Radar System Engineering

SEC.12.4] I-F AMPLIFIER DESIGN 443
noise due to the resistance in the coils themselves can be kept negligibly
small by using coils of moderate Q. The coupling between primary and
secondary is preferably magnetic, since capacitive coupling gives a somewhat
poorer noise figure. The amount of coupling is usually fixed at a
value that gives the best noise figure. This is necessarily greater than
critical coupling, and in the case of a 6AC7 seems to be about transitional
—that is, just before a double hump appears in the pass band. The
bandwidth with this amount of coupling depends upon the crystal resistance
and the mixer and tube capacities. Since the crystal resistance is
small (around three or four hundred ohms), the bandwidth is usually
adequate, being around 10 Me/see between half-power points. Considerably
greater bandwidths can be obtained by paying attention to the
mixer capacity.
A number of tube types have been tried in the first stage; however,
most radar receivers today use the 6AC7, 6AK5, or 717. The 6AK5 and
717 are very similar in electrical performance and give a lower noise
figure than the 6AC7; however, the latter is still widely used.
TABLE 12.1.—AvERAGE NOISE FIGURE
Circuit
Tube type
1st
stage
2nd
stage
Intermediate
frequency,
Mc,/sec
Over-all
receiver
bandwidth,
Me/see
Average
noise,
db.
Grounded-cathode pentode to
grounded-cathode pentode
6AC7 6AC7
6AK5 6AK5
6AK5 6AK5
30
30
60
1.5
6
16
3,9
3.3
6.5
Grounded-cathode triode to
grounded-grid triode
6AK5 6AK5
6AK5 6AK5
6AK5 6J6
6J4 6J4
30
30
60
180
1.5
8
12
3
15
2,2
3.5
5.5
The o~eratin~ voltage and current for the first tube are frequently
influenced by design considerations other than noise figure. In generai,
the plate and screen voltage should be low (75 to 120 volts are common)
and the cathode current should be as high as is permitted by the tube
ratings.
The noise figure that can be obtained by use of the circuit shown in
Fig. 12.4 depends upon the intermediate frequency and the over-all
receiver bandwidth. Since it varies considerably with individual tubes
of the same type and manufacture it is desirable to quote average figures.
Some representative values are given in Table 12.1.
It has long been realized that a large portion of the shot noise in a
pentode is due to the interception of electrons by the screen grid. Hence.