Radar System Engineering

SEC.12.4] I-F AMPLIFIER DESIGN 445
mediate frequency considerably greater than this is required in order that
i-f sine waves can be filtered out of the video amplifier. Local-oscillator
noise can be minimized by the use of a high intermediate frequency;
however, the balanced mixer (cf. Vol. 16 of this series) provides a better
solution to this problem. Many of the present AFC systems require the
use of a high intermediate frequency to prevent locking on the wrong
sideband. Finally, components such as condensers and coils become
smaller as the frequency is raised, a distinct advantage in lightweight
airborne radar sets. On the other hand, there are at least two very cogent
reasons for favoring a low intermediate frequency: (1) the noise figure
of the i-f amplifier is smaller at lower frequencies, and (2) manufacture
and maintenance is considerably simplified because variations in tube and
wiring capacitances, as well as in tuning inductances, affect the over-all
receiver response much less; Thus the choice of an intermediate frequency
is a compromise. Frequencies II
of 30 Me/see and 60 Me/see have been
chosen for most of the present-day ~
radar sets. The i-f amplifier bandwidth
is not an important factor in
the choice of the intermediate frequency.
It is just as easy to achieve
a bandwidth of, for example, 5 Me/see
I
at a center frequency of 5 Me/see as it
= E+
FIG.l?.6.—Single-tuncdi-f amplifier.
is at 60 Me/see.
An i-f amplifier consists of a number of cascaded stages. Figure 12.6
is a circuit diagram of a type of stage in common use.
This is known as a single-tuned stage, since there is one tuning
inductance per stage. It has the advantage of being simple, easy to
manufacture and align, and noncritical in adjustment. It is particularly
useful in i-f amplifiers of over-all bandwidth less than 3 M c/see. It
becomes expensive at wider bandwidths, although an improvement in
tube performance would raise this figure proportionately.
The inductance L is tuned to resonate at the intermediate frequency
with the combined output and input capacity plus stray capacity to
ground due to sockets and wiring. It is placed in the grid circuit to
provide a low-resistance path to ground. Thus, when the grid draws
current during a strong signal, it does not accumulate a bias; hence the
gain is not reduced and the amplifier remains sensitive to weak signals.
The gain G of the single-tuned stage shown in Fig. 12.6 is given by the
expression
G = g.RL, (2)
combined responseof the video and i-f amplifier)will be taken authe equivalenti-f
amplifierbandwidthunlessotherwisestated.