Oscillators – Automatic frequency stabilization using a phase or frequency... – Plural oscillators controlled
Reexamination Certificate
2000-11-02
2002-06-11
Mis, David (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Plural oscillators controlled
C331S018000, C327S147000
Reexamination Certificate
active
06404288
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an arrangement for generating a first alternating signal and a second alternating signal that is in a predetermined, fixed frequency relation to the first signal. Such an arrangement is used, for example, as a time base generator for a level meter that operates according to the radar principle and whose measuring system is based on the time domain reflectometry (TDR) measuring principle. The TDR measuring principle is known from the field of cable testing, for example, and shows similarities with the method of operation of radar equipment. In a known TDR level meter, an extremely short electrical pulse carried via two electric conductors running essentially straight is emitted into a container holding a medium such as a liquid, a powder or a granulate, the level of which is to be determined. The short electrical pulse emitted via the two conductors into the container is reflected at the surface of the medium, and the reflected portion of the short electrical pulse is again detected by a measuring transducer of the measure device. The reflected portion of the short electrical pulse depends on the dielectric constant of the medium and increases with it. In this connection, the transition time of the signal is proportional to the distance of the measuring transducer from the surface of the medium in the container. Changing surrounding conditions such as increasing or decreasing surrounding pressure or increasing or decreasing temperature do not impair the measuring accuracy of the TDR level meter. In addition, the signal's transition time is independent of the dielectric constant of the medium whose level is to be measured.
The TDR measuring principle is thus based on the fact that the transition times—quite short under certain circumstances—of an electromagnetic signal are measured. If the container is almost completely filled with the medium in such a way that the surface of the medium is, for example, only 15 cm below the measuring transducer of the TDR level meter, the entire path of the electromagnetic signal from the measuring transducer to the surface of the medium and back again is only 30 cm, corresponding to a transition time of 1 ns for the short electrical pulse. To be able to measure such short transition times at all, a sampling process is used for which two fast, alternating signals are generated that have a certain frequency difference from each other in the range of a few hertz. Measuring is then carried out in such a way that the measurement is started at a time zero at which the two fast, alternating signals are vibrating in-phase. In this connection, the signal vibrating at the higher frequency indicates the timing for emitting the actual measuring signal, i.e., the short electrical pulse, into the container. Thus, for example, a short electrical pulse is always generated and emitted into the container at the beginning of a period of the signal vibrating at the higher frequency. The signal vibrating at the lower frequency lags behind the signal vibrating at the higher frequency by a certain amount per is vibration period, namely some 4 ps per period for a frequency difference of, e.g., 4 Hz and an oscillation frequency of about 1 MHz for the two signals. This period of 4 ps thus indicates the digital time frame or the digital time base with which the transition time of the short electrical pulse emitted in the container and reflected at the surface of the medium is measured.
While the signal vibrating at the higher frequency indicates the timing for emitting the short electrical pulse into the container, the signal vibrating at the lower frequency determines at what point in time a reflected signal can be detected for an extremely short period. Thus, according to the above-described example, as of the time when the two alternating signals are in-phase, after 1000 vibration periods, a reflected pulse would be detected that would have had a transition time of 4 ns, corresponding to a total path of 1.2 m and thus a level for the medium to be measured of 60 cm under the measuring transducer of the TDR level meter.
Time base generators with essentially the following design have previously been used to generate a time base for a TDR level meter:
With two oscillators that are each operated at an oscillation frequency of several times 10 MHz, a first oscillation signal and a second oscillation signal are generated. These two oscillation signals are divided, on the one hand, by digital frequency dividers with a corresponding frequency division so as to obtain, for example, a signal of 1 MHz and a second signal of 1.000004 MHz, i.e. the first signal and the second signal have a frequency difference of 4 Hz. The oscillation signals of the two oscillators, on the other hand, are fed to a third and a fourth digital frequency divider, respectively, which digitally divide the two oscillation frequencies of the various oscillators so as to obtain a common, identical synchronization frequency from the two different oscillation frequencies. The synchronization frequency generated from the third digital frequency divider and the synchronization frequency generated from the fourth digital frequency divider are fed to a phase indicator which, depending on the phase difference between the two received signals, transmits a voltage to a voltage-controlled oscillator which, in turn, regulates the frequency of the second oscillator in such a way that the frequencies of the two oscillators are in a predetermined, fixed frequency relation to each other, in such a way that the two signals generated and emitted by the time base generator have a predetermined, fixed frequency difference, 4 Hz in the present case. In this way, the permanently preset frequency difference between the two signals is always maintained regardless of frequency changes of the first oscillator, e.g. due to temperature influences. In this connection, it should be noted that due to the ratio of the two signals generated and emitted by the time base generator, which is quite close to 1:1, the maintenance of the permanently preset frequency ratio between the two signals in practical operation, in which only slight fluctuations of the first oscillator's frequency occur due to external influences, also meets the requirement for the frequency difference between the two signals to remain constant.
The principle of such a time base generator is thus based on the fact that the first oscillator is excited with a first frequency but may experience at least slight deviations from its preset value due to ambient influences such as temperature changes. However, since it is essential for the accuracy of the time measurement that there be, relative to the first oscillator's frequency, a precisely predetermined, always constant frequency difference that represents the time frame for the time base, the second oscillator's frequency must also be made to follow accordingly if the first oscillator's frequency changes. For this, the above-described design, in which a PLL (phase locked loop) is used, serves to adjust the frequency of the second oscillator to the proper value in each case. In general, a PLL essentially consists of a phase indicator and a voltage-controlled oscillator. In this connection, the phase indicator obtains, on the one hand, a reference signal to whose frequency the signal in the PLL should be adapted and, on the other hand, a signal from the voltage-controlled oscillator, which can be fed directly or further processed, to the phase indicator. The phase indicator then puts out a voltage signal—depending on the phase difference of the two signals fed to it—to the voltage-controlled oscillator, which signal thus serves to control its output frequency. It is then possible with such a control circuit of a PLL to lock-in a frequency generated in the PLL to another reference frequency. In the present case, the following are components of the PLL: the phase indicator, the voltage-controlled oscillator, the second oscillator as well as the third
Bletz Achim
Thollet Alexandre
Cesari and McKenna LLP
Krohne A.G.
Mis David
LandOfFree
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