Radar System Engineering

444 7’11E RECEIVING S YS1’EM—RADAR RECEIVERS [SEC. 12.4
a triode is less noisy than a pentode. The difficulty in using a triode lies
in finding a circuit that is stable, uncritical in adjustment, yet with
enough gain to swamp out noise originating in the following stage. A
circuit that has recently been developed is shown in Fig. 12”5. The input
transformer T is similar to the one used for pentodes. In order to realize
the ultimate noise-figure capabilities of the circuit, the Q of the coils
must be kept high.
The circuit shown in Fig. 12”5 consists of a grounded~athode triode
working into a grounded-grid triode. The input impedance of a
grounded-grid amplifier stage is very low, being approximately l/gn, or
200 ohms for a 6AK5. Since this impedance loads the first stage so
heavily that its voltage gain is about 1, there is no tendency for it to
oscillate, even without neutralization. The loading is so heavy that the
interstage bandwidth is very great.
Since the value of L2 is thus noncritical,
the circuit is, in fact, fixed-
L
tuned. Inductance La is an i-f
choke of almost any characteristics.
Thus the circuit is stable
and uncritical; it only remains to
be shown that the noise contribution
of the second triode is
small. This is not obvious, and a
rigorous proof is beyond the scope
of this book (see Vol. 18 of this
—
series). In order to minimize the
Fm. 12.5.-—Triodeinput circuit.
second stage noise, the impedance
seen when looking back from the cathode of the second triode must be
large compared to the equivalent noise resistance at this cathode. To
make this impedance as high as possible, an inductance L1 is connected
between plate and grid of the first triode. Inductance L1 resonates with
the plate-grid capacity at the intermediate frequency. This inductance
is not needed for stabilit y but does improve the noise figure about 0.25 db.
Noise figures obtainable with the double-triode circuit depend on
several factors; representative values are shown in Table 12.1. Improvements
of 2 db or more over the pentode circuit are usual.
Before describing the i-f amplifier, a brief dkcussion of some of the
factors involved in choosing the intermediate frequency will be given.
The over-all receiver bandwidth should be from 1 to 10 Me/see to pass
the pulses ordinarily encountered in microwave radar sets. 1 An 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