The Use and Calibration of the Kern ME5000 Mekometer - SLAC ...

ME5000 Operation
found, F2 = FL.+ For a few distances in the range 20-25 m only two frequencies, F1 and F3, are found.
At ‘each nodal point the frequency is determined by a coarse measurement followed by a fine
measurement. _
3.13. Coarse search in normal mode
Using the onboard coarse search function (SF144) the Frequency is increased by predefmed
increments, starting from the lower end of the bandwidth, until a zero point (zero phase difference) is
found. Each zero point is indicated by a reduction in the light intensity falling on the detector diode in the
instrument. When the outgoing and return signals are exactly in phase no light falls on the diode, Fig. 7.
Light
Intensity
3.1.3. Fine search in normal mode
,,..- ------- . .._.,____ ------. .,.. ohsrscterlstic curve
,..’ -._,____,,.,_.._...-. /----
,,’
'i
..z
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i'
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FIGURE 7. Light intensity at the receiver diode
Frequency
The onboard function SF145 performs the fine search routine. Due to background noise it is difficult
to pick up the minimum signal itself. Instead, the instrument superimposes a 2 kHz wobbler frequency.
The amplitude of this signal is 9~6.7 kHz for high range (>500 m) and i35 kHz for low range (20 - 500
m). When the oscillation of this 2 kHz signal covers the zero point itself it produces a 4 kHz signal. This
means that the coarse search must locate the zero point to within half the bandwidth of this wobbler
frequency. If the mean frequency does not exactly coincide with the zero point a 2 kHz signal is also
produced, Fig. 8(a). When the mean frequency does coincide with the zero point this 2 kH.z signal
disappears, Fig. S(b). The wobbler frequency is adjusted in steps of 162 Hz until the zero point is located.
This smallest step of 162 Hz is equivalent to a change in wavelength of 0.1 p.
(a) Non-tuned case
(b) Tuned case
sea table
FIGURE 8. Determination of the zero point using a 2 kHz wobbler signal
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