Abstract:
An absolute distance meter for measuring a distance to a target may include a synthesizer including a first quadrature modulator and structured to receive a reference signal having a reference frequency and output a first signal having a first frequency and a second signal having a second frequency, a laser structured to output a laser beam, wherein the laser beam is modulated by the second signal, an optical system for directing the laser beam toward the target, a reference phase calculating system structured to calculate a reference phase based on signals having the first frequency and the second frequency, a target optical detector structured to receive at least a portion of the laser beam returned from the target and structured to output a measured electrical signal having the second frequency based on the at least a portion of the laser beam, and a measure phase calculating system structured to calculate a measure phase based on the measured electrical signal and the first signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 60/909,099 filed Mar. 30, 2007, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to a device capable of making absolute distance measurements. As instrument capable of absolute distance measurement is distinguished from one that measures incremental distance in that it can immediately measure distance to a target of interest even if the beam path has been broken. Another way of saying this is that an absolute distance measuring device can immediately measure distance to a target at an arbitrary location. The target may be a cooperative target such as a retroreflector or a non-cooperative target such as a diffuse surface. 
         [0003]    One method of measuring absolute distance is to modulate laser light, send the light to a remote target, detect it upon return, and determine its phase of modulation. The phase of the returning laser light, which is called the measure phase, is compared to a reference phase, which is derived either from an electrical or optical signal from within the instrument. The difference in the measure and reference phases is used to calculate the distance to the target. 
         [0004]    To measure the phase of a modulated signal following optical detection, one technique that can be used is to create another frequency, referred to as the LO frequency, that is mixed with the laser modulation frequency, referred to as the radio frequency (RF). After filtering, the result of the mixing process is to produce a frequency, called the intermediate frequency (IF), whose value is equal to difference in the LO and radio frequencies. This IF is sampled in an analog-to-digital converter (ADC) an integer number of times per cycle. The numerical values from the ADC are sent to a processing device that uses a single-point Digital Fourier Transform (DFT) algorithm or its equivalent to find the phase of the measure and reference signals. 
         [0005]    The chief complexity in this approach is generating two frequencies, the LO and radio frequencies, that are relatively close in value so that the IF is small enough so to permit reasonably small sampling rate for the ADC. Furthermore, the signals must be generated in a way that avoids generation of spurious signals that can corrupt the measurement results. One method that has been used for generating two closely spaced frequencies is with a double phase-locked loop (PLL) approach; however, this approach requires a complex design. A second method is to use a series of mixers to upconvert two baseband signals of slightly different frequency to two much higher frequencies having the same slight frequency difference. The main disadvantage of this approach is that it requires numerous mixers and filters, all of which must be shielded from one another to prevent crosstalk. The resulting assembly is relatively large and complicated. A third method uses a quadrature modulator to generate a single sideband signal. By carefully adjusting the phase, offset, and amplitude of the low-frequency signals that are applied to the quadrature modulator, it is possible to ensure that the modulator will reject the unwanted sideband and carrier by approximately 50 dB. However, for optimum performance of an ADM system, this rejection should be at least 70 dB and preferably 90 dB. 
         [0006]    What is needed is an absolute distance meter that measures phase by a simplified method that neither has the complexity of the dual-PLL or multiple-mixer approach nor the performance shortcomings of the quadrature-modulator approach. 
       SUMMARY OF THE INVENTION 
       [0007]    At least an embodiment of an absolute distance meter for measuring a distance to a target may include a synthesizer comprising a first quadrature modulator and structured to receive a reference signal having a reference frequency and output a first signal having a first frequency and a second signal having a second frequency, a laser structured to output a laser beam, wherein the laser beam is modulated by the second signal, an optical system for directing the laser beam toward the target, a reference phase calculating system structured to calculate a reference phase based on signals having the first frequency and the second frequency, a target optical detector structured to receive at least a portion of the laser beam returned from the target and structured to output a measured electrical signal having the second frequency based on the at least a portion of the laser beam, and a measure phase calculating system structured to calculate a measure phase based on the measured electrical signal and the first signal. 
         [0008]    At least an embodiment of a synthesizer for use in an absolute distance meter may include a phase locked loop structured to receive a reference signal having a reference frequency and output a phase locked loop signal having a phase locked loop frequency, a first signal generator structured to output a first generated signal having a first generated frequency, a second signal generator structured to output a second generated signal having a second generated frequency, a first quadrature modulator structured to receive the phase locked loop signal and the first generated signal and structured to output a first sideband signal, and a second quadrature modulator structured to receive the phase locked loop signal and the second generated signal and structured to output a second sideband signal, wherein the phase locked loop frequency is higher than the reference frequency, and the first generated frequency and the second generated frequency differ by a predetermined intermediate frequency. 
         [0009]    At least an embodiment of a method of making an absolute distance measurement of a target may include generating a first signal having a first frequency and a second signal having a second frequency using a first quadrature modulator, outputting a laser beam from a laser, wherein the laser beam is modulated by the second signal, directing the laser beam to the target, detecting at least a portion of the laser beam returned from the target and generating a measured electrical signal having the second frequency based on the at least a portion of the laser beam, calculating a measure phase based on the measured electrical signal and the first signal, calculating a reference phase based on signals having the first frequency and the second frequency, determining the absolute distance measurement based on a difference between the reference phase and the measure phase. 
