Low power wideband LO using tuned injection locked oscillator

A tunable Injection-Locked Oscillator (ILO) having a wide locking range is used in a Local Oscillator (LO) of a wideband wireless transceiver to generate differential signals. The ILO includes a resonator with an adjustable natural oscillating frequency. In one example, the ILO is part of a quadrature divider that can lock onto a Phase-Locked Loop (PLL) output signal in a wide frequency band while achieving lower power consumption and lower phase noise than a differential latch type divider. The ILO is tuned by disabling a Voltage-Controlled Oscillator (VCO) from driving the ILO, adjusting the natural oscillating frequency, making a measurement indicative of the natural oscillating frequency, and determining whether the measurement is within a predetermined range. If the measurement is below the predetermined range, capacitances of resonators within the ILO are decreased, whereas if the measurement is above the predetermined range, capacitances of the resonators are increased.

BACKGROUND INFORMATION

1. Technical Field

The present disclosure relates to low power wideband local oscillators.

2. Background Information

Wireless transceivers generally use one or more local oscillator circuits to generate signals referred to as Local Oscillator (LO) signals. These LO output signals are used by other circuitry in the transceiver either to upconvert a baseband signal in frequency for transmitting, or to downconvert an RF signal in frequency during receiving. The LO output signals often must involve quadrature signals. A divider circuit is therefore traditionally used to divide down a higher frequency signal by two and thereby to output both an In-Phase (I) LO output signal and a Quadrature-Phase (Q) LO output signal, where the I and Q LO output signals are both at half the frequency of the signal supplied to the divider but where the I and Q signals are ninety degrees out of phase with respect to one another. In a narrowband transceiver, an injection-locked type divider can be used for this purpose of generating the I and Q signals. As compared to a differential latch type divider, the injection-locked divider may exhibit lower power consumption and lower phase noise. The injection-locked divider however has a locking range due to the natural oscillating frequency of a part or parts of the circuit. A reference signal is supplied to a Phase-Locked Loop (PLL) such that the PLL outputs a signal of twice the frequency of the desired quadrature signals. The PLL output signal is tunable. The PLL output signal is made to inject energy into the injection-locked oscillator in such a way that a resonator of the injection-locked oscillator oscillates at the frequency or at a frequency fraction of the input PLL signal. For example, the ILO divider may oscillate at a frequency that is ½, or ⅓, or ¼, etc., of the frequency of the PLL output signal. The result is that the injection locked oscillator outputs I and Q signals of the desired output frequency, where this frequency is tunable by controlling the PLL appropriately.

Using an injection locked oscillator to perform frequency division works well in many applications, but using the injection locked oscillator has a drawback in that the oscillator generally can only lock to signals in a narrow frequency band. Locking of the injection locked oscillator is limited in this way because the oscillator has a high Q resonator. Where the device that uses the I and Q LO output signals is the receiver and/or transmitter within a wideband transceiver, such an injection locked oscillator cannot lock at all the PLL signal frequencies required. This is especially true when a large population of devices is considered, over temperature, and process and supply variations that the transceivers will experience. There are techniques for making an injection locked oscillator type divider more adaptable, but these techniques degrade divider performance. Accordingly, in wideband transceivers a differential latch type divider is generally employed to generate the LO output signals. However, a differential latch divider circuit has a higher power consumption and phase noise than an injection locked oscillator even though it can divide down the PLL signal over a wider frequency range.

SUMMARY

A Local Oscillator (LO) of a wideband wireless transceiver generates differential quadrature LO output signals. The LO includes a Phase-Locked Loop (PLL) that drives a quadrature divider. The quadrature divider includes a tunable Injection-Locked Oscillator (ILO) that locks onto the PLL output signal, and divides down the frequency of the PLL output signal by two (or three, or four, etc.), thereby generating both an In-Phase (I) LO output signal and a Quadrature-Phase (Q) LO output signal. Depending on the frequency band and channel in which the wideband wireless transceiver is to communicate, the natural oscillating frequency of the ILO is set to be a predetermined fraction (for example, one half, one third, one quarter, etc.) of the frequency of the PLL output signal.

The ILO includes a pair of resonators where each resonator has a natural oscillating frequency that can be adjusted by a multi-bit digital control signal. The multi-bit digital control signal is used to set the natural oscillating frequency of the ILO to be an appropriate fraction of the frequency of the PLL output signal. By setting the natural oscillating frequency in this way, the ILO is tunable over a wide frequency range. The ILO can therefore lock onto PLL output signals over a wide frequency range. The quadrature divider exhibits lower power consumption and lower phase noise than if the quadrature divider were of a differential latch type. Despite lower power consumption and lower phase noise, the ILO can lock onto a wide range of PLL output frequencies required for operation of the wideband wireless transceiver.

