System and method for implementing an oscillator

In one embodiment, a system for generating an oscillating signal includes a transconductance amplifier comprising a single-ended output and a differential input. The system also includes only one feedback loop coupled to the transconductance amplifier. The feedback loop includes a low pass filter configured to receive the output of the transconductance amplifier. Also, the feedback loop includes a high pass filter configured to receive the output of the first low pass filter and output a signal to only one terminal of the differential input of the transconductance amplifier.

TECHNICAL FIELD

This invention relates generally to electronic circuits and more particularly to a system and method for implementing an oscillator.

BACKGROUND

Oscillators are useful in a variety of applications, such as communication. The Wien-Bridge oscillator is an implementation that has been widely used, but is limited. This oscillator operates using a voltage-mode feedback which requires an operational amplifier. The Wien-Bridge oscillator suffers from the inability to operate in high frequencies effectively because of the parasitic effects of a voltage-voltage amplifier (such as an operational amplifier).

Prior solutions have used a transconductance amplifier with multiple feedback loops with the feedback loops providing both positive and negative feedback. These suffer from certain disadvantages. Due to the multiple feedback loops, such solutions operate slowly. In addition, such solutions have a tendency to become unstable, especially when generating high frequency signals.

SUMMARY

In one embodiment, a system for generating an oscillating signal includes a fully differential transconductance amplifier. The fully differential transconductance amplifier includes a first transistor and a second transistor. The system also includes only one feedback loop coupled to each output terminal of the fully differential transconductance amplifier such that each of the first transistor and the second transistor are coupled to two capacitors. Each feedback loop comprises a low pass filter configured to receive a signal from one output terminal of the fully differential transconductance amplifier. The low pass filter includes a varactor coupled, in parallel, to a resistor. Each feedback loop also comprises a high pass filter. The high pass filter is configured to receive the output of the low pass filter and output a signal to the input of the fully differential transconductance amplifier. The high pass filter comprises a variable resistor coupled, in series, to a capacitor. The system also includes a third transistor operable to bias the fully differential transconductance amplifier. Further, the system includes a set of resistors, not comprised by the low pass or high pass filters, coupled to the fully differential transconductance amplifier operable to detect a common-mode voltage from the differential outputs of the transconductance amplifier.

In another embodiment, a system for generating an oscillating signal includes a transconductance amplifier comprising at least one output and at least one input. The system also includes only one feedback loop coupled to the transconductance amplifier. The feedback loop includes a low pass filter configured to receive the output of the transconductance amplifier. Also, the feedback loop includes a high pass filter configured to receive the output of the first low pass filter and output a signal to only one terminal of the at least one input of the transconductance amplifier.

In one embodiment, a method for generating an oscillating signal includes (a) low pass filtering an input signal to generate a low pass filtered signal. The method also includes (b) high pass filtering the low pass filtered signal to generate a high pass filtered signal. In addition, the method includes (c) amplifying, by a transconductance amplifier, the high pass filtered signal to generate an amplified signal. The method also includes forming only one positive feedback loop by routing the amplified signal such that the input signal of step (a) comprises the amplified signal and repeating steps (a) through (c).

Depending on the specific features implemented, particular embodiments may exhibit some, none, or all of the following technical advantages. High frequency oscillating signals may be generated using passive components. The oscillator may be implemented in a relatively small portion of a semiconductor substrate. Other technical advantages will be readily apparent to one skilled in the art from the following figures, description and claims.

DETAILED DESCRIPTION

FIG. 1Aillustrates a block diagram of one embodiment of oscillator100. Oscillator100includes transconductance amplifier110. Transconductance amplifier110is coupled to low pass filter120such that the output of transconductance amplifier110is received by the input of low pass filter120. The output of low pass filter120is coupled to the input of high pass filter130. The output of high pass filter130is coupled to the input of transconductance amplifier110forming a feedback loop.

In some embodiments, transconductance amplifier110operates in a current mode. Transconductance amplifier110may receive a voltage signal, amplify the signal, and output a current proportional to the received voltage signal. In various embodiments, utilizing transconductance amplifier110may allow for amplifying signals at high frequencies, such as those in the gigahertz range.

In some embodiments, low pass filter120and high pass filter130may include a variety of active and/or passive components. Examples of passive components may include capacitors, varactors, resistors, and variable resistors. Filters120and130also may be implemented using active components by being configured according to Sallen-Key or Multiple Feedback topologies.

FIG. 1Bis a flowchart illustrating one embodiment of the operation of oscillator100ofFIG. 1A. In general, the steps illustrated inFIG. 1Bmay be combined, modified, or deleted where appropriate, and additional steps may also be added to the example operation. Furthermore, the described steps may be performed in any suitable order.

At step150, a signal is filtered using low pass filter120. The signal may be noise resident in the circuit or a signal injected into the oscillator depicted inFIG. 1A. At step160, the output of step150is filtered using high pass filter130. This may occur in order to isolate a desired frequency in the signal. At step170, the output of step160is amplified using transconductance amplifier110; the amplified signal is then filtered using low pass filter120at step150. This completes the positive feedback loop and the output of step160may continue to be filtered and amplified by repeating steps150-170. In some embodiments, as the positive feedback loop continues to operate, the signal measured at the input to the transconductance amplifier continues to grow until it reaches a steady state, thereby generating an oscillating signal of a desired frequency. The desired frequency of oscillator100may be designated by adjusting the characteristics of filters120and130. The cutoff frequencies of filters120and130may be determined such that a desired frequency band remains after a signal pass through filters120and130.

