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APPENDIX C:
Calibration
The user may occasionally find the need to recalibrate the ZM-30 instrument, such as when new software updates
are available that change the way in which calibration data are used, or when the characteristics of the DDS signal
source change in any regard. In these cases, the instrument may be quickly and easily recalibrated by following a
short sequence of operations located in the “Calibrate” function, located beneath the CONFIG pushbutton.
1.
Select the Calibrate function, located within CONFIG.
2.
See “Calibrate now?” displayed in the LCD. Confirm by selecting “yes” with the dial. (Press the dial
when “yes” is displayed.)
3.
See “Open load” displayed on the LCD. Ensure that nothing is connected to the RF output jack, then
confirm by selecting ”yes” with the dial.
4.
See a display of numbers representing the reflectometer channel voltages for Vf, Vr, Vz and Va. Ensure
that the first three hex numbers are generally “high” numbers above 90 (9x, Ax, Bx, Cx, Dx, or Ex), and
that the last number (Va) is very close to zero (e.g., 04).
5.
If the displayed numbers are in the acceptible range, as explained above, press the Dial to continue.
However, if the “3 high and one low” condition of numbers is not present, or if any channel is very high in
the hex number range (e.g., from F0 to FF), then you cannot proceed with calibration and you should
contact Support.
6.
See “Shorted load?” displayed on the LCD. Place a short circuit on the RF output jack and select “yes”
with the dial.
7.
See “270-ohm load on?” displayed on the LCD. Place the supplied 274-ohm resistive load on the RF
output connector and confirm by selecting “yes”.
8.
See “Make SWR=5.4” displayed on the first line of the LCD, and a number on the second line. Adjust the
dial to make the number on the second line be as close as possible to 5.4. Press the dial when this number
is 5.4.
9.
See “50-ohm load on? displayed on the LCD. Place a 50-ohm load on the RF output connector and
confirm by selecting “yes” with the dial.
10.
See “Calibration done! Saving data” displayed on the LCD and the red BUSY LED will turn on for several
seconds.
11.
When BUSY turns off, “Exit” will be displayed on the LCD. Confirm by pressing the dial and control will
again be placed in the Impedance mode, with measurements being made using the newly-created
calibration data.
Aside: “What is being calibrated?”
In order to normalize the gains of the four reflectometer channels with respect to each other, the software creates
“gain correction factors” for the Vr, Vz and Va channels. When each channel is multiplied by its respective
correction factor, it will equal the value of the Vf channel, as determined under the specific conditions of
open/short/50-ohm loads presented during the calibration sequence. These correction factors are then used during
the normal operation of reading raw Vr, Vz and Va signals to compute the SWR, R and X values displayed on the
LCD in Impedance mode. The correction factors are saved away to nonvolatile memory such that they can be used
each time the instrument is turned on
.