Patent Publication Number: US-2015070099-A1

Title: System to linearize a frequency sweep versus time

Description:
TECHNICAL FIELD OF INVENTION 
     This disclosure generally relates to a system for generating a variable frequency, and more particularly relates to a using an integrator to vary the control signal provided to a voltage controlled oscillator (VCO) so the frequency value changes linearly over time. 
     BACKGROUND OF INVENTION 
     It is known to use a voltage controlled operator (VCO) to generate a signal that includes a series of frequency ramps. Such a signal is sometimes called a ‘fast chirp’ waveform and is used in a radar system marketed by Delphi Inc. of Troy, Mich. as a Simultaneous Transmit and Receive Pulse-Doppler (STAR-PD) radar system. This waveform consists of a series of frequency ramps that sweep frequency, for example, from seventy-seven Giga-Hertz (77 GHz) to seventy-six Giga-Hertz (76 GHz) in forty-six microseconds (46 us), and repeats that sweep every fifty microseconds (50 us). It is preferable that the frequency sweep for a radar system using a fast chirp type signal be highly linear (i.e.—constant slope versus time), for example, less than 0.5% non-linearity to optimize the performance of the radar system. 
     Typical automotive radar systems use a VCO that has a relatively non-linear voltage-to-frequency characteristic as such VCO&#39;s are relatively inexpensive. That is, a linear change in a control signal into the VCO will not result in a linear change in frequency. Two commonly-used approaches to generate a more linear frequency ramp or sweep are to use a phase-lock-loop (PLL), or to control the VCO open-loop with a control signal directly from a digital-to-analog converter (DAC). PLLs undesirably increase the cost of a radar system. Open loop control with a DAC will typically use some prior knowledge about the VCO non-linear characteristic curve to generate a non-linear control voltage designed to produce a linear frequency ramp. However, as the DAC outputs a stair-step waveform with discontinuities, the signal output by the VCO generally has unwanted spurs in the spectrum. A higher resolution DAC may be used to reduce the frequency spurs, but this undesirably increases the cost of the radar system. A filter can be used to smooth the control voltage signal, but this undesirably increases the minimum time between the end of one chirp and the beginning of the next chirp. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment, a system for generating a variable frequency is provided. The system includes a voltage controlled oscillator (VCO) and an integrator. The VCO is configured to output a frequency signal with a frequency value dependent on a voltage value of a control signal. The integrator is configured to vary the control signal provided to the VCO. The ramp rate of the integrator is varied so the frequency value changes at a substantially constant frequency rate over a period of time. 
     Further features and advantages will appear more clearly on a reading of the following detailed description of the preferred embodiment, which is given by way of non-limiting example only and with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present invention will now be described, by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagram of a system for generating a variable frequency in accordance with one embodiment; 
         FIG. 2  is a graph of a signal output by the system of  FIG. 1  in accordance with one embodiment; and 
         FIG. 3  is a diagram of a system for generating a variable frequency that is an alternative to the system of  FIG. 1  in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a non-limiting example of a system  10  for generating a variable frequency signal FO such as a chirp signal  12  ( FIG. 2 ). As will become apparent in the description that follows, the frequency signal FO advantageously exhibits a highly linear characteristic. That is, the change in frequency value  14  versus time has a substantially constant slope or frequency rate FR over a period of time TC. 
     The system  10  includes a voltage controlled oscillator (VCO), hereafter often referred to as the VCO  16 . In general, the VCO  16  is configured to output a frequency signal FO with a frequency value  14  that is dependent on a voltage value  18  of a control signal VC. As minimizing the cost of the system  10  is desirable, especially for automotive radar sensors, using an inexpensive VCO is preferred. However, as inexpensive VCO&#39;s generally have a non-linear voltage-to-frequency conversion characteristic, but a constant frequency slope is desired for a radar sensor, an inexpensive way to generate the control signal VC is to have a voltage value  18  that varies over time in such a way as to compensate for the non-linear voltage-to-frequency conversion characteristic of the VCO. 
     Prior attempts of using a digital-to-analog converter (DAC) to directly generate the control signal VC had undesirable results as the stair-step nature of the signal output by the DAC caused spurious noise in the spectrum of the frequency value  14 . Accordingly, a way to generate a control signal VC that is smoother (i.e.—with reduced discontinuities) is desired. To this end, the system  10  includes an integrator  20  configured to vary the control signal VC provided to the VCO. In general, the integrator  20  is configured so a ramp rate of the integrator  20  (i.e. the ramp rate of the output voltage VO) is varied so the frequency value  14  changes at a substantially constant frequency rate FR over a period of time TC ( FIG. 2 ). 
     As will be recognized by those in the electronics arts, when the switch SW 1  is closed, the output voltage VO is equal to the bias voltage VB. It should also be recognized that when the switch SW 1  is open, the rate at which the output voltage VO changes is a function of the difference between input value  22  of the input voltage VI and the bias value  28  of the bias voltage VB, the resistance value  24  of the resistor R 1 , and the capacitance value  26  of the capacitor C 1 . If the bias value  28 , the resistance value  24  and the capacitance value  26  are constant, then the ramp rate of the integrator, that is the rate at which the output voltage VO changes, is based on or determined by the input value  22  of an input signal VI to the integrator. 
