02 introduction of experimental device

The selection of the clock:
    The frequency of the clock should be as higher as possible and is stable enough. Firstly, the laser light occurred to us. For example, some advanced stable 1030nm laser light which adopts deep cooling measure, its linewidth can reach 1MHz, this is not easy comparatively. We can regard the linewidth as stability of short-term frequencyand It is: △f/f =3.4×10^-9 (suppose relative to 1 second but it can't reach 1 second. Linewidth is caused by the heat movement of atoms. It's irregular noise ). But, The short-term stability of a usual "Oven Controlled Crystal Oscillator" (OCXO) can reach △f/f=10^-10 (relative to 1 second). In the long-stability of frequency, that a phase-locking technology of Frequency was adopted, at present, the frequency excursion of the best laser light is 100MHz per hour. however, this performance  of the OCXO is 1ppm per year, not to mention the other factors, such as volume and price. At the present time, two laser lights can not get stable interference images. So that, OCXO is the best choice, especially in its characteristic can not be interfered by  the temperature of environment. Here, we select 327.68MHz OCXO.

Figure 2.01 Oven Controlled Crystal Oscillator (OCXO)

  The main characteristic parameters of the OCXO (supplied by the manufacturer)

The production and installation of the revolving bar
    We changed the sine wave which mixer output into square wave and measured the width of the square wave by high speed counter? The high speed counter continually increases under the drive of an 80MHz clock, the parallel port of the counter there is a flip-latch. When the rising edge of the square wave arrive, the flip-latch instant record the count value and send to the processor. The processor subtract previous count value from this, then gets the net value of the square wave. Fig. 2.02 is the circuit frame in revolving bar.

Fig. 2.02 function circuits and its installation position in the revolving bar
A ; B - clocks; T- transmission line; F- mixer; L - filter; E - voltage comparator ; C - processor

    The OCXO should be go through more than 480 hours of aging. In order to obstruct the external electromagnetic interference, the OCXO need to be covered by copper shield and iron shield, in the most outer, an insulation layer and certain thick of Thermal Insulation are need in order to avoid the temperature change by air current. All the circuits are powered by the battery which fixed in the revolving bar. The battery operation floating charge state and keep the power of the circuit uninterrupted forevermore and avoid resetting. In the proceeding of collection, the only contact to the outer of the device is the drive. The drive could also be removed if there were no the air drag. So the device of the revolving bar is an absolutely independent system.

Average the circumference into eight parts
    The data of width which measured is sorted to record, the base that be sorted is the direction of the clock moving, i.e. the position of the revolving bar then and there. As the Fig. 2.03 shows, the infrared photo interrupter I is installed on the revolving bar. There eight cut-light-pieces S0~S7 are fixed on the support. When the photo interrupter passes the S0, the address is reset and return to zero. When it passes one of the S1~S7, the address increase by one. So, when the revolving bar rotates, the address will be 0, 1, 2, 3, 4, 5, 6, 7. It will repeat in this manner.

Fig. 2.03 the function of the revolving bar recognizing angle
I - infrared photo interrupter. S0~7 – cut-light-pieces (copper pieces fixed on the support)

Working process
    During the rotation, the processor examine at once the address of the revolving bar when it gets a data of width and save the data according as the address. After the collection, stop rotating and take out the storer, read the data by computer to analyse.

Fig. 2.04 overlooks of the experimental set

Zhan, Li Kui 6/18/2007

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