils of moderate secondary is preferably what poorer value that noise figure. The gives the best noise critical coupling, —that is, just bandwidth magnetic, Q. The since capacitive amount a double with this amount This primary and gives a some- is usually is necessarily fixed at a greater than seems to be about transitional hump appears in the of coupling depends upon the crystal capacities. pass Since the crystal small (around three or four hundred adequate, being around 10 Me/see greater bandwidths between coupling of coupling figure. 443 can be kept negligibly coupling and in the case of a 6AC7 before ance and the mixer and tube siderably DESIGN ohms), between can be obtained band. The resist- resistance is the bandwidth is usually half-power points. Conby paying attention to the mixer capacity. A number of tube types have been most radar receivers today use the 6AC7, 717 are very electrical similar in figure than the 6AC7; however, TABLE 1st stage pentode The influenced o~eratin~ by to voltage design and a lower and give 2nd stage 6AK5 6AK5 6AK5 6J4 current considerations for the other used. Intermediate frequency, Mc,/sec 6AK5 6AK5 6J6 6J4 and noise FIGURE 6AC7 6AC7 6AK5 6AK5 6AK5 6AK5 to grounded-cathode pentode Grounded-cathode triode grounded-grid triode The 6AK5 NOISE type however, or 717. the latter is still widely 12.1.—AvERAGE Circuit in the first stage; performance Tube Grounded-cathode tried 6AK5, Over-all receiver bandwidth, Me/see Average noise, db. 1.5 30 30 60 3,9 3.3 6.5 6 16 1.5 30 30 60 180 15 2,2 3.5 5.5 8 12 3 first tube than are frequently noise figure. In generai, the and plate the common) ratings. Fig. and screen low cathode current voltage should should be as behigh permitted The noise obtained figure 12.4 depends receiver bandwidth. intermediate of the same type manufacture Some representative by in as (75is to use of 120 volts are by the tube that can be the upon Since varies and the it frequency and the overconsiderably with individual tubes all figures. it is desirable to quote averagein given 12.1. values are Table It has long been that a large of the realizedis due to the interception of portion a pentode electrons by the screen ingrid. circuit shot shown noise Hence. 7'11E RECEIVING 444 S YS1'EM—RADAR The difficulty triode is less noisy than a pentode. a in finding enough a circuit gain to that swamp circuit that has recently transformer out The circuit ohms originating in in Fig. its voltage even without Since gain low, this neutralization. being the Since the value l/gn, a or first stage — seen when looking large compared to Thus the uncritical; shown impedance circuit the the an is stable remains noise second to contrtriode small. This is not obvious, rigorous proof is beyond of book this i-f is and a the scope (see Vol. 18 of In order to minimize this the second stage noise, the impedance as high as possible, capacity at the intermediate is not needed for stabilit y but does improve obtainable with the an inductance L1 is connected Inductance L1 resonates with frequency. This inductance the noise figure about 0.25 db. double-triode circuit depend several factors; representative values are shown in Table 12.1. ments of 2 db or more over the pentode circuit are usual. Before describing the i-f amplifier, a brief factors involved in choosing the intermediate over-all is characteris- it only that of La any back from the cathode of the second triode must be the equivalent noise resistance at this cathode. To plate and grid of the first triode. figures almost series). Fm. 12.5.-—Triodeinput circuit. pulses so of L2 is thus non- Inductance of ibution the triof critical, the circuit is, in fact, fixed- be The coils 1, there is no tendency for it to The loading is so heavy that the interstage bandwidth is very great. and Noise the impedance loads tics. the plate-grid Q of approximately choke this the input tuned. between A a grounded~athode The impedance is about with The input circuit, of triode. stage is very a 6AK5. yet stage. In order to realize the 12"5 consists L make in using a triode lies adjustment, in the following of 12.4 [SEC. is shown in Fig. 12"5. capabilities a grounded-grid amplifier for that oscillate, shown into grounded-grid heavily noise uncritical T is similar to the one used for pentodes. working 200 stable, been developed the ultimate noise-figure must be kept high. ode is RECEIVERS receiver ordinarily bandwidth encountered should dkcussion frequency be from in microwave on Improve- of some will be 1 to 10 Me/see radar sets. 1 An of the given. to pass inter- 1In this chapter i-f amplifier bandwidth will be taken between the half-power points; video amplifier bandwidths will be measured between the frequencies at which the videe rmponse is 3 db down. The over-all receiver bandwidth (i.e., the