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Radar System Engineering

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678 MOVING-TARGET INDICATION [SEC. 16.21<br />

oscillator. A similar type of stability is required of the coherent oscillator.<br />

The apparatus designed by S. Robertsl for measuring oscillator<br />

stability has two functions to perform. The first is the quantitative<br />

determination of the rate of change of frequency. Because a precise<br />

calculation of phase error requires the assumption that the modulating<br />

frequencies are small, the second function is to check the accuracy of this<br />

assumption. This is done by changing the time constant of the measuring<br />

circuit.<br />

A block diagram of the apparatus used for testing local-oscillator<br />

stability is given as Fig. 16”36. The difference frequency of 7(I kc/see is<br />

obtained by mixing the power from two oscillators in a crystal rectifier.<br />

One of these oscillators can be a reference standard against which the<br />

cl<br />

Oscillator<br />

under<br />

test j<br />

t<br />

Crystal<br />

mixer<br />

J<br />

Diffe~dtiator<br />

70kc/sec<br />

— — Discriminator — frequency<br />

amplifier<br />

weighting<br />

circuit<br />

f<br />

—<br />

,<br />

Recti~lng<br />

voltmeter<br />

FIG. 16.36.—Block diagram of oscillator stability tester.<br />

other is compared. The output signal from the mixer is amplified and<br />

applied to a frequency discriminator circuit. The voltage applied to the<br />

discriminator is monitored by means of a vacuum-tube voltmeter and is<br />

always adjusted to the same “value. The voltage output of the discriminator<br />

is linearly related to the difference frequency. This discriminator<br />

output voltage is applied to a differentiating circuit consisting of a<br />

resistor and condenser. The average voltage across the resistor, proportional<br />

to the average rate of change of the difference frequency, is<br />

measured by a voltmeter. Headphones assist in the identification of the<br />

frequency of any modulation that may exist.<br />

To measure the stability of a coherent oscillator, it is allowed to run<br />

freely. For the crystal mixer of Fig. 16.36 is substituted a 6SA7 mixer,<br />

whose third grid is connected to the output signal of the coherent oscillator.<br />

Signals from a quartz-controlled standard oscillator are applied to<br />

the first grid of the mixer. The ditierence frequency between the coherent<br />

oscillator and the standard is chosen as 70 kc/see.<br />

I Lot. cit.

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