PRINCIPLES OF OPERATION

TM 11-6625-3059-10

PRINCIPLES OF OPERATION

b. Radio Frequency (RF) Circuits. The RF IN signal to be measured is connected to the radio frequency circuits

through a front panel RF IN connector. Within the rf circuits the input signal is applied through an electronically

controlled attenuator to a sampling mixer. The attenuator is controlled by a signal from the amplitude modulation (AM)

circuits, and it is effective for input signals above approximately 100 millivolts rms. Attenuation prevents sampling mixer

overload. The sampling mixer mixes the input signal and signal supplied either from an internal osciIllator or from an

external source (EXT OSC) as selected by the operator, to produce a 1.0 M Hz intermediate frequency signal. If the

internal oscillator is selected, the oscillator frequency is swept over the oscillator tuning range automatically by the

frequency modulation circuits until frequency lock (1.0 MHz intermediate frequency) is attained; the oscillator frequency

is then maintained precisely at this point by the local oscillator control in the frequency modulation circuits. If the

external source is selected, the external oscillator must be set precisely to a frequency that will produce the 1.0 MHz

intermediate frequency when mixed with the radio frequency input signal. The 1.0 MHz intermediate frequency signal

developed by the sampling mixer is supplied to the amplitude modulation circuits and to the frequency modulation

circuits.

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