CMOS power oscillator with frequency modulation

CMOS power oscillator and a method of frequency modulating a CMOS power oscillator. The oscillator comprises a transformer-based feedback CMOS power oscillator circuit formed on a chip-substrate, the oscillator circuit including a transformer coupled to a transistor; means for modulating the capacitance of the transformer to the chip-substrate for frequency modulating an output of the power oscillator.

FIELD OF INVENTION

The present invention relates broadly to a complementary metal oxide semiconductor (CMOS) power oscillator with frequency modulation, and to a method of frequency modulating a CMOS power oscillator.

BACKGROUND

The rapidly growing market of personal communication systems, radio medical implanted systems, and wireless hearing aids provides an increasing demand for more integrated and more efficient radio frequency (RF) integrated circuits (IC's). These IC's are required to operate with supply voltages under 2V and sometimes down to 1V with minimum current consumption at frequencies up to several GHz. Such applications typically contain a combination of several modules including a power amplifier, an oscillator, for example a voltage controlled oscillator (VCO), and modulator.

For example Class E power amplifier circuits are very suitable for high efficiency power amplification applications in the radio-frequency and microwave ranges. However, due to the inherent asymmetrical driving arrangement, existing Class E amplifier circuits suffer significant harmonic contents in the output voltage and current, and usually require substantial design efforts in achieving the desired load matching networks for applications requiring very low harmonic contents.

The basic Class E circuit is typically implemented using discrete components including a transistor, which is connected with an RFC to the supply voltage and to the load network. The load network is made up of a capacitor shunting the transistor and a series tuned inductor capacitor resonant circuit. The transistor is driven hard enough to act like a switch. The principle of Class E power amplifiers is to avoid by design the simultaneous existence of high voltage and high current in the switch, even in the case of a long switching time. That would imply 100% efficient conversion of dc to RF energy.

Frequency modulation is typically implemented via a varactor and is based on an LC-tank circuit. However, this requires additional discrete components to match the load network resulting in lower power efficiency. Typical solutions include using two identical resonant circuits, which encounters the same problem of matching inductors and capacitors, as well as using symmetrically driven push-pull Class E amplifier for high power applications.

A need therefore exist for providing an alternative oscillator design with frequency modulation capability, which seeks to address one or more of the above mentioned problems.

SUMMARY

In accordance with a first aspect of the present invention there is provided a CMOS power oscillator comprising a transformer-based feedback CMOS power oscillator circuit formed on a chip-substrate, the oscillator circuit including a transformer coupled to a transistor; means for modulating the capacitance of the transformer to the chip-substrate for frequency modulating an output of the power oscillator.

The means for modulating may comprise a patterned ground shield (PGS) layer formed in the chip-substrate and coupled to an input circuit for receiving a modulating signal.

The power oscillator may further comprise a conducting layer formed in the chip-substrate for shielding the PGS layer and the transformer.

The input circuit may comprise a MOS FET.

The modulating signal may alternately set the PGS to floating and to grounding for modulating the capacitance of the transformer to the chip-substrate for frequency modulating an output of the power oscillator.

The means for modulating may comprise a deep N-well formed in the chip-substrate and coupled to an input circuit for receiving of a modulating signal.

The deep N-well may comprise a p-n junction.

The modulating signal may alternate the p-n junction capacitance and resistance for modulating the capacitance of the transformer to the chip-substrate for frequency modulating an output of the power oscillator.

The oscillator circuit may further include a variable capacitance coupled between an output terminal of the power oscillator and ground for varying an output carrier frequency of the power oscillator.

The variable capacitor may comprise a varactor for implementing a voltage controlled oscillator (VCO) with frequency modulation capabilities.

The transformer may provide a feedback path between the drain and the gate of the transistor.

A first port of the transformer may be connected for RF grounding and drain bias feeding, and a second port of the transformer is connected for RF grounding an gate bias feeding.

A third port of the transformer may be connected to the drain of the transistor, and a fourth port of the transformer is connected to the gate of the transistor.

Parameters of the transistor and parameters of the transformer may be chosen to pre-set the output carrier frequency of the power oscillator.

In accordance with a second aspect of the present invention there is provided a method of frequency modulating a CMOS power oscillator, the method comprising providing a transformer-based feedback CMOS power oscillator circuit formed on a chip-substrate, the oscillator circuit including a transformer coupled to a transistor; and modulating the capacitance of the transformer to the chip-substrate for frequency modulating an output of the power oscillator.

DETAILED DESCRIPTION

FIGS. 1(a) and (b) show a circuit schematic and a die microphotograph respectively of an on-chip power oscillator structure100. The structure100is fabricated using a conventional 0.18 μm CMOS process technology, with six metal TiW/Al-1% Si/TiW interconnects on a lossy silicon substrate102of 10 Ωcm. A three and a half turn circular spiral transformer104with metal trace width of 10 μm, a spacing of 2 μm and an inner diameter of 100 μm (total size: about 270×270 μm2) is formed on the substrate102(FIG. 1(b)). The transformer metal traces106are formed by a top metal layer of 2 μm thickness, and two embedded metal layers on the substrate102are stacked together with a dense resistive via array108to form an underpass.

The input and output ports112,114of the transformer104are connected to respective ground-signal-ground (GSG) pads116. A ground guard-ring structure118is laid out for better grounding. The equivalent circuit200for the on-chip transformer104is presented inFIG. 2. The circuit200consists of three parts: I) self-inductances, self-resistances (L1, L2, R1, R2); II) coupling capacitances (C12, C13, C23), and III) substrate effect parasitics, including oxide capacitances (Cs1, Cs2, Cs3), substrate capacitances (Cs11, Cs22, Cs33) and substrate resistances (Rs1, Rs2, Rs3). The mutual inductance between the metal traces is described by parameter K. Accurate parameters of the transformer circuit200model can be easily extracted from measured S-parameters. The feedback topology is chosen for a power oscillator design.

