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Radar System Engineering

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f?iEc.16.4] ALTERNATIVE METHODS FOR OBTAINING COHERENCE 637<br />

Taking all possible combinations, we get 23 different types of circuit.<br />

These areshown in Figs. 1613to16”16.<br />

The circuit shown in Fig. 1613a. has advantages over the other<br />

schemes. In the first place, the signals are added at intermediate<br />

Modulator — Transmitter TR<br />

L<br />

Mixer<br />

Stable Iwal<br />

— —<br />

oscillator<br />

Mixer<br />

I f !<br />

I<br />

(a)<br />

1<br />

Mixer<br />

I<br />

If<br />

transmitter<br />

*<br />

I 1 /<br />

Power ~<br />

TR<br />

amplifier<br />

I<br />

1=<br />

Echo signals<br />

I<br />

—<br />

Stable local<br />

Mixer<br />

oscillator<br />

Reference<br />

signal<br />

Modulator LO Mixer<br />

I<br />

L<br />

CoherentI.f<br />

oscillator<br />

t<br />

Receiver<br />

t<br />

FIG. 16.16 .—I-f locking, r-f addition. (a) I-f locking by transmitter, r-f addition.<br />

(b) I-f Iockhg by oscillator, r-f addition.<br />

(b)<br />

frequency, which allows amplification of the signals before addition.<br />

Second, i-f locking is superior to r-f locking. The latter is hard to do in<br />

the case of the transmitter, because of the large amount of power required;

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