In one specific example, the tunable ILO includes a first resonator and a second resonator. Each resonator has a variable capacitor element. A natural oscillating frequency of the tunable ILO is adjusted by receiving a multi-bit digital control signal that adjusts the capacitances of the variable capacitor elements, thereby adjusting the natural oscillating frequencies of the resonators and of the ILO. Each variable capacitor element includes a set of capacitance elements. Each capacitance element includes two capacitors and a switch. The switch is controlled by a corresponding bit of the multi-bit digital control signal. By appropriate control of the multi-bit digital control signal, selected ones of the capacitance elements are selectively switched in or switched out of the variable capacitor element, thereby setting the overall capacitance of the variable capacitor element.

The natural oscillating frequency of the tunable ILO is typically adjusted during operation of the wideband wireless transceiver to be half the frequency of the PLL output signal injected into the ILO. In order to perform this adjustment, a processor of the wireless wideband transceiver first determines an impending change in frequency of the LO output signal. The processor may, for example, use band and channel information to look up an acceptable range of measurement values. Next, a Voltage-Controlled Oscillator (VCO) of the PLL that normally drives the ILO is disabled from driving the ILO. After the VCO is disabled, the natural oscillating frequency of the ILO is adjusted so that the natural oscillating frequency of the resonators within the ILO will be closer to the natural oscillating frequency required for the operation band and channel selected for communication by the wideband wireless transceiver. In one example, this adjustment is performed by setting the value of the multi-bit digital control signal supplied to variable capacitance elements in the resonators in the ILO. In another example, this adjustment is performed by setting an analog control voltage going to varactors in the resonators in the ILO. Next, a measurement is made that is indicative of the natural oscillating frequency of the resonators. If the measurement is not within the acceptable range, then the adjustment and measurement process is iteratively repeated until the natural oscillating frequency is within the acceptable range. Once the natural oscillating frequency as measured is determined to be within the acceptable range, then the VCO is enabled so that the VCO again drives the ILO.

The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and does not purport to be limiting in any way. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth herein.

DETAILED DESCRIPTION

FIG. 1is a simplified diagram of a wireless transceiver100in accordance with one aspect. In this case, the wireless transceiver100is a Wi-Fi (“wireless fidelity”) transceiver for wireless communication. Wi-Fi is a wireless technology standard for wireless local area network (“WLAN”) and wireless devices, and refers to certain types of WLAN that operate in compliance with specifications in the IEEE 802.11 family. The IEEE 802.11 family includes 802.11(a), 802.11(b), 802.11(n), 802.11(ad), 802.11(ac), and 802.11(g), among others, and specifies an over-the-air interface for wireless communication. Transceiver100includes, among other parts not illustrated, an antenna101, a Radio Frequency (RF) transceiver integrated circuit102, and a baseband processor integrated circuit103. The transceiver100can receive a signal104on antenna101. The signal104passes from the antenna101through an antenna switch105, and then to a receive chain106. The receive chain106includes a Low Noise Amplifier (LNA)107that amplifies the signal and a mixer108that downconverts the signal. The downconverted signal is filtered by baseband filter109and converted into digital form by Analog-to-Digital Converter (ADC)110such that the resulting digitized information passes to the baseband processor integrated circuit103for further processing by processor111. In some examples, ADC110is a part of the baseband processor integrated circuit103. How the mixer108of the receive chain106downconverts the signal is controlled by local oscillator112. In the event the transceiver100is transmitting a signal, information to be transmitted is generated in digital form by processor111, and is communicated by a bus mechanism114to a transmit chain115in the RF transceiver integrated circuit102. The transmit chain115includes a Digital-to-Analog Converter (DAC)116that converts the digital information into an analog information. The analog information is filtered by baseband filter117and is then upconverted by mixer118. The upconverted signal is then amplified by power amplifier119, passes through antenna switch105and is transmitted from antenna101in the form of wireless communication113. In some examples, DAC116is a part of the baseband processor integrated circuit103. How the mixer118of the transmit chain115upconverts the signal is controlled by local oscillator112. In this case, the local oscillator112supplies quadrature signals in the form of an In-Phase (I) differential signal on conductors120and121, and a Quadrature-Phase (Q) differential signal on conductors122and123. The Q differential signal is ninety degrees out of phase with respect to the I differential signal. In addition, as described below, the local oscillator112involves an Injection-Locked Oscillator (ILO)112. This ILO is calibrated or tuned by an amount of digital control circuitry124. The processor111communicates with and controls the digital control circuitry124via bus mechanism114and digital control conductors125. The processor111of the baseband processor integrated circuit103can access and execute programs of processor executable instructions126that are stored in memory127. In addition, the memory127stores an ILO tuning table128as described below.

