Patent Publication Number: US-2023134987-A1

Title: Low-Noise Oscillator Amplitude Regulator

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 17/480,633 filed Sep. 21, 2021, which is a continuation of U.S. patent application Ser. No. 16/862,084 filed Apr. 29, 2020, which issued as U.S. Pat. No. 11,152,945 on Oct. 19, 2021, which is a continuation of U.S. Ser. No. 16/245,804 filed on Jan. 11, 2019, which issued as U.S. Pat. No. 10,673,441 on Jun. 2, 2020, which is a continuation of U.S. patent application Ser. No. 15/577,973, filed on Nov. 29, 2017, which issued as U.S. Pat. No. 10,218,361 on Feb. 26, 2019, which is a national stage application of PCT/EP2016/060873, filed May 13, 2016, which is a continuation of U.S. patent application Ser. No. 14/731,487, filed Jun. 5, 2015, which issued as U.S. Pat. No. 9,473,151 on Oct. 18, 2016, the disclosure of each of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The solution presented herein relates generally to frequency generation, and more particularly to reducing phase noise and power consumption of high frequency generation circuits. 
     BACKGROUND 
     Oscillators are widely used in various electronic devices, e.g., to provide reference clocks, mixing frequencies for telecommunication signals, etc. A negative resistance-based oscillator represents one type of oscillator architecture typically used for the generation of higher frequency signals, such as used in wireless communication devices. Examples of negative resistance-based oscillators include, but are not limited to crystal oscillators, Surface Acoustic Wave (SAW)-based oscillators, etc. Negative resistance-based oscillators comprise an oscillator core having a resonant circuit operatively connected to a negative resistance circuit. The resonant circuit oscillates at the desired resonant frequency, and the negative resistance circuit cancels the resistive losses of the resonant circuit. In effect, the negative resistance circuit eliminates the natural damping of the resonant circuit, and therefore enables the oscillator core to continuously oscillate at the desired resonant frequency. 
     The successful operation of electronic devices containing such oscillators requires accurate and reliable amplitude control. In particular, amplitude control is necessary due to the fact that different Q-values, e.g., of different resonant circuits, as well as different PVT (Process, Voltage, and Temperature) conditions for any one oscillator may cause wide amplitude variations. For example, an oscillator having a high-Q resonant circuit will have higher amplitude oscillation than an oscillator having a low-Q resonant circuit. Further, an oscillator running in a linear mode requires continuous regulation of the amplitude to prevent the oscillator amplitude from quickly falling to zero or increasing to a level limited by the non-linear effects, e.g., voltage clipping, of the oscillator. Such voltage clipping can greatly deteriorate oscillator performance, increase the risk of parasitic oscillation, increase the current consumption (depending on circuit topology), and generally make the behavior of the oscillator more unpredictable. Accurate and reliable amplitude control will equalize the amplitude variations across a wide range of Q-values and PVT conditions, as well as ensure good noise performance, provide low current consumption, avoid parasitic oscillation, and possibly prevent damage to active and passive components. 
     A negative feedback loop provides one way to control the amplitude of the oscillator output, where the negative feedback loop senses the amplitude of the oscillator output and then adjusts the amplitude by controlling an operating point of the oscillator core. For example, controlling the current through active transistor devices of the oscillator core controls the transconductance g m  of the oscillator core to control the negative resistance, and thus controls the oscillator amplitude. However, such negative feedback loops may introduce noise into the oscillator core, particularly when the negative feedback loop has a high gain. Further, the nonlinear properties of the oscillator core will convert the input noise to both AM (Amplitude Modulation) and PM (Phase Modulation) noise. While increasing the loop gain of the negative feedback loop will reduce the AM noise, such an increased loop gain will not only increase the power consumption, but will also fail to reduce the PM noise. While reducing the bandwidth of the negative feedback loop will also reduce the noise, such a bandwidth reduction, however, will increase the startup time of the oscillator, and may also undesirably increase the size (consumed chip area) of any filter required to filter the oscillator input signal. Thus, such bandwidth reduction is also not desirable. 
