Abstract:
Systems and methods synthesize a signal from the odd harmonic frequency components of an input signal. An exemplary embodiment synthesizes a first signal with a digital to analog converter (DAC), generates a second signal from the first signal, and filters a selected one of the odd harmonic frequency components through a band pass filter to produce an output signal. The first signal is defined by a first frequency. The second signal is defined by the first frequency and includes a fundamental frequency component and plurality of odd harmonic frequency components. The output signal has a frequency substantially equal to the frequency of the selected odd harmonic frequency component.

Description:
BACKGROUND OF THE INVENTION 
   Direct Digital Synthesis (DDS) devices create waveform outputs with an accurate and tunable frequency. In some applications, the DDS output waveform frequency cannot be tuned high enough for the application. For example, radiation hardened systems and/or devices may use a DDS device. However, due to input clock rate limitations, the frequency of an output signal generated by the radiation hardened DDS device may not be sufficiently high for some applications, such as when the output waveform is used as a clock signal source for a system that requires a relatively high clock rate. Accordingly, it is desirable to increase the frequency of the output waveform. 
   SUMMARY OF THE INVENTION 
   Systems and methods of synthesizing a signal from the odd harmonic frequency components of an input signal are disclosed. An exemplary embodiment has a digital to analog converter (DAC), a comparator, and a band pass filter. The DAC is operable to generate a first signal defined by a first frequency. The comparator is communicatively coupled to the DAC, is operable to receive the first signal from the DAC, and is operable to generate a substantially square-shaped waveform signal in response to receiving the first signal, wherein the substantially square-shaped waveform signal comprises a fundamental frequency component and plurality of odd harmonic frequency components. The band pass filter is communicatively coupled to the comparator, is operable to receive the substantially square-shaped waveform signal from the comparator, and is operable to filter the substantially square-shaped waveform signal to produce an output signal corresponding to a selected one of the odd harmonic frequency components of the substantially square-shaped waveform signal received from the comparator, wherein the band pass filter output signal has a frequency substantially equal to the frequency of the selected odd harmonic frequency component. 
   In accordance with further aspects, an exemplary embodiment synthesizes a first signal with a digital to analog converter (DAC), generates a second signal from the first signal, and filters a selected one of the odd harmonic frequency components through a band pass filter to produce an output signal. The first signal is defined by a first frequency. The second signal is defined by the first frequency and comprises a fundamental frequency component and plurality of odd harmonic frequency components. The output signal has a frequency substantially equal to the frequency of the selected odd harmonic frequency component. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred and alternative embodiments are described in detail below with reference to the following drawings: 
       FIG. 1  is a block diagram of an embodiment of the direct digital simulation (DDS) harmonic waveform generation system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a block diagram of an embodiment of a direct digital simulation (DDS) harmonic waveform synthesizing system  100 . Embodiments of the DDS harmonic waveform synthesizing system  100  generate a waveform, such as, but not limited to, a clock signal, that may be used as an input to other devices. The output waveform corresponds to an odd harmonic of the output waveform of a digital to analog (DAC) converter  102 . The DDS harmonic waveform synthesizing system  100  comprises the DAC converter  102 , an optional low pass filter  104 , a first comparator  106 , a band pass filter  108 , and an optional second comparator  110 . The output signal V OUT    112  from the DDS harmonic waveform synthesizing system  100  may be used as an input signal to another system and/or device (not shown). 
   A digital input  114  provided to the DAC converter  102  results in generation of, or synthesis of, an analog signal  116  at the output  118  of the DAC converter  102 . In one embodiment, the DAC converter  102  is a radiation hardened converter. Due to the radiation hardening, the frequency of the output analog signal  116  is limited to a relatively low value (in comparison to non-radiation hardened DACs). In alternative embodiments, any suitable DAC converter may be used, such as, but not limited to, a DAC converter  102  that is not radiation hardened. 
   In some applications, the analog signal  116  generated by, or synthesized by, the DAC converter  102  may require smoothing or the like. Accordingly, the output analog signal  116  is input to the optional low pass filter  104 . The low pass filter  104  outputs a signal  120  at its output  122 . In this exemplary embodiment, the output signal  120  may be generally described as a sine wave. 
   The signal  120  from the low pass filter  104  is input to the first comparator  106 . The first comparator  106  compares the voltage of the signal  120  to a comparison voltage, V COMP . When the voltage of the signal  120  is greater than the voltage of V COMP , a comparator output signal  124  at the output  126  is a logical high. When the voltage of the signal  120  is less than the voltage of V COMP , the comparator output signal  124  at the output  126  is a logical low. Accordingly, the comparator output signal  124  has a square-shaped waveform in this exemplary embodiment. The comparator output signal  124 , due to its substantially square-shaped waveform, comprises a fundamental frequency and plurality of odd harmonic frequency components. 
   The square-shaped waveform of the comparator output signal  124  may be suitable as a clocking signal for some devices (not shown). For example, the comparator output signal  124  may have a frequency that is suitable for another device. However, in some situations, the frequency of the comparator output signal  124  may not be sufficiently high enough for other applications. For example, but not limited to, a relatively high frequency clocking signal may be required in the other application. Accordingly, synthesis of a higher frequency signal is generated by embodiments of the DDS harmonic waveform synthesizing system  100 . 