In one example, the wireless transceiver100ofFIG. 1is a Wi-Fi transceiver that can operate either in the 802.11(a) band or in the 802.11(b) band. The 802.11(a) band extends from approximately 4.8 GHz to 5.8 GHz, whereas the 802.11(b) band extends from approximately 2.412 GHz to 2.484 GHz. Within each of these bands the wireless transceiver100may be communicating wireless signals over a selected one of multiple channels, where each channel is separated from the nearest adjacent channel by 20 MHz. The baseband processor integrated circuit103is aware of and determines which communication channel will be used, and controls the local oscillator112of the RF transceiver integrated circuit102appropriately.

FIG. 2is a block diagram of local oscillator112and digital control circuit124ofFIG. 1. Local oscillator112receives a reference clock signal FREF129on conductor130, and outputs the In-Phase differential signals IN131and IP132on conductors120and121, and the Quadrature-Phase differential signals QN133and QP134on conductors122and123. The I signal is a differential signal, and the Q signal is a differential signal. The local oscillator112includes a Phase-Locked Loop (PLL)135and a quadrature divider136. There are many ways the PLL135can be implemented, but in the illustrated example, the PLL135includes a Phase Frequency Detector (PFD)137, a Charge Pump (CP)138, a loop filter139, a Voltage-Controlled Oscillator (VCO)140, and a loop divider141. PFD137compares the phase of the reference clock signal FREF129with a feedback divider signal FDIV142from the loop divider141. Depending on the relative phase of these two signals, PFD137generates up and down signals, UP and DN. The charge pump138converts the UP and DN signals into a Direct Current (DC) signal that is in turn filtered by loop filter139and converted into a VTUNE signal143. VCO140outputs an oscillating differential signal involving signal VON144on conductor145, and signal VOP146on conductor147. The frequency of the VCO output signals depends on the VTUNE signal143supplied to the VCO140.

The digital control circuit124ofFIG. 2receives an acceptable measurement count range signal148on conductors125, and outputs a VCO ON/OFF signal149on conductor150, as well as a resonator control signal RES CTRL151on conductors152. The resonator controls signal RES CTRL151is a multi-bit digital control signal, and reference numeral152indicates multiple digital conductors. In addition, the digital control circuit124communicates with processor111of the baseband processor integrated circuit103by conductors125and the local bus mechanism114. The digital control circuit124includes a measurement circuit153and a state machine154. The measurement circuit153in the illustrated example includes a differential-to-single buffer circuit155that receives the differential in phase signals IN131and IP132on conductors120and121, and outputs a corresponding singled-ended clock signal to the clock input lead of counter156. The counter156has a clear input lead157coupled to receive the COUNTER ENABLE/CLRB signal158from toggle flip-flop159. The counter156supplies a multi-bit digital parallel output MEASUREMENT CNT OUT signal160on conductors161to the state machine154. The state machine154is clocked by the signal FREF129.

FIG. 3is a more detailed circuit diagram of the quadrature divider circuit136ofFIG. 2. The quadrature divider circuit136receives the multi-bit digital control signals RES CTRL151on conductors152, and outputs the I and Q differential signals on conductors120,121,122, and123. The quadrature divider136includes first portion162and a second portion163. Each of these two portions includes a resonator. Resonator164is the resonator for first portion162. Resonator165is the resonator for second portion163. The first portion162further includes cross-coupled transistors166and167, as well as quadrature signal injection transistors168and169having gates that receive signals QN133and QP134, respectively. Similarly, the second portion further includes cross-coupled transistors170and171, as well as quadrature signal injection transistors172and173having gates that receive signals IN131and IP132, respectively. The first and second portions162and163, together along with N-channel transistors174and175, and coupling capacitors176and177constitute an Injection-Locked Oscillator (ILO)178. The ILO178, as illustrated inFIG. 3, is a conventional ILO except for the variable nature of capacitances within the resonators, and typically includes biasing circuitry (not shown) that biases N-channel transistors174and175. The first portion162and second portion163oscillate at the same frequency, but output signals I and Q that are ninety degrees out phase with respect to each other.