     As noted above, negative resistance-based oscillators are particularly useful for high frequency applications, and may be particularly important for mmW (millimeter wave) communication. Also, specifically for reference oscillators based on e.g., crystal or SAW resonators, the use of even higher frequencies is anticipated, from todays 10&#39;s of MHz to 100&#39;s of MHz and possibly even frequencies approaching the GHz range. The generation of such higher frequencies generally results in higher power consumption. Further, the generation of such higher frequencies also presents design challenges due to increased tolerances of the resonators, increased noise, increased component sizes, longer startup times, and/or larger impacts from parasitic elements of the circuitry and associated package. Thus, there remains a need for improved higher frequency generation circuits that do not incur higher power consumption, higher noise, and/or longer start-up times. 
     SUMMARY 
     The solution presented herein generates high frequency signals with lower power consumption and lower noise by controlling an oscillator amplitude using two feedback paths. A first feedback path provides continuous control of the oscillator amplitude responsive to an amplitude detected at the oscillator output. A second feedback path provides discrete control of the amplitude regulating parameter(s) of the oscillator responsive to the detected oscillator amplitude. Because the second feedback path enables the adjustment of the amplitude regulating parameter(s), the second feedback path enables an amplifier in the first feedback path to operate at a reduced gain, and thus also at a reduced power and a reduced noise, without jeopardizing the performance of the oscillator. 
     One exemplary embodiment comprises a frequency generation circuit comprising an oscillator, a detector, a first feedback path, and a second feedback path. The oscillator comprises an oscillator output, a first control input, and a second control input. The detector is configured to detect an amplitude of the oscillator output. The first feedback path operatively connects the detector to the first control input, and is configured to provide time-continuous control, responsive to the detected amplitude, of the amplitude of the oscillator output by continuously controlling a first control signal applied to the first control input. The second feedback path operatively connects the detector to the second control input, and is configured to provide time-discrete control, responsive to the detected amplitude, of one or more amplitude regulating parameters of the oscillator by providing time-discrete control of a second control signal applied to the second control input. 
     Another exemplary embodiment comprises a method of controlling an oscillator comprising an oscillator output, a first control input, and a second control input. The method comprises detecting an amplitude of the oscillator output, and providing time-continuous control, responsive to the detected amplitude, of the amplitude of the oscillator output by continuously controlling a first control signal applied to the first control input. The method further comprises providing time-discrete control, responsive to the detected amplitude, of one or more amplitude regulating parameters of the oscillator by providing time-discrete control of a second control signal applied to the second control input. 
     Another exemplary embodiment comprises a computer program product stored in a non-transitory computer readable medium for controlling an oscillator of a frequency generation circuit. The oscillator comprises an oscillator output, a first control input, and a second control input. The computer program product comprises software instructions which, when run on the frequency generation circuit, causes the frequency generation circuit to detect an amplitude of the oscillator output, and provide time-continuous control, responsive to the detected amplitude, of the amplitude of the oscillator output by continuously controlling a first control signal applied to the first control input. The software instructions, when run on the frequency generation circuit, further cause the frequency generation circuit to provide time-discrete control, responsive to the detected amplitude, of one or more amplitude regulating parameters of the oscillator by providing time-discrete control of a second control signal applied to the second control input. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a block diagram of a frequency generation circuit according to one exemplary embodiment. 
         FIG.  2    shows an amplitude control method according to one exemplary embodiment. 
         FIG.  3    shows a block diagram of the first feedback path of the frequency generation circuit of  FIG.  1    according to one exemplary embodiment. 
         FIG.  4    shows a block diagram of the second feedback path of the frequency generation circuit of  FIG.  1    according to one exemplary embodiment. 
         FIG.  5    shows another amplitude control method according to one exemplary embodiment. 
         FIG.  6    shows simulation results achievable with only a first feedback path having a high gain. 
