Radar System Engineering

134 C-W RADAR SYSTEMS [SEC. 56
transmitter, and the doppler frequency. The d-c component is removed
by passage through a transformer, or otherwise, and the doppler frequency
is used to actuate the indicator.
An incidental but important advantage of this modification is that
it enables the use of a suitable and easily adjustable crystal current.
These modifications are introduced in a manner indicated in Fig. 5.5
which shows a block diagram of the system. Everything is straightforward,
except for the local-oscillator power which is obtained by
modulating the transmitter and selecting a suitable sideband by means
of a bandpass filter. This insures a constant intermediate frequency and
avoids any tuning problems. Frequency-doubling is employed in the
modulator to avoid difficulties in keeping leakage from the crystal
oscillator out of the i-f amplifier.
Transmitting
antenna
Receiving
antenna
3000 Me/see
I
Oscillator
1
30 Mclsec
i.f
amplifler
2nd
detector
FIG. 5.5.—Block diagram of simple doppler-type radar system.
Design Procedure. —Having blocked out a proposed system, we may
now sketch the design procedure that fixed the various apparatus constants
and dimensions.
Design commenced with the choice of an indicator. A voltmeter
will certainly work, but experience has shown that a speaker or pair of
earphones, in conjunction with the operator’s hearing sense, is much
more effective provided the frequencies to be detected lie in the range
between a few hundred and a few thousand cycles per second. 1 Within
this range, the ear easily recognizes pure tones in the presence of hiss
even when the pure note is much too weak to be seen on an oscillograph.
This phenomenon is analogous to the averaging performed by the eye
when looking at an A-scope, as discussed in Chap. 2. Rough experiments
show that the ear will detect a pure tone in the presence of hiss
when the tone is stronger than the noise in a 200-cps bandwidth. If
the ear can do this over the range 200 to 3000 cps, then by using the
I A detailed study of the aural detection problem can be found in RRL Report
No. 411-86, May 5, 1944,