Abstract:
Methods and systems for improving frequency estimation without increasing digital counter resolution. An example system mixes and filters a known carrier signal with the signal containing the frequency of interest, in order to bring the frequency domain image closer to baseband, and then performs the frequency estimation. This allows much better resolution without the need to increase the counter frequency.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Some accelerometers require a high accuracy frequency estimation of an output waveform. The better the quality of the frequency estimation, the better the ability to accurately estimate acceleration. One of the problems associated with frequency estimation using digital counters is that resolution is limited by the counter frequency. 
       SUMMARY OF THE INVENTION 
       [0002]    The present invention provides a way to improve the frequency estimation without increasing the counter resolution. The invention mixes and filters a known carrier signal with the signal containing the frequency of interest, in order to bring the frequency domain image closer to baseband, and then performs the frequency estimation. This allows for much better resolution without the need to increase the counter frequency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0004]      FIG. 1  is a block diagram of a portion of a resonating beam accelerometer system; 
           [0005]      FIG. 2  illustrates a block diagram of a frequency estimation device included in the system shown in  FIG. 1 ; and 
           [0006]      FIGS. 3-7  illustrate an example of time and frequency domain signals at various stages within the frequency estimation device of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0007]    The present invention provides for accurate frequency estimation of a resonating beam sensor, such as a double-ended tuning fork. As shown in  FIG. 1 , an example system  20  includes a sensor  22 , a frequency estimation device  24 , and an output device  26 . The sensor  22  outputs a signal to the frequency estimation device  24 . The frequency estimation device  24  estimates the frequency of the received signal and outputs that value to the output device  26 . 
         [0008]    The frequency estimation device  24  receives the signal from the sensor  22 , process it by mixing it down, filtering it, and digitizing it. The digitized signal is then processed by a digital circuit included in the device  24  which includes a counter in order to estimate the frequency of the mixed down signal. The frequency by which the signal from the sensor  22  was mixed down is then added to the digitally estimated frequency in order to produce an accurate estimate. The frequency estimation is significantly more accurate for the mixed down signal than it would be for the original signal from sensor  22  because the digital counter resolution would be better in estimating the mixed down signal than the original. 
         [0009]      FIG. 2  illustrates an embodiment of the frequency estimation device  24 . The frequency estimation device  24  receives the sensor signal from the sensor  22  at a (optional) first low pass filter  34 . A reference signal generated by generator/synthesizer  36  is mixed with the output of the first low pass filter  34  at a mixer  40 . This first low pass filter is designed to block frequencies above the range permitted by the mixer  40 . The synthesizer  36  generates the reference signal (analog) based on a frequency value (digital) sent from the adaptive reference frequency selector  54 . An example of the output of the low pass filter  34  is illustrated in  FIG. 3  in both the time and frequency domains. An example reference signal is illustrated in  FIG. 4 . The output of the mixer  40  (example shown in  FIG. 5 ) is sent to a second low pass filter  42 . Because the output of the mixer  40  includes both a high and low frequency components, the second low pass filter  42  is set in order to filter out the high frequency component. An example signal in the time and frequency domains outputted by the second low pass filter  42  is illustrated in  FIG. 6 . Next, at a square-up circuit  46  the output of the second low pass filter  42  is converted into a square wave, such as that shown by example in  FIG. 7 . A digital frequency estimation component  48 , such as a digital counter, then determines the frequency of the output of the square-up circuit  46  and sends that determination to a summation device  50  that combines it with the frequency value originally sent to the synthesizer  36  by the adaptive reference frequency selector  54 . 
         [0010]    The output of the summation device  50  is now an accurate value of the frequency of the original signal that was received from the sensor  22 . The output of the summation device  50  is then sent to one or more output devices  26  and to the adaptive reference frequency selector  54  that is controlled by a controller  56 . 
         [0011]    In one example, the controller  56  determines how often to change the reference frequency. This may be desired if the sensor signal needed to be tracked more closely. The controller  56  can select the reference frequency that is optimum for the mixer operation. 
         [0012]    In one embodiment, the adaptive reference frequency selector  54  receives the output of summation device  50  in order to adjust the reference frequency for optimum mixer operation. 
         [0013]    Frequency mixing of two signals (at the mixer  40 ) is equivalent to multiplying 2 signals in the time domain. The result of the multiplication is two components: one component has a frequency equal to the sum of the two input frequencies; and the other component has a frequency equal to the difference of the two input frequencies. In one example, the signal of interest is band limited (e.g., 1 kHz) and its center frequency is known e.g., 15 kHz). The signal of interest (signal from sensor) is used as the first input to the mixer  40 . The second input to the mixer  40  is generated by the synthesizer  36 ) to be a single sinusoid whose frequency exceeds the center frequency of the signal of interest by about a factor of two times (or greater) the bandwidth of the signal of interest (15 kHz+[1 kHz×2]=17 kHz) to prevent aliasing. After mixing, the two components that are generated are the sum of the two input frequencies (15 kHz+17 kHz=32 kHz) and the difference of the two input frequencies (17 kHz−15 kHz=2 kHz). The mixed signal is then filtered to remove the higher frequency component (the 32 kHz component). The resulting filtered signal (2 kHz) is used for frequency estimation. This process allows for a better frequency estimation, since the digital counter resolution is better for a low frequency component (2 kHz) than for a higher frequency component (15 kHz). 
         [0014]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.