Low-Noise Oscillator Amplitude Regulator

Abstract

An electronic device comprises a first feedback circuit operatively connected to an amplitude detector and a first control input of an oscillator. The first feedback circuit is configured to control an amplitude of an output of the oscillator by continuously applying a first control signal to the first control input in response to an amplitude detected by the amplitude detector. The electronic device further comprises a second feedback circuit operatively connected to the amplitude detector and a second control input of the oscillator. The second feedback circuit is configured to modify one or more amplitude regulating parameters of the oscillator by providing a second control signal in response to the amplitude being beyond an upper or lower amplitude threshold, and refrain from modifying the one or more amplitude regulating parameters when the amplitude is within the upper and lower amplitude thresholds.

Claims

1. An electronic device comprising: a first feedback circuit operatively connected to an amplitude detector and a first control input of an oscillator, the first feedback circuit configured to control an amplitude of an output of the oscillator by continuously applying a first control signal to the first control input in response to an amplitude detected by the amplitude detector; and a second feedback circuit operatively connected to the amplitude detector and a second control input of the oscillator, the second feedback circuit configured to: modify one or more amplitude regulating parameters of the oscillator by providing a second control signal in response to the amplitude detected by the amplitude detector being beyond an upper or lower amplitude threshold; and refrain from modifying the one or more amplitude regulating parameters when the amplitude detected by the amplitude detector is within the upper and lower amplitude thresholds.

2. The electronic device of claim 1, further comprising the oscillator and the amplitude detector.

3. The electronic device of claim 1, wherein the second feedback circuit is further configured to control the second control signal to prevent the one or more amplitude regulating parameters of the oscillator from changing when the oscillator powers on.

4. The electronic device of claim 1, wherein the second feedback circuit is further configured to control the second control signal to change the one or more amplitude regulating parameters responsive to an upcoming random access channel transmission event, an upcoming radio transmission event, and/or an upcoming radio reception event.

5. The electronic device of claim 1, wherein the first feedback circuit is further configured to control the amplitude of the oscillator output, responsive to the detected amplitude, by continuously controlling a gain of the oscillator.

6. The electronic device of claim 1, wherein the one or more amplitude regulating parameters comprises an oscillator bias current.

7. The electronic device of claim 1, wherein the one or more amplitude regulating parameters comprises a number of oscillator cells controlling transconductance of the oscillator.

8. The electronic device of claim 1, wherein the one or more amplitude regulating parameters comprises a bias point of one or more oscillator cells controlling transconductance of the oscillator.

9. The electronic device of claim 1, wherein the one or more amplitude regulating parameters comprises a variable resistance connected in parallel with the oscillator.

10. The electronic device of claim 1, wherein the second feedback circuit is further configured to provide the second control signal only at discrete times when the oscillator is stable.

11. A method of regulating oscillator amplitude, the method comprising: controlling an amplitude of an output of an oscillator by using a first feedback circuit to continuously applying a first control signal to a first control input of the oscillator in response to an amplitude detected by the amplitude detector; and controlling one or more amplitude regulating parameters of the oscillator using a second feedback circuit to: modify one or more amplitude regulating parameters of the oscillator by providing a second control signal in response to the amplitude detected by the amplitude detector being beyond an upper or lower amplitude threshold; and refrain from modifying the one or more amplitude regulating parameters when the amplitude detected by the amplitude detector is within the upper and lower amplitude thresholds.

12. The method of claim 11, wherein controlling the one or more amplitude regulating parameters using the second feedback circuit further comprises using the second feedback circuit to prevent the one or more amplitude regulating parameters of the oscillator from changing when the oscillator powers on.

13. The method of claim 11, wherein using the second feedback circuit to modify the one or more amplitude regulating parameters comprises changing the one or more amplitude regulating parameters responsive to an upcoming random access channel transmission event.

14. The method of claim 11, wherein using the second feedback circuit to modify the one or more amplitude regulating parameters comprises changing the one or more amplitude regulating parameters responsive to an upcoming radio transmission event and/or an upcoming radio reception event.