         FIG.  7    shows simulation results achievable with only a first feedback path having a low gain. 
         FIG.  8    shows exemplary simulation results achievable with the solution presented herein. 
         FIG.  9    shows exemplary simulation results when the first feedback path has different gains. 
         FIG.  10    shows exemplary simulation results of the noise improvement achievable with the solution presented herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a block diagram of a frequency generation circuit  100  according to one exemplary embodiment. For simplicity,  FIG.  1    only shows the elements of the frequency generation circuit  100  necessary to facilitate the description provided herein. It will be appreciated by those skilled in the art that the frequency generation circuit  100  may include additional components and/or signal connections not shown in  FIG.  1   . 
     Frequency generation circuit  100  includes an oscillator  110  coupled to control circuitry  115  that controls the amplitude of the oscillator output. Oscillator  110  includes a first control input (CTRL 1 ), a second control input (CTRL 2 ), and an output (OUT). The oscillator  110  may comprise a crystal oscillator, or any other negative resistance-based oscillator that includes a resonant circuit  112  operatively connected to a negative resistance circuit  114 . In one exemplary embodiment, the resonant circuit  112  may comprise a crystal, and the negative resistance circuit  114  may comprise an amplifier (not shown). First and second control signals, S 1  and S 2 , applied to the respective first and second control inputs control the amplitude of the signal S o  at the output of the oscillator  110 . In particular, the first control signal S 1  provides time-continuous control of the amplitude of S o , while the second control signal S 2  provides time-discrete control of one or more amplitude regulating parameters of the oscillator  110 , as described further below. Exemplary amplitude regulating parameters include, but are not limited to, an oscillator bias current, a number of active oscillator g m  cells, a bias point of one or more of the oscillator g m  cells, and/or a variable resistance connected in parallel with a core of the oscillator  110 . Because the second control signal S 2  controls the configuration of the oscillator  110 , S 2  enables the relaxation of the requirements that would otherwise be placed on the time-continuous amplitude control provided by the first control signal S 1 . 
     The control circuitry  115  generates the first and second control signals S 1 , S 2  responsive to the oscillator output signal S o  according to the exemplary method  200  of  FIG.  2   . More particularly, the control circuitry  115  comprises a detector  120 , a first feedback path  130 , and a second feedback path  140 . The detector  120 , which is coupled between the oscillator output and the inputs of the first feedback path  130  and the second feedback path  140 , detects an amplitude A of the oscillator output signal S o  (block  210 ). The first feedback path  130  provides time-continuous control of the amplitude of the oscillator output signal S o  by continuously controlling the first control signal S 1  responsive to the detected amplitude A (block  220 ). The second feedback path  140  provides time-discrete control of one or more amplitude regulating parameters of the oscillator  110  by controlling, in discrete time, the second control signal S 2  responsive to the detected amplitude A (block  230 ). For example, the second control signal may provide time-discrete control of the parameter(s) controlling the operation of the negative resistance circuit  114 . By controlling the amplitude regulating parameter(s) of the oscillator  110 , the second feedback path  140  allows the first feedback path  130  to operate at a lower gain, and therefore at a lower power and with less noise. 
       FIG.  3    shows a block diagram of the first feedback path  130  according to one exemplary embodiment. In this embodiment, the first feedback path  130  includes an amplifier  132  and a filter  134 . The detected amplitude A, as well as a reference amplitude A ref , are input to amplifier  132 . Amplifier  132  amplifies the amplitude error A err  formed from the difference between the detected amplitude A and the reference amplitude A ref , and filter  134  helps reduce the noise input to the oscillator  110  by low-pass filtering the amplified signal to generate the first control signal S 1 . The first control signal S 1 controls the gain of the oscillator core by controlling the gain of the negative resistance circuit  114 . In so doing, the first control signal S 1  controls the amplitude of the oscillator output signal S o . 
