X
X
L
L
D
D
C
C
M
M
A
A
N
N
.
.
d
d
o
o
c
c
3
3
-
-
1
1
5
5
R
R
E
E
V
V
E
E
Press the up or down arrow keys to select either 1, 5, or 10 MHz as the input frequency of the external
oscillator. Press "FUNC/ENTR" to enter your choice. Now the display will show:
Ext Osc TUNING
SLOPE +1.00e-06
Press the up or down arrow keys to toggle the sign of the external oscillator voltage control tuning slope
which is entered in units of fractional frequency offset per volt. Press the right arrow key to move to the
next digit. Either press the up or down arrow keys or alternately, directly enter numbers from the
keypad for each of the remaining digits. The cursor will automatically advance to the next position
when a number is entered directly. When the display shows the desired choice, press "FUNC/ENTR"
to enter your choice. Now the display will show:
Ext Osc DAC
NOMINAL 0.50
Press the up or down arrow keys or directly enter the values desired for each digit to set the initial
starting value of the external oscillator control voltage. The number input here sets the decimal
fractional value of the full scale DAC output voltage swing to be used for the initial control voltage
setting. The control voltage output may be configured to be either 0 to +10 Volts or -5 to +5 Volts via
jumper JP5 on the 87-6XX GPS-XL board. Valid input range is 0.00 to 1.00. Once the desired value
has been set, press "FUNC/ENTR". The display will show:
Ext Osc TEMP
STAB 5.00e-10
Using the same combination of arrow keys and direct digit entry, set the 0
°
to 60
°
C temperature
stability of the external oscillator that is being controlled.
This value is used by the XL-DC to determine the optimal control loop averaging time so that
performance under environmental stress of up to 8.3
°
C per hour temperature change does not
degrade the output frequency stability significantly below that of the GPS system stability.
Inputting of better or worse temperature stability than the external oscillator actually has allows
adjustment by the user of the control loop averaging time being used. This may be desirable when the
user knows that the environment differs significantly from the rather stringent 8.3
°
C per hour
assumption made by the XL-DC.
As an example, if the environment is known to exhibit maximum temperature changes on the order of
1
°
C per hour and the user would desire more filtering of the de-stabilizing effects of SA from the
outputs of the XL-DC, a smaller value for the temperature stability of the external oscillator could be
entered. The control loop averaging time would then be lengthened relative to the default averaging
time assuming the true temperature stability were entered, by the square root of the ratio of the true
temperature stability divided by the temperature stability actually entered.
After pressing "FUNC/ENTR" to set the external oscillator temperature stability, the display will show:
Save Ext Osc
Parameters? No
Summary of Contents for 151-600
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