CRS PHASE DISCRIMINATOR A3700

TM 11-5820-670-30

1-18. CRS HUNT DISCRIMINATOR A3600, CRS PHASE DISCRIMINATOR A3700, AND REFERENCE

OSCILLATOR A2000. (CONT)

The hunt discriminator is tuned to a 5.625 MHz center frequency. It will therefore generate output

signals when its if. input is substantially under or over 5.625 MHz, which condition corresponds to a

large local oscillator error.

The hunt discriminator generates two kinds of signals, one of which is applied to the local oscillator

through the damping network to coarse tune the oscillator. When the local oscillator is far off fre-

quency (eg, -250 kHz), as it could be when changing channels, the hunt discriminator outputs a

negative dc voltage. This voltage can be measured at TP3003 with a dc voltmeter. When changing

channels, the local oscillator could be off frequency by some positive number of cycles, in which case

a positive voltage could be measured at TP3003. The plus or minus dc signal is applied to the local

oscillator as an error voltage, which corrects the local oscillator's frequency. Therefore, the dc

voltage at TP3003 is only momentary; that is, it disappears when the local oscillator approaches the

correct frequency.

Besides the dc error voltage, the hunt discriminator outputs an ac signal riding on the dc level. This

occurs because the local oscillator can drift around its steady state tuned frequency. The rate of drift

produces an ac signal from the hunt discriminator at the oscillator's drift rate. However, this ac com-

ponent is almost completely attenuated in the phase discriminator's damping network, and therefore

has no effect on the local oscillator.

The hunt discriminator is capable of outputting an error signal as great as 2.6 vdc measured at

TP3001 when a ground is applied to TP3701 (shorting the phase discriminator's output) and the local

oscillator is running 350 kHz off frequency. Strong dc output from the hunt discriminator rapidly

biases the local oscillator to within approximately 100 kHz of its correct frequency. Once the Iocal

oscillator is running 100 kHz from center frequency, the hunt discriminator is not sensitive enough

to fine tune the oscillator further. At this point, the phase discriminator begins to take control.

CRS PHASE DISCRIMINATOR A3700

Phase discriminator A3700 contains a ring modulator similar to that found in transmitter phase

discriminator A8200. It compares the phase of the signals from buffer and driver amplifiers A3700,

and outputs a dc error voltage when a phase difference exists. An ac component will ride the dc Ievel

due to oscillator drift, but the ac component is attenuated in the damping network and, therefore, is

not measurable at TP3001.

Once the hunt discriminator forces the local oscillator close to its correct frequency, the phase

discriminator maintains a 3.5 kHz oscillator tolerance. Because of its sensitivity to small errors in

the oscillator frequency, the phase discriminator performs a fine tuning function.

REFERENCE OSCILLATOR A2000

The output of reference oscillator A2000 is controlled by one of two crystals: a 5.60 MHz crystal and

another at 5.65 MHz. The crystals are selected by a switch through the action of the radio's gear train.

The switch contains contacts that correspond to 20 possible 50 kHz increments that can be selected

between whole-number frequencies. All frequencies whose decimal digits end in zero utilize the 5.65

MHz crystal. For example, the rotary switch selects 5.65 MHz crystal for 30.00, 30.10, 30.20, 30.30

MHz, etc. Decimal frequencies ending in five use 5.80 MHz crystal such as 30.05, 30.15, 30.25 MHz.

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