     Amplifier  132  establishes the gain of the first feedback path  130 . Because various environmental conditions, oscillator properties, and/or the age of the oscillator  110 , may impact the ability of the first control signal S 1  to sufficiently control the amplitude of the oscillator output signal S o , conventional systems tend to set the gain of amplifier  132  to account for a wide range of conditions, even if some of the more extreme conditions are very rare. For example, higher temperatures may reduce the gain of the oscillator core relative to what that gain would be with the same input control signal at regular operating temperatures. Conventional solutions address this problem by making sure the gain of amplifier  132  is high enough to enable the oscillator core to handle even extreme temperature conditions without dropping the amplitude of the oscillator output S o  below a desired level. Such high gain conditions, however, cause amplifier  132  to consume more power and to insert more noise into the oscillator core than would otherwise be necessary for many operating conditions. 
     The solution presented herein incorporates the second feedback path  140  into the control circuitry  115  to control the amplitude regulating parameter(s) of the oscillator  110 , which allows the first feedback path  130  to be designed and configured for a lower gain. Such gain reduction in the first feedback path  130  will enable the frequency generation circuit  100  to operate at a lower power and will reduce the noise level input to oscillator  110 . To that end, the second feedback path  140  controls one or more amplitude regulating parameters responsive to the detected amplitude A of the oscillator output signal S. For example, if the detected amplitude A drops too low, indicating that the first control signal is unable to sufficiently amplify the oscillator amplitude, the second feedback path  140  may adjust the amplitude regulating parameters, e.g., by increasing the bias current, increasing the number of active oscillator gm cells, and/or increasing a bias point of one or more of the active g m  cells. Alternatively or additionally, the second feedback path  140  may adjust the amplitude regulating parameters by increasing the resistance of a variable resistance connected in parallel with the oscillator core, e.g., using a variable resistor  116  connected across differential outputs of the oscillator  110 . In another example, if the detected amplitude A rises too high, indicating the amplitude of the oscillator output signal S o , is too high, the second feedback path  140  may decrease the bias current, decrease the number of active oscillator g m  cells, decrease a bias point of one or more of the active g m  cells, and/or decrease the resistance of the variable resistor  116  connected in parallel with the core of the oscillator  110 . In either case, the second feedback path  140  adjusts the amplitude regulating parameter(s) for the current operating conditions as indicated by the detected amplitude A to enable the oscillator  110  to maintain the desired amplitude at the output without requiring the first feedback path  130  to have a high gain. 
     Because the gain of amplifier  132  is designed to handle most operating conditions, the control provided by the second feedback path  140  may be implemented in a time-discrete manner. For example, the second feedback path  140  may include a control circuit  142 , as shown in  FIG.  4   . Control circuit  142  may control the amplitude regulating parameter(s) of the oscillator in a time-discrete manner by only controlling the amplitude regulating parameter(s) when the detected amplitude A satisfies one or more predetermined conditions, e.g., threshold conditions. For example, the control circuit  142  may control the second control signal S 2  to control the amplitude regulating parameter(s) only when the detected amplitude A exceeds an upper threshold T U  or is lower than a lower threshold T L . In addition, the control circuit  142  may control the second control signal S 2  to control the amplitude regulating parameter(s) only under certain operating conditions and/or responsive to an event trigger. For example, control circuit  142  may control the second control signal S 2  to allow the amplitude regulating parameter(s) to change when the oscillator  110  powers on and/or when the oscillator  110  is acting in response to some communication event trigger. However, because changing the amplitude regulating parameters during, e.g., active communications, could disrupt the phase and/or frequency of the oscillator  110 , the control circuit  142  may control the second control signal S 2  to prevent the amplitude regulating parameter(s) from changing during such periods to prevent this disruption. The control circuit  142  may therefore use, in addition to the threshold conditions, power on/off events and/or communication event triggers to provide additional time-discrete control of the oscillator&#39;s amplitude regulating parameter(s). 
