TB 9-4931-539-35
time to obtain the difference. It is equally correct to compute the difference in
microseconds by subtracting the start time from the stop time. The magnitude of the
phase deviation will remain the same. Only the polarity of the FFE result will change.
To alleviate confusion, use whichever method is preferred consistently and disregard
the polarity sign in the FFE result. The computation of phase difference using the chart
recorded is basically identical to that using the MICROSECONDS counter reading. The
chart paper is broken down into a series of grids. Reading vertically, up and down, the
right-hand side of the paper is numbered from 1 to 12. This numeric sequence is
repetitious and represents a certain amount of elapsed time. The time lapse between
numerals (Phase Tracking Rate) is dependent on the position of the SERVO TIME
CONSTANT SEC switch. Reading the chart paper horizontally gives the phase
deviation. Full scale can be either 10 or 100 s, depending on the scale setting of the
rear panel switch. Ideally, if there is no frequency deviation between the received VLF
signal and the local oscillator over a specific time period, a straight line (track) would be
produced indicating a zero phase shift (drift). Tracks to the left indicate that the local
standard is low in frequency ( ) while tracks to the right indicate that it is high in
-
frequency (+). By observing the elapsed time and converting it to seconds and noting the
phase deviation in microseconds, fractional frequency error can be calculated. (NOTE:
Frequency comparisons should always be made during the daylight hours when VLF
propagation is extremely stable.)
used to adjust the local oscillator if required. Adjustment is NOT necessary as long as
the local standard is within 5 parts in 1010. There are two adjustments for both models,
105A and FS-323. These are the COARSE FREQUENCY ADJUST and the FINE
FREQUENCY ADJUST. The COARSE FREQUENCY ADJUST is used to change the
oscillator's frequency by large increments and is normally made only when the local
oscillator is initially installed or has been repaired. Adjustment is accomplished by first
setting the FINE FREQUENCY ADJUST control to 250 (midrange) and then connecting
the output of the oscillator to an oscilloscope. Next, adjust the COARSE FREQUENCY
ADJUST control for a stable display on the oscilloscope. The COARSE FREQUENCY
ADJUST control is normally kept covered to prevent accidental adjustment or
tampering. Control range of the COARSE FREQUENCY ADJUST is 1 part in 106.
Ordinarily, the FINE FREQUENCY ADJUST control is the adjustment utilized to
maximum range of 500 parts in 1010. To illustrate how the actual adjustment is made,
use the previously computed FFE example of 2.36 x 10-9 Since the local standard is not
within 5 parts in 10-10, an adjustment is required. The computed FFE is either
subtracted from or added to the current FINE FREQUENCY ADJUST control setting.
It will be added if the MICROSECONDS counter reading has decreased (indicating a
decrease in nominal frequency of the local standard). It will be subtracted if the
MICROSECONDS counter reading has increased (indicating an increase in the nominal
frequency of the local standard). For instructional purposes, assume that the FINE