     In this non-limiting example the input value  22  is determined by a digital to analog convertor (DAC), hereafter the DAC  30 . If the input value  22  of the input voltage VI is varied by varying the output of the DAC, then the ramp rate of the output voltage VO can be varied, and so the output voltage VO will have a piece-wise linear interpolation characteristic. Analysis has shown that using a control signal VC with such a piece-wise linear interpolation shape provides a frequency signal FO with fewer spurious frequency components when compared to using directly the stair-step type waveform output by a DAC as the control signal VC. 
     It is recognized that there are other ways to vary the ramp rate or integration rate of the integrator  20 . For example, the resistance value  24  of the resistor R 1 , the capacitance value  26  of the capacitor C 1 , or the bias value  28  of the bias voltage VB could be varied independently or cooperatively, with or without varying the input value  22  of the input voltage VI. The choice of varying the ramp rate of the integrator  20  by varying the input value  22  of the input voltage VI was selected based on convenience. The bias voltage VB may be a voltage divider network connected to a reference voltage such as a supply voltage for the various electronics so that the bias value  28  is fixed, or a second DAC so that the bias value  28  is adjustable. 
     Continuing to refer to  FIG. 1 , the system  10  may include a controller  32  configured to control the DAC  30 , and other aspects of the system  10  as will be further described below. The controller  32  may include a processor (not shown) such as a microprocessor or other control circuitry such as analog and/or digital control circuitry including an application specific integrated circuit (ASIC) for processing data as should be evident to those in the art. The controller  32  may include memory, including non-volatile memory, such as electrically erasable programmable read-only memory (EEPROM) for storing one or more routines, thresholds and captured data. The one or more routines may be executed by the processor to perform steps for controlling signals output by the controller  32  for linearizing the output of the VCO  16  as described herein. 
     The non-linear characteristics of the VCO  16  may be pre-programmed into the controller  32  and used to control the input value  22  of the input voltage VI output by the DAC in order to generate a linearized frequency signal FO. As used herein, linearized means that the slope indicated by the frequency rate FR is substantially constant, where substantially means that the linearity performance of the system  10  meets the linearity requirements of whatever the system is applied to, a radar sensor for example, which may require a linearity performance characteristic of less than 0.5%. 
     Alternatively, the controller  32  may be configured to detect or receive the frequency signal FO and determine the non-linearity characteristics of the VCO  16  by direct measurement. Such a configuration may be advantageous if the non-linearity characteristics of the VCO  16  change, for example, over time or over temperature. If the non-linearity characteristics do change over time or temperature, the controller can ‘learn’ those characteristics and thereby further refine control of the DAC  30  to better linearize the frequency signal FO. If the application does not need a highly linearized frequency sweep versus time performance characteristic, or a frequency signal FO with an amount of spurious frequency components in the spectrum of the frequency signal FO is acceptable, then the addition of the integrator may be unnecessary. 
     The system  10  may also include a gain-and-offset stage  34  to further adjust the value of the control signal VC based on the output voltage VO. The system may also include a filter  36  to further smooth the signal provided to the VCO  16 . It should be recognized that the filter  36  is generally characterized as a low-pass-filter, and that such a filter would not need to be as aggressive as a filter that would be necessary if the output of the DAC  30  was applied directly to the VCO  16  without the smoothing effects of the integrator  20 . In other words, without the integrator  20 , the characteristics of the filter  36  would likely include a higher cut-off frequency, and/or a higher order than what would be sufficient for the filter  36  to smooth the piece-wise linear interpolation signal from the integrator  20 . 
     The system may also include a frequency divider  38  configured to divide the frequency signal FO output by the VCO  16  to a divided frequency FD that is sampled by controller  32  to determine the frequency value  14 . The controller  32  can thus determine the voltage-frequency characteristic curve of the VCO, in order to determine the desired control signal to be created by the Integrator  20 . 
     Referring now to  FIG. 2 , for a radar sensor, suitable values for the period of time TC, the silent time TS and the time duration TD are 46 us, 4 us, and 300 us, respectively. Suitable values for the frequency value  14  include a maximum frequency FMAX of 77 GHz, and a minimum frequency value of 76 GHz. These values are only example values as it is recognized that other values may be suitable for radar sensors, or for other electrical systems that need a signal with a linear frequency sweep. 
       FIG. 3  illustrates an alternative to the system  10  shown in  FIG. 1 , hereafter referred to as the system  110 . In this example, the Low speed DAC and gain block K are replaced with a sample and hold circuit formed by the switch SW 3  and the capacitor C 2 . It is recognized that the bias voltage VB could also be replaced by a similar sample and hold circuit. Alternatively, the integrator  20  illustrated in  FIGS. 1 and 3  could be replaced with a sample and hold type circuit that applies a voltage to a capacitor, and then a variable current sink would draw current from that capacitor to generate the output voltage VO. 
     Accordingly, a system  10  and system for generating a linearized variable frequency from a voltage controlled oscillator (VCO) that can be characterized as non-linear is provided. The resolution of the DAC  30  is selected so that in cooperation with the integrator a control signal VC can be provided to the VCO  16  such that the frequency signal FO varies the frequency value  14  over time more linearly than would be the case if the DAC  30  was used without the smoothing effects of the integrator  20 . Alternatively, by adding the integrator  20  to the system  10 , the performance/resolution requirements of the DAC may be relaxed and thereby reduce the cost of generating a signal such as the chirp signal illustrated in  FIG. 2 . 
     While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.