Radar System Engineering

448 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC.124
advantage that cascaded double-tuned circuits do not narrow as rapidfy
as cascaded single-tuned circuits because of the rectangular form of the
response curve (Fig. 12-8). For example, nine cascaded single-tuned
stages, each of 6-Me/see bandwidth, would have an over-all bandwidth
of 1.7 Me/see. However, nine cascaded double-tuned circuits, each 6
Me/see wide, would combine to give a 3.2-Me/see over-all bandwidth.
The over-all bandwidth of n cascaded double-tuned circuits is given by
over-all bandtidth = single stage bandwidth
1.1 q;
The usefulness of double-tuned circuits in wideband amplifiers has
been demonstrated by their incorporation in a radar receiver having 120
2
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Frequency
(a) Single tuned (b) Double tuned (c) Stagger tuned
FIG. 128.-Amplifier response cum es.
db of i-f amplifier gain and an over-all bandwidth of 12 Me/see. This
was accomplished with only nine 6AK5 tubes in the i-f amplifier (Sec.
12. 11). On the other hand, there are disadvantages in using doubletuned
circuits. They are difficult to align, special equipment being
required. Though this can be circumvented to some extent in wideband
amplifiers by the use of fixed tuning, the sensitivity to variations in tube
capacity and coil inductance is much greater than that of singIe-tuned
circuits. This is particularly true when the loading is on one side only,
so that this arrangement is ordinarily used only in very wideband amplifiers
(15 Me/see or over).
Another means of achieving large bandwidth is by using staggertuned
circuits. 1 Consider two cascaded single-tuned circuits of the type
previously described, choosing the load resistors so that each has a bandwidth
of 4 Me/see as shown in Fig. 12.8. The combined response curve
of the two stages will be the product of the responses of the individual
stages and is shown by the dashed line,
1H. Wallman, RL Report A-o. 524, Feb. 23, 1944.