A natural oscillating frequency of resonators164and165can be adjusted such that the ILO178can lock to a PLL135output signal that is variable over a wide frequency range. The resonators164and165include inductive elements and capacitive elements coupled in parallel. Resonator165is of substantially the same structure as resonator164. In the illustrated example, the inductive elements of resonator164are inductor179and inductor180. The capacitive element is a digitally-controlled variable capacitor element181. The variable capacitor element181is a network of parallel-connected capacitance elements182,183, and184(identified by the dotted circles). Each capacitance element includes two capacitors and a switch. The switches185-187are controlled by digital control bits of the resonator control multi-bit digital control signal RES CNTRL151to selectively switch in or switch out capacitance elements182-184thereby varying the capacitance of the variable capacitor181. In this example, the capacitors of resonators164and165have a binary weighted sizing of 10-femtofarad granularity and can be calibrated to have capacitances between approximately 0-femtofarads and 150-femtofarads (for example, 10-femtofarads, 20-femtofarads, 30 femtofarads, . . . , 150 femtofarads). The capacitances listed above assume ideal switches and capacitors. In this way, the state machine154of the digital control circuit124can change the capacitances of the resonators164and165, and therefore the natural oscillating frequency of the resonators by changing a value of the resonator control signal RES CNTRL151. The quadrature divider136advantageously can operate at a low supply voltage. A supply regulator188receives an unregulated battery voltage of 1.3V from battery supply conductor191and supplies the ILO178with a regulated 0.7V DC supply voltage on supply conductor189. Although the resonators164and165of the ILO178may be relatively high Q resonators, their natural oscillating frequencies are controllable and adjustable by the state machine154of the digital control circuit124. In this way, processor111can adjust the natural oscillating frequency of the resonators164and165such that the ILO178can lock to a PLL135output signal that is variable over a wide frequency range such that the ILO178is usable as a local oscillator in a wideband wireless receiver while still obtaining advantages attendant in an ILO of relatively low power and phase noise.

FIG. 4is a simplified waveform diagram that illustrates how the digital control circuit124can make a measurement indicative of the natural oscillating frequency of the ILO178. The counter156of the measurement circuit153is cleared when the COUNTER ENABLE/CLRB signal158is low. The counter starts counting on the rising edge of the COUNTER ENABLE/CLRB signal158and counts rising edges of the In-Phase signal (represented here by signal IN131) and this counting of edges continues for one complete period of the signal FREF129and concludes at the time indicated by arrow190, at the falling edge of COUNTER ENABLE/CLRB signal158. The number of edges is recorded in the digital value of the MEASUREMENT CNT OUT signal160on conductors161ofFIG. 2. At the same time, the state machine154reads the measurement count and uses the measurement value to determine whether or not to adjust the natural oscillating frequency of the ILO178ofFIG. 3.

FIG. 5is a table that illustrates how the ILO resonator is tuned in different operating conditions of the wireless transceiver100ofFIG. 1. In this example, the wireless transceiver100is operable in either the 802.11(a) band and 802.11(b) band, and in each of these bands the transceiver100can operate in a selectable one of various different channels. In the table ofFIG. 5, the band is indicated in the left column and channel is indicated in the middle column of the table. For each combination of band and channel, the ILO resonator tuning table ofFIG. 5that is stored in memory127of the baseband processor integrated circuit103includes data indicating an acceptable measurement count range. Each of these count ranges includes a first count value that indicates a low value of the count range and a second count value that indicates an upper value of the count range. The processor111of the baseband processor integrated circuit103determines the band and channel to be used as the wireless transceiver100operates and looks up and supplies the acceptable measurement count range information148via the local bus mechanism114to the digital control circuit124ofFIG. 1so that the digital control circuit124can in turn adjust the natural oscillating frequency of the local oscillator112appropriately. In this way, even though the resonators164and165of the ILO178may be relatively high Q resonators so that the ILO178has a relatively narrow frequency locking range, the processor111can adjust the natural oscillating frequency by changing the capacitance in the resonators164and165such that the narrow locking range corresponds to the VCO output signal required for communication in the necessary band and channel.

