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

ME5000 Test Measurements
4. FREQUENCY MEASUREMENTS
Since the accuracy of any EDM instrument is at least partly dependent upon the accurate knowledge
of the modulation frequency, it is important to ensure that there is no significant frequency drift over time.
The basic equation for dete mining the distance is shown in (3). The distance D equals a whole number m
of half-wavelengths U2, plus some fiactionfof an additional half-wavelength.
k x
D= mpfz (3)
Three different methods are used in EDM instruments in order to fmd f, the fractional part of a half-
wavelength.
1. I&a-red instruments, such as the DM503, use a fued t?equency and measure the phase
difference between the reference signal and the returning beam. Such an approach, though the
simplest and cheapest, introduces cyclic error.
2. The frequency of the ME3000 Mekometer is allowed to vary as changing atmospheric conditions
change the dimensions of the reference cavity. A system of prisms is moved within the
instrument so as to increase the path length until the fractional component a is eliminated. This
approach eliminates cyclic error.
3. The ME5000 Mekometer varies the frequency until there is an integral number of wavelengths in
the distance. Like the ME3000, this approach eliminates cyclic error.
-The frequency cahbration of the ME5000 is more straightforward than that of the earlier ME3000
Mekometer. Because the carrier wave of the ME5000, a HeNe laser (h = 632.8 nm), is continuously
modulated the modulation frequency can be directly measured by any frequency counter capable of
operating at 500 MHz (Fig. 2). Since there is no reference cavity in the ME5000, the modulation
frequency is not dependent on the ambient temperature and pressure as is the ME3000.
A SMA jack, similar to that on the ME3000, is located on the underside of the ME5000. A SMA-
BNC adapter is included .with the instrument so that a standard c0axia.l cable can be connected to this
outlet. The function switch should be set to either Battery or Remote. Set to Battery, the frequency
currently set is shown on the display. Set to Remote, any frequency within the range of the instrument
(460 - 5 10 MHz) can be set externally.
SMA-BNC
Adapter
FIGURE 2. Schematic for ME5000 frequency calibration
600 MHz
Frequency
counter
At SLAC we use a HP 5342A microwave frequency counter in one of the Klystron test labs. It is
calibrated annually and is reliable to a few tens of Hertz. As long as power is supplied to the counter, the
crystal is kept at a stable temperature even when the main power switch is off.
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