Download 442 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC. 12.4

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