15. The method of claim 11, wherein controlling the amplitude using the first feedback circuit comprises controlling the amplitude of the oscillator output responsive to the detected amplitude by continuously controlling a gain of the oscillator.

16. The method of claim 11, wherein the one or more amplitude regulating parameters comprises an oscillator bias current.

17. The method of claim 11, wherein the one or more amplitude regulating parameters comprises a number of oscillator cells controlling transconductance of the oscillator.

18. The method of claim 11, wherein the one or more amplitude regulating parameters comprises a bias point of one or more oscillator cells controlling transconductance of the oscillator.

19. The method of claim 1, wherein the one or more amplitude regulating parameters comprises a variable resistance connected in parallel with the oscillator.

20. The method of claim 1, wherein controlling the one or more amplitude regulating parameters using the second feedback circuit comprises providing the second control signal only at discrete times when the oscillator is stable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 shows a block diagram of a frequency generation circuit according to one exemplary embodiment.

[0012] FIG. 2 shows an amplitude control method according to one exemplary embodiment.

[0013] FIG. 3 shows a block diagram of the first feedback path of the frequency generation circuit of FIG. 1 according to one exemplary embodiment.

[0014] FIG. 4 shows a block diagram of the second feedback path of the frequency generation circuit of FIG. 1 according to one exemplary embodiment.

[0015] FIG. 5 shows another amplitude control method according to one exemplary embodiment.

[0016] FIG. 6 shows simulation results achievable with only a first feedback path having a high gain.

[0017] FIG. 7 shows simulation results achievable with only a first feedback path having a low gain.

[0018] FIG. 8 shows exemplary simulation results achievable with the solution presented herein.

[0019] FIG. 9 shows exemplary simulation results when the first feedback path has different gains.

[0020] FIG. 10 shows exemplary simulation results of the noise improvement achievable with the solution presented herein.

DETAILED DESCRIPTION

[0021] 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.

[0022] 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.sub.1), a second control input (CTRL.sub.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.sub.1 and S.sub.2, applied to the respective first and second control inputs control the amplitude of the signal S.sub.o at the output of the oscillator 110. In particular, the first control signal S.sub.1 provides time-continuous control of the amplitude of S.sub.o, while the second control signal S.sub.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.sub.m cells, a bias point of one or more of the oscillator g.sub.m cells, and/or a variable resistance connected in parallel with a core of the oscillator 110. Because the second control signal S.sub.2 controls the configuration of the oscillator 110, S.sub.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.sub.1.

[0023] The control circuitry 115 generates the first and second control signals S.sub.1, S.sub.2 responsive to the oscillator output signal S.sub.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.sub.o (block 210). The first feedback path 130 provides time-continuous control of the amplitude of the oscillator output signal S.sub.o by continuously controlling the first control signal S.sub.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.sub.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.

[0024] 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.sub.ref, are input to amplifier 132. Amplifier 132 amplifies the amplitude error A.sub.err formed from the difference between the detected amplitude A and the reference amplitude A.sub.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.sub.1. The first control signal S.sub.1controls the gain of the oscillator core by controlling the gain of the negative resistance circuit 114. In so doing, the first control signal S.sub.1 controls the amplitude of the oscillator output signal S.sub.o.

[0025] 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.sub.1 to sufficiently control the amplitude of the oscillator output signal S.sub.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.sub.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.

[0026] 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.sub.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.sub.o, is too high, the second feedback path 140 may decrease the bias current, decrease the number of active oscillator g.sub.m cells, decrease a bias point of one or more of the active g.sub.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.

[0027] 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.sub.2 to control the amplitude regulating parameter(s) only when the detected amplitude A exceeds an upper threshold T.sub.U or is lower than a lower threshold T.sub.L. In addition, the control circuit 142 may control the second control signal S.sub.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.sub.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.sub.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's amplitude regulating parameter(s).

[0028] 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.sub.o (block 210). If the detected amplitude A exceeds an upper threshold T.sub.U (block 232) or is less than a lower threshold T.sub.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.

[0029] 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.sub.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.

[0030] 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.

[0031] 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.sub.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.

[0032] 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.