     The exemplary method  250  of  FIG.  5    provides a more detailed approach for controlling the oscillator  110  at startup. In this exemplary method  250 , the oscillator  110  is powered on (block  202 ), and the process waits until the oscillator  110  stabilizes (block  204 ). Once the oscillator  110  stabilizes (block  204 ), the detector  120  detects the amplitude A of the oscillator output signal S o  (block  210 ). If the detected amplitude A exceeds an upper threshold T U  (block  232 ) or is less than a lower threshold T L  (block  234 ), the control circuit  142  in the second feedback path  140  determines the oscillator  110  is unable to maintain a desired amplitude with the current configuration. In response, the control circuit  142  therefore alters one or more amplitude regulating parameters of the oscillator  110  (block  236 ). Blocks  210 ,  232 , and  234  may be repeated once the oscillator  110  stabilizes again (block  204 ). This repetition may be indefinite, or may terminate after some predetermined maximum number of iterations. 
       FIGS.  6 - 10    show simulation results to demonstrate the advantages of the solution presented herein.  FIGS.  6  and  7    first show the oscillation amplitude achievable when the control circuitry  115  does not include the second feedback path  140 . In this case, the amplitude regulating parameters of the oscillator  110  are fixed and the first feedback path  130  provides the only amplitude control.  FIG.  6    provides results when amplifier  132  in the first feedback path  130  is configured to operate with a high gain that results in a relatively high loop gain, e.g., greater than  10 , versus the results in  FIG.  7    where the amplifier  132  operates with a lower gain that results in a relatively low loop gain, e.g., less than 5. As shown by  FIG.  6   , the higher loop gain implementation provides a very low amplitude variation, e.g., 50-55% of the full swing. However, the high gain necessary to achieve this low amplitude variation results in high power consumption and high noise levels. The lower loop gain implementation enables lower power consumption and noise levels, but as shown in  FIG.  7   , this lower loop gain implementation has a relatively high amplitude variation, e.g., 48-68% of the full swing.  FIG.  8    shows the results when the second feedback path  140  is included with the control circuitry  115  to enable time-discrete adjustment of the amplitude regulating parameter(s) of the oscillator  110 . In this simulation, the first feedback path  130  has a low gain and the second feedback path  140  is used to control two extra amplitude regulating parameters, e.g., the bias tail current and/or the number of g m  cells in the oscillator core, as shown by the three curves in  FIG.  8   . As shown by  FIG.  8   , the solution presented herein results in a lower amplitude variation (52-60%), which was previously not achievable when the first feedback path  130  had a lower loop gain. Thus, the solution presented herein provides the lower noise and power consumption benefits more typically associated with lower loop gain implementations while also providing the amplitude control benefits more typically associated with higher loop gain implementations. 
       FIG.  9    shows simulation results demonstrating how the gain of amplifier  132  may be selected to achieve the desired trade-off between amplitude control and noise/power reduction. The results in  FIG.  9    demonstrate the oscillator amplitude performance for six scenarios, which are qualitatively specified at each point, e.g., “high loop gain,” “low loop gain including second feedback path,” etc. The first four scenarios show the amplitude performance for high/low loop gain and high/low Q scenarios when the second feedback path  140  is not included. The last two scenarios show the amplitude performance for low loop gain and high/low Q scenarios when the second feedback path  140  is included. 
       FIG.  10    shows simulation results demonstrating the noise performance for the same six scenarios as in  FIG.  9   , and thus demonstrates the noise improvement provided by the solution presented herein. In particular, the top two plots show the operation of the frequency generation circuit  100  when the amplitude regulating parameters are fixed and the loop gain of the first feedback path  130  is high. The bottom plot shows the results when the second feedback path  140  is used to modify the bias current and the g m  cells of the oscillator core when the loop gain of the first feedback path  130  is low. The solution presented herein therefore provides a frequency generation circuit having the amplitude control benefits associated with high gain negative feedback and the power and noise benefits associated with low gain negative feedback. 
     The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.