FIG. 2is a circuit diagram illustrating one embodiment of oscillator100a, which is a particular implementation of oscillator100. Oscillator100aincludes transconductance amplifier110athat is coupled to low pass filter120aand high pass filter130a. High pass filter130aincludes resistor240connected in series with capacitor230. Low pass filter120aincludes capacitor210connected in parallel with resistor220. The frequency of the signal measured at node250may be determined by utilizing the following formula:

ω02=1RS⁢CS⁢RL⁢CL
where Rsand Rlare the resistances of resistors240and220, respectively; where Csand Clare the capacitances of capacitors230and210, respectively; and where ω02is the square of the frequency of the signal measured at node250. Thus, a desired frequency may be determined by changing the values of resistors220and240and/or capacitors210and230. In some embodiments, the gain (gm) of transconductance amplifier110amay be determined utilizing the following formula:

The depicted embodiment ofFIG. 2illustrates an oscillating circuit with only one feedback loop: the output of transconductance amplifier110ais passed through capacitor230before reaching the input of transconductance amplifier110a. In some embodiments, a single-ended (rather than a differential) output of the transconductance amplifier may be utilized in forming the feedback loop. In some embodiments, this architecture reduces parasitic capacitance and improves the likelihood of generating high quality high frequency signals. Such an architecture may also increase power efficiency. In some embodiments, an optimal configuration of resistor and capacitor values may be where Rsis equal to 2Rland Csis equal to Cl, respectively. In various embodiments, the architecture depicted inFIG. 2may be implemented in a relatively small portion of a semiconductor substrate since it does not include inductors.

Low pass and high pass filters120aand130aare depicted as including resistors and capacitors. These components may be variable so that the filters may be tuned to desired frequencies. Examples of variable components that may be used include varactors and variable resistors. In some embodiments, other suitable components may be used to form the filters, such as active components. For example, digital filters may be utilized to form filters120aand130a.

FIG. 3illustrates a fully differential embodiment of oscillator100b, which is a particular implementation of oscillator100. Transistor310is coupled to low pass filter120b. Low pass filter120bincludes varactors330coupled in parallel to resistors320. Node380may be used to control the capacitance of varactors330by applying a voltage (Vtune) to node380. High pass filter130bis coupled to low pass filter120b. High pass filter130bincludes capacitors350coupled, in series, to variable resistors360. High pass filter130bis coupled to transconductance amplifier110bwhich is operating in a fully differential mode. Transconductance amplifier110bincludes transistors370. Resistors340are coupled to transistors370and operate to detect a common-mode voltage from the differential outputs and use it to bias transistors370. Transistor310sets a total current with a bias voltage (Vb) and is used to bias transistors370. Transistor310may also provide a shielding effect from power supply noise.

In operation, when transistor310is activated, any difference between nodes390and392(which could arise due to noise or mismatch) is detected, filtered (utilizing low pass filter120band high pass filter130b), and amplified (utilizing transconductance amplifier110b). Since the outputs of transistors370are routed to capacitors350and capacitors350are coupled to the input of transistors370, positive feedback loops are formed. In time, the system may reach a steady state such that an oscillating signal may be measured across nodes390and392which will have a frequency determined by low pass filter120band high pass filter130b.

In some embodiments, a desired frequency band may be determined by utilizing varactors330and variable resistors360. For example, the cutoff frequency of high pass filter130bmay be determined by manipulating the resistance of variable resistors360. The cutoff frequency of low pass filter120bmay be determined by changing the voltage level at Vtunesuch that the capacitance across varactors330is changed. As described above, changing the cutoff frequencies of filters120band130bmay allow control over what frequencies are present in the signal outputted by oscillator100b.

The depicted embodiment ofFIG. 3illustrates a positive feedback loop for each of the outputs of transconductance amplifier110b. Each of these outputs are coupled to capacitors350. In turn, capacitors350are coupled to the inputs of transconductance amplifier110b, forming positive feedback loops. In some embodiments, this may be advantageous in that it allows the circuit to reliably generate high frequency oscillating signals. In various embodiments, the architecture depicted inFIG. 3may be implemented in a relatively small portion of a semiconductor substrate since it does not include inductors.

In some embodiments, varactors330may include varactor diodes. Varactors330may also be implemented using active components. Other suitable components may be used. In various embodiments, varactors330may be substituted with capacitors that have fixed capacitances.

In some embodiments, variable resistors360may comprise a digital resistor bank. They may also comprise potentiometers. Other suitable components may be used. In various embodiments, variable resistors360may be substituted with resistors with fixed resistances.

In some embodiments, transistor310may be n-type while transistor370may be p-type. Transistor310may be p-type while transistor370may be n-type. Transistors310and370may use MOSFET, JFET, or other suitable semiconductor technologies.

FIG. 4illustrates several embodiments of transconductance amplifiers that may be used in the oscillators described above. Amplifier410is an embodiment of a differential-input, differential-output transconductance amplifier. Amplifier420is an embodiment of a differential-input, single-output transconductance amplifier. Amplifier430is an embodiment of a single-input, single-output transconductance amplifier. Amplifiers410-430provide examples of transconductance amplifiers that may be used with the oscillators described inFIGS. 1-3. Other suitable configurations of a transconductance amplifier may be utilized in the oscillators described above.

Although several embodiments have been illustrated and described in detail, it will be recognized that modifications and substitutions are possible without departing from the spirit and scope of the appended claims.