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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