Radar System Engineering

SEC. 16.4] ALTERNATIVE METHODS FOR OBTAINING COHERENCE 635
initially. The two lines are mounted close together in a thermally
insulated box so that the temperature changes equally for both.
16.4. Alternative Methods for Obtaining Coherence.—Figures 16%
and 16.9 show two clifferent ways of producing coherence between echo
signals and a reference signal: in the first method, the oscillator whose
phase is locked runs at radio frequency and the signals are added at
radio frequency; in the second, the locking and adding are both done at
the intermediate frequency.
Mcdulator Transmitter TR
I
I
I
I
v
Mixer
Locking
Coherent l-f
oscillator
Stable local
oscillator
Reference signal
(a)
Mixer
Echo signals
Receiver
t
Mdulator
Power — TR
amplifier
t $
1
If
transmitter
— Mixer —
i
Stable local
oscillator
I
I Coherent l-f Reference
Receiver
t
Locking oscillator signal Echo signals
(b)
FIG. 16. 13.—I-f locking, i-f addition. (a) I-f locking by; transmitter, ‘-f addb,ion.
(b) I-f locking by oscillator, i-f addition.
It is also possible to lock at radio frequency and add at intermediate
frequency. Further, the oscillator can be made to lock the phase of the
transmitter instead of vice versa. The various arrangements can
therefore be classified according tfi
1. Whether the transmitter locks the oscillator or vice versa.
2. Whether the locking takes place at radio frequency or intermediate
frequency.
3. Whether the signals are added at radio frequency or intermediate
frequency.
t