FIG. 6is a simplified flowchart of an operation of the wireless transceiver100ofFIG. 1in accordance with one novel aspect. A processor of the wireless transceiver determines (step201) that the local oscillator output signal is to change in frequency in such a way that the ILO natural oscillating frequency will be changed. The processor then determines how the ILO is to be adjusted (step202) such that the new ILO natural oscillating frequency is obtained. For example, the processor111may determine based on the band and channel to be used in a communication what the corresponding acceptable measurement count range is by reading the ILO resonator tuning table128from memory127, and then communicating the count values indicating the measurement count range to the state machine154of the digital control circuit124. Next, the state machine154of the digital control circuit124disables the VCO140(step203) from driving the ILO178. In one example, the state machine154disables the VCO140from driving the ILO178by asserting VCO ON/OFF signal149on conductor150so that the VCO140no longer outputs an oscillating signal. Because the output of the VCO140is AC coupled to signal injection N-channel transistors174and175ofFIG. 3by AC coupling capacitors176and177, the fact that the VCO output signal no longer oscillates prevents the VCO140from changing the conductance of the N-channel transistors174and175and therefore prevents the VCO140from affecting the resonance of the ILO178. Next, the state machine adjusts the ILO resonator (step204) of the ILO so that the natural oscillating frequency of the resonators is adjusted to be closer to the natural oscillating frequency required according to the band and channel selected. For example, this is done by changing the resonator control digital signal RES CTRL151and changing the capacitances of the variable capacitance elements in the resonators164and165. Next, the measurement circuit in the digital control circuit makes a measurement (step205) indicative of the natural oscillating frequency of the ILO. As explained above in connection withFIG. 4, this may involve counting the number of rising edges of the In-Phase signal that occur in a fixed amount of time such as in one period of the reference clock signal FREF129. In the present example, the signal FREF129has a precise frequency and is generated and received from a crystal oscillator oscillating at 19.2 MHz. Once the measurement is made, the state machine determines (step206) whether the measurement indicates whether the ILO has been properly adjusted. If the measure indicates that the ILO has not been properly adjusted, (for example, the measurement count is not in the acceptable measurement count range148received by the state machine154from the processor111) then processing proceeds to step207. If the state machine determines that the ILO natural oscillating frequency is high (step207), then the state machine adjusts the ILO resonator (step209) so that the ILO natural oscillating frequency decreases. This involves increasing the amount of capacitance of the variable capacitor elements. On the other hand, if the ILO natural oscillating frequency is determined not to be high (step207), then the state machine adjusts the ILO resonator so that the natural oscillating frequency increases (step208). Regardless of whether the ILO natural oscillating frequency has been increased or decreased in steps208or209, processing returns to step205. The measurement circuit153of the digital control circuit124makes another measurement indicative of the natural oscillating frequency of the ILO (step205) and processing proceeds to step206. In this way the digital control signal iteratively adjusts the natural oscillating frequency of the resonators of the ILO until the measurement indicates that the ILO has been properly adjusted (step206). If the state machine determines that the ILO has been properly adjusted, (for example, the value of MEASUREMENT CNT OUT160falls within the acceptable measurement count range looked up from table128), then the digital control signal enables the VCO (step210) so that the VCO again drives the ILO.

Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. The tunable ILO may be used in other types of transceivers and is not limited to use in the Wi-Fi transceiver ofFIG. 1. In one example, the tunable ILO is part of a wireless transceiver that operates in a different frequency band than Wi-Fi. In another example, the tunable ILO is part of a transceiver (without an antenna) used for a wireline application. There are several different types of resonators and they can be adjusted in different ways. In one example, the natural oscillating frequency of the resonator is adjusted by changing the capacitance of a varactor within the resonator by changing a voltage level. In another example, the natural oscillating frequency of the resonator can be adjusted by changing an inductance in the resonator. In yet another example, the resonator is constructed using Rotating Metal On Metal (RTMOM) capacitors. Furthermore, switches in a variable capacitor element of an ILO resonator are not limited to the configuration ofFIG. 3, and the switches may be constructed with P-channel field-effect transistors, N-channel field effect transistors, or transmission gates. Additionally, the counter that makes the “MEASUREMENT CNT OUT” measurement ofFIG. 4may measure through a prescaler, where the prescaler is part of the loop divider of a PLL. In that case, differential signals IN and IP as output from the quadrature divider (ILO) are inputs to the prescaler, not VOP and VON from the VCO (as illustrated inFIG. 2.). The signal output by the prescaler is an input signal to the counter that outputs the “MEASUREMENT COUNT OUT” ofFIG. 4. There are also different types of state machines and different ways of constructing them, and the state machine154ofFIG. 2is but one type. In one example, the state machine is a hardwired, dedicated state machine. In another example, the state machine is a processor that executes instructions. Accordingly, various modifications, adaptations, and combinations of the various features of the described specific embodiments can be practiced without departing from the scope of the claims that are set forth below.