Tektronix SPG8000 Technical Reference page 172

SYSTEM button menu
160
TUNE. Displays the following information about the tuning status of the internal
oscillator. Use the left (◄) or right (►) arrow button to view all of the oscillator
tuning status information.
Tune value: Displays how far the instrument is deviating from the calibrated
internal frequency of the internal oscillator in order to lock to an external
reference input. One value of this readout is to determine if the instrument
oscillator needs to be calibrated. If the instrument is locked to a traceable
reference, then any tune value indicates error in the instrument calibration.
For example, if the instrument is locked to GPS and the Tune readout is more
than ±0.1 e-6, then you should consider calibrating the internal frequency of
the instrument using the CALIBRATE OVEN menu item in the SYSTEM
button menu. (See page 135.)
The first part of the Tune readout indicates the fractional amount of deviation
from the calibrated frequency. For example, if you use a reference signal to
lock to a video source that is 5 parts per million high, then the readout will
say "+5.00e-6". This allows you to measure the frequency of the reference
input and to see if the input is near the end of the available tuning range. If the
instrument is in internal reference mode, the tune value should be 0.00 e-6 and
the integer values will equal the calibration value (CAL readout).
NOTE.
The TUNE display will show non-zero deviation values only when the
signal reference source is not configured as INTERNAL in the REF button menu.
(See page 57.)
The integer values indicate the actual tuning control word. The allowed range
of the tuning control word is 0 to 4,194,304. The first number is the current
value of the word and the second number is the target value for the tuning
control loop. In most cases, these two values are the same. However, in the
case where the instrument needs to make a large frequency change to achieve
signal lock, then depending on the instrument mode, the value may slew
slowly to the target value to stay inside the allowed slew rate for reference
systems. In this case, the first number will slew over seconds or minutes until
it equals the value of the second number. If the instrument is taking a long
time to lock to an external reference signal, the Tune readout may provide
valuable information about what is happening.
Phase value: Displays the phase between the GPS pulse per second and the
internal master time. A Tektronix Customer Service Representative may
request these values for troubleshooting installation issues, should they occur.
DDS value: Displays the DDS code that is tuning the oscillator. This is
useful for checking if the instrument is getting close to the end of the oven
oscillator tune range.
SPG8000 Technical Reference