   As noted above, the square-shaped waveform of signal  124  is known to contain odd harmonic frequency components. The largest magnitude odd harmonic is the third harmonic. To generate a higher frequency signal, the odd harmonic frequency component of the square-shaped waveform of signal  124  is selectively passed through the band pass filter  108 . Accordingly, the comparator output signal  124  is input to the band pass filter  108 . The frequency range of the band pass filter  108  is set to correspond to the desired odd harmonic frequency of the square-shaped waveform of signal  124 . Thus, the fundamental frequency component and the non-selected odd harmonic frequency components are attenuated or blocked, while the selected odd harmonic frequency component passes through the band pass filter  108 . 
   In this exemplary embodiment, the band pass filter  108  frequency range corresponds to the third harmonic frequency of the square-shaped waveform of signal  124 . Accordingly, the third harmonic frequency component of the square-shaped waveform of signal  124  passes through the band pass filter  108 , while the fundamental frequency of the square-shaped waveform of signal  124  and its other harmonic frequency components are attenuated (blocked). 
   The output signal  128  provided at the output  130  of the band pass filter  108  is a signal corresponding to the selected third harmonic frequency of the square-shaped waveform of the comparator output signal  124 . The waveform of the output signal  128  is conceptually illustrated as a sine-shaped waveform with an amplitude less than the square-shaped waveform of the comparator output signal  124 . Further, the frequency of the output signal  128  is substantially equal to three times the frequency of the square-shaped waveform of the comparator output signal  124  since the output signal  128  corresponds to the third harmonic frequency component of the comparator output signal  124 . 
   In some embodiments, the higher frequency output signal  128  from the band pass filter  108  may be directly used by some systems and/or devices (not shown). However, in other situations, the output signal  128  may not have a sufficient amplitude or edge rate to perform an intended task. For example, a clocking signal with a frequency corresponding to the frequency of the output signal  128 , but with a greater amplitude and sharper edges, may be needed for a specific application. 
   Accordingly, the output signal  128  from the band pass filter  108  is input to an optional second comparator  110 . In this exemplary embodiment, when the output signal at output  130  is greater than ground, the output signal  132 , corresponding to V OUT    112 , is a logical high. When the output signal  128  is less than ground, the output signal  132 , corresponding to V OUT    112 , is a logical low. 
   In this exemplary embodiment, a pull-up resistor  134  is used to pull the voltage of the output signal V OUT    112  to a value corresponding to V c  when the output of the second comparator  110  is at a logical high. Thus, the amplitude of the output signal V OUT    112  may be controllable to a desired value based upon selection of V c . Accordingly, an amplified output signal V OUT    112  is generated from the output signal  128 . Further, other circuitry (not shown) may be employed to provide one or more other controllable voltages from the output signal V OUT    112 . 
   The above-described embodiments were described as using the third harmonic frequency component of a generated signal to generate the output signal V OUT    112 . In some situations, the frequency of the third harmonic frequency component of the square-shaped waveform of comparator output signal  124  may not be high enough for a particular application. Accordingly, the band pass filter  108  may have its band pass frequency set to correspond to another one of the odd harmonic frequencies (fifth harmonic, seventh harmonic, ninth harmonic, etc.) of the comparator output signal  124 . The frequency of the output signal  128  would then correspond to the frequency of the selected odd harmonic. Thus, the fundamental frequency component, the third harmonic frequency component, and the non-selected odd harmonic frequency components, are blocked by the band pass filter  108 . 
   For example, the band pass filter  108  may be tuned to the fifth harmonic frequency component, thus blocking the fundamental frequency component, the third harmonic frequency component, and the other odd harmonic frequency components of the comparator output signal  124 . Accordingly, an output signal  128  is generated having a frequency corresponding to the fifth harmonic frequency component of the square wave, which is substantially five times the frequency of the square-shaped waveform of the comparator output signal  124 . This output signal  128  may be amplified, if necessary, to a desired voltage using the second comparator  110 . 
   In the exemplary embodiment of the DDS harmonic waveform synthesizing system  100 , the first comparator  106  is an open collector or open drain type comparator. Accordingly, there is no active voltage pull-up for the output  126 . Accordingly, an external pull-up circuit  136  is employed to pull the output of the first comparator  106  to the voltage V C-OUT . 
   Since the output signal V OUT    112  of the exemplary embodiment of the DDS harmonic waveform synthesizing system  100  is itself a square wave form, the output signal V OUT    112  has its own set of odd harmonic frequency components. Accordingly, if even higher frequency signals are required, the output signal V OUT    112  may be input to another band pass filter (not shown) and its odd harmonic frequency components may be processed as described above to generate a higher frequency output signal. 
   Alternative embodiments may use any suitable type of device or circuit, such as, but not limited to, integrated circuit transistors, for the first comparator  106  and/or the second comparator  110 . For example, two transistors may be operated cooperatively in a push-pull configuration so that the above-described output signals  124  and/or  132  are generated. Any suitable configuration of transistors or other components may be used to generate the above-described output signals  124  and/or  132 . 
   The second comparator  110  of the above-described embodiment of the DDS harmonic waveform synthesizing system  100  had its comparison voltage set to ground. In an alternative embodiment, another comparator voltage may be used, similar to V COMP  used by the first comparator  106 . 
   Further, the above-described output signals  124  and/or  132  were described as having a square wave form. Other wave forms that have discernable odd harmonic frequency components may be used by alternative embodiments. 
   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.