         [0010]    At least an embodiment of a method of generating sideband signals may include receiving a reference signal having a reference frequency in a phase locked loop, generating a phase locked loop signal having a phase locked loop frequency, generating a first generated signal having a first generated frequency, generating a second generated signal having a second generated frequency, receiving the phase locked loop signal and the first generated signal in a first quadrature modulator, receiving the phase locked loop signal and the second generated signal in a second quadrature modulator, outputting a first sideband signal from the first quadrature modulator based on the phase locked loop signal and the first generated signal, and outputting a second sideband signal from the second quadrature modulator based on the phase locked loop signal and the second generated signal, wherein the phase locked loop frequency is higher than the reference frequency, and the first generated frequency and the second generated frequency differ by a predetermined intermediate frequency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Referring now to the drawings, exemplary embodiments are shown which should not be construed to be limiting regarding the entire scope of the disclosure, and wherein the elements are numbered alike in several FIGURES: 
           [0012]      FIG. 1  is a block diagram of an exemplary measuring device and system; and 
           [0013]      FIG. 2  is a block diagram view of the synthesizer components and the signal frequencies that are generated. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    As shown in  FIG. 1 , ranging device  100  comprises frequency reference  10 , synthesizer  20 , laser  50 , collimating lens  60 , beam-splitting means  62 , optical detectors  70 ,  80 , mixers  72 ,  82 , analog-to-digital converters (ADCs)  74 ,  84 , and divide-by-N function  76 ,  86 . Frequency reference  10 , which is preferably an oven controlled crystal oscillator (OCXO), sends a high stability signal of frequency f REF  to synthesizer  20 . Synthesizer  20  produces signals at frequencies f LO  and f RF . The signal with frequency f REF  is an example of a reference signal having a reference frequency. The signal with frequency f LO  is one example of a first signal having a first frequency, and the signal at frequency f RF  is an example of a second signal having a second frequency. 
         [0015]    The signal at frequency f RF  modulates some characteristic of laser  50 , preferably the optical power of the laser beam. This type of modulation is commonly known as intensity modulation. Laser beam  90  passes through collimating lens  60 . A first part of this laser beam, i.e., a target beam, then passes through beam splitting means  62  and travels to target  200 . On the return path, the laser beam is redirected by beam-splitting means  62  to strike optical detector  80 . A second part of laser beam  90  from collimator lens  60 , i.e., a reference beam, is directed by beam-splitting means  62  to optical detector  70 . Hence that portion of the laser beam received by optical detector  80  has made a round trip to target  200 , while that portion received by optical detector  70  has remained within the ranging device  100 . Beam-splitting means  62  may be made of glass, as illustrated in  FIG. 1 , or it may be a fiber optic assembly comprising one or more fiber splitters or similar devices. 
         [0016]    The electrical signals from optical detectors  70  and  80  contain the frequency f RF . It will be understood that by a signals “containing” or “having” the frequency f RF  does not necessarily mean that these signals contain only frequency f RF . For example, it will be understood that the signals may include other frequencies that may be excluded. These signals pass into mixers  72  and  82 , respectively. Mixer  72  is one possible example of a reference mixer and mixer  82  is one possible example of a target mixer. The signal at frequency f LO  from synthesizer  20  enters mixers  72 ,  82 . The function of the two mixers is to produce sum and differences frequencies. The higher of these two frequencies is filtered out, either by a filter specifically created for this purpose or incidentally as a result of bandwidth limitations of the components that follow the mixer. The lower of the two frequencies that leaves the mixer is the intermediate frequency (IF), which is equal to f IF =|f LO −f RF |. In other words, the mixers  72 ,  82  output an intermediate signal having an intermediate frequency. The IF is sent to the analog-to-digital converter (ADC), where it is sampled at the rate of the clock that is derived from the frequency reference by passing through the divide-by-N component. The rate of the sample clock is equal to a multiple of the intermediate frequency f IF . 
         [0017]    The digital samples that are output from ADCs  74 ,  84  are sent to processing device  78 ,  88 , which are preferably a microprocessor (uP) or digital signal processing (DSP) chip. The devices  78  and  88  are preferably combined in one electrical chip. Processing devices  78 ,  88  perform calculations to the phase of the IF signals from mixers  72 ,  82 . Generally these calculations are based on the discrete Fourier transform (DFT) and are selected to efficiently extract the phase of the signal received by the ADC. Processors  72 ,  82  are said to extract the reference phase and measure phase, respectively. The difference phase is obtained by subtracting the reference phase from the measure phase. The phase is divided by 2π and the result is multiplied by the ambiguity interval to determine the relative distance traveled within that ambiguity interval. The relative distance traveled can be determined by a distance calculator such as a processor or any other suitable device or structure. The ambiguity interval is defined as the speed of light in vacuum divided by twice the product of the modulation frequency and the group index of refraction of air. If more than one ambiguity interval is present, then another must be provided to establish which ambiguity interval the target is in. This is usually done by providing one or more additional modulation frequencies to the laser. These modulation frequencies may be applied sequentially or simultaneously depending on the particular measurement requirements. In addition, prior to first use of absolute distance meter  100 , a compensation procedure is performed to determine compensation parameters. These compensation parameters usually include a phase offset term and may also include cyclic or intensity correction terms. 
         [0018]    In  FIG. 1 , the reference phase calculated by processor  78  is based on the phase the modulated laser light output from optical detector  70 . An alternative is to apply radio frequency f RF  directly to mixer  72  without first undergoing conversion to light in laser  50  and conversion back to electricity in optical detector  70 . In other words, a mixing signal is applied to mixer  72 . Each of the two alternative approaches has its merits. The approach shown in  FIG. 1  has the advantage of eliminating common-mode laser noise. The all-electrical approach, on the other hand, reduces size and cost. 
         [0019]    Synthesizer  20  shown in  FIG. 2  comprises phase-locked loop (PLL)  22 , signal generators  28 ,  30 , and quadrature modulators  24 ,  26 . Phase-locked loop  22  receives a signal at frequency f REF  from frequency source  10  and generates a signal at a much higher frequency f PLL . In other words, the signal at frequency f PLL  can be one example of a phase locked loop signal having a phase locked loop frequency. As an example, f REF  may be 20 MHz and f PLL  may be 2560 MHz. Signal generators  28 ,  30  generate signals f 1 , f 2 , i.e., first and second generated signals whose frequencies are separated by the desired IF. For example, if the desired f IF  is 10 kHz, then the frequencies created by signal generators  28 ,  30  might be f 1 =5.005 MHz and f 2 =4.995 MHz.  FIG. 1  shows that there are two signals f 1  called f 1I  and f 1Q  and two signals f 2  called f 2I  and f 2Q . The subscripts I and Q in these symbols refer to in-phase (0 degrees) and quadrature (90 degrees), respectively. In other words, the signals f 1I  and f 1Q  have the same frequency but differ in phase by approximately 90 degrees. 
         [0020]    The purpose of quadrature modulators  24 ,  26  is to produce single sideband signals f LO  and f RF , respectively. In  FIG. 2 , the single sideband signals have frequencies that are equal to the sum of the PLL and signal-generator frequencies. This frequency component is said to be the upper sideband. The lower sideband, which has a frequency equal to the difference of the PLL and signal-generator frequencies, could equally well have been selected. It is desirable that the unwanted sideband and the carrier component, whose frequency is equal to f PLL , be as small as possible. Another way of saying this is that the rejection of the undesired sideband and carrier signal should be as high as possible. To maximize rejection of the unwanted sidebands and carrier, the characteristics of the signals from signal generators  28 ,  30  are manipulated to give the ideal phase difference, sinusoidal amplitude, and DC offset between the I and Q components that are put into quadrature modulators  24 ,  26 . These ideal values have been achieved when the unwanted sideband and carrier in the output signal are shown on an RF spectrum analyzer to be as small as possible. If the signals from signal generators  28 ,  30  are properly adjusted for phase, amplitude, and offset, the unwanted sideband and carrier should be approximately 50 dB or more below the desired sideband. 
         [0021]    It is possible to obtain the desired IF (for example, 10 kHz) by using a single quadrature modulator. For example, it would be possible to use the quadrature modulator to generate a single sideband signal for the LO and the phase-locked-loop signal only to modulate laser  50 . In this case, f LO =f PLL  and f RF =f PLL +f 2 . However, the mixing product f IF  from mixers  72 ,  82  will then have unwanted sideband and carrier signals that are only approximately 50 dB smaller than the desired signal. Consequently, cyclic errors are larger and measurements noisier than desired. 
         [0022]    These problems are avoided by adding a second quadrature modulator, as shown in  FIG. 2 . As a specific example, suppose that the PLL frequency is f PLL =2560 MHz and the signal generator frequencies are f 1 =5.005 MHz and f 2 =4.995 MHz. Assuming that the upper sidebands are desired, the resulting LO and radio frequencies are then f LO =f PLL +f 1 =2565.005 MHz and f RF =f PLL +f 2 =2564.995 MHz. When these signals pass through mixers  72 ,  82 , the resulting difference frequency is f IF =10 kHz. The unwanted sidebands will have frequencies 2555.005 MHz and 2554.995 MHz. These unwanted sidebands can mix with one another, but because each is down by 50 dB, the mixing product will be down by 100 dB, which is not a problem. These unwanted sidebands can also mix with the desired sidebands, but then the frequency difference is approximately 10 MHz, which is easily filtered out from the desired 10 kHz signal. 
         [0023]    By using two quadrature modulators as shown in  FIG. 2 , it is possible to obtain a compact and low-cost absolute distance meter that has low cyclic error and low noise. 
         [0024]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation. 
         [0025]    The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.