Returning toFIG. 1(b), the on-chip transformer104is used as a RF signal feedback and bias supply paths between the drain119and gate120of a power transistor122to reduce substrate coupling and resistance loss to achieve a high efficiency. Ports117,121of the primary and secondary sides of the on-chip transformer104are connected to the drain119and the gate120of the CMOS power transistor122, respectively. The ports114,112are connected to capacitors128,130respectively for RF grounding, as well as for drain and gate bias feeding, respectively. The output port of the power oscillator100is from the RF terminal132connected to the drain119via capacitor134.

For considerations of the circuit design, the size of the transistor122and the number of turns of the transformer104determines the oscillating carrier frequency, as the transformer provides a feedback path that forms a resonant loop for the desired oscillating carrier frequency. The size of the transistor122also determines the RF output power level (calculation based on transistor Poutmax<about 0.1 W/mm and efficiency), with a larger size transistor122providing more power gain to the oscillator100, while the operating carrier frequency decreases due to a higher Cgs. Therefore, the output power and operating frequency are a trade-off between dimensions of the transformer104and the size of the transistor122.

An NMOS transistor122with gate length of 0.18 μm and total width of 550 μm is used for the power oscillator100. The transistor RF model is created using a Bsim3 model for simulation together with extracted substrate and gate network parameters. Simulation was carried out using extracted RF models of the transistor122, the transformer104and the capacitors128,130,134. The waveforms of the output voltage (curve300) and current (curve302) are shown inFIG. 3. The voltage waveform (curve300) shows that the circuit operates in Class-E mode. The carrier frequency is at about 2.45 GHz with an output power of 15.5 dBm and a phase noise of about −122 dBc/Hz at 100 kHz offset at Vds=1.8 V and Ids=29.8 mA.

The fabricated oscillator100with a die size of 0.6×0.7 mm with the GSG test pad116, was also measured using a HP 8563E spectrum analyzer with phase noise measurement option and battery power supply. The oscillator100was placed in a small shielded chamber during the measurements. The measured results shown inFIG. 4demonstrate that the output power is about 15.3 dBm with a phase noise of about −113 dBc/Hz at 100 kHz offset from a carrier frequency of about 2.446 GHz at Vds=1.8 V and Ids=28.7 mA.

The carrier frequency (curve500) and DC-to-RF conversion efficiency (curve502) as a function of gate voltage were also measured and are shown inFIG. 5. The results show that the carrier frequency drifts down slightly while the gate voltage increases from 0.47 to 0.89 V, and the peak efficiency of the DC-to-RF conversion of about 66% occurs at Vgs=0.71 V. This is believed to be due to the increase in transconductance, gm, resulting in an increase in feedback power level while the gate voltage increases. The nonlinear part in the output spectrum, especially the third-harmonic signal, will reduce the carrier output power at higher gate voltages, as the gate voltage increases, and the increase in gate capacitance induces a decrease in carrier frequency. Further increase in gate voltage results in multi-oscillating frequencies.

Returning now toFIG. 1(b), the oscillator100can be used as a Class E power amplified type circuit with the oscillator in switching mode, and exhibits low phase noise, high efficiency and high power. The transformer104is used to generate a feedback path to meet the oscillation loop requirement: loop-phase equal to 360° and amplitude is greater than 1, while the transistor122provides the loop power gain and part of the loop phase shift. This results in a Class E power amplified type circuit with very low phase noise.

The oscillator100can be modulated by feeding a modulating signal to a Patterned Ground Shield (PGS) layer600of the substrate102(visible through transparent oxide layers of the substrate102) which affects the transformer104on the top layer, thus forming an oscillator with modulation. PGS layers are typically used for isolating a circuit on top of a substrate from the rest of the substrate and around the circuit.FIG. 6shows a schematic cross-sectional view of the transformer104, illustrating the location of the PGS layer600underneath the transformer104. The modulation signal602is provided to the PGS layer600via a transistor604. The modulation signal602is utilized to modulate the gate of the transistor604and to modulate the channel resistance. While the channel resistance is very high, the PGS layer600behaves as a floating metal layer, and while the channel resistance is very low, the PGS layer600behaves as if it is connected to ground. Due to the proximity of the PGS layer600to the transformer104, the modulation signal602will modulate the distance between the transformer104and ground606to change the capacitance between the transformer104and the substrate102, which in turn modulates the effective inductance of the transformer104.

The PGS layer600can be set to floating or grounding to modulate the capacitance of the transformer104to the substrate102. The effective inductance of the transformer104is modulated by the modulating signal602and the carrier frequency of the oscillator100is thus modulated by the modulating signal.FIG. 7is a schematic circuit diagram illustrating the influence of the modulated capacitance of the transformer104to the substrate102, indicated as arrows700to704inFIG. 7.FIG. 8shows an FM signal spectrum800for a modulating signal having a pulse frequency of about 30 kHz and width of about 900 ns, and a modulated voltage of about 0.7V while the drain voltage is about 1.5V and the gate voltage is about 0.7V.

FIG. 9shows a circuit schematic of a CMOS process technology VCO structure900with FM modulation, which is a modification of the oscillator structure100ofFIG. 1(a). The modification consists of connecting a variable capacitor in the form of a MOS varactor902between the RF output904and the RF ground906.FIG. 10shows the simulated frequency (curve1000) and output power (curve1002) respectively as a function of the controlled voltage applied to the MOS varactor (compare902inFIG. 9).FIG. 10demonstrates that the circuit can function as a VCO. Further simulations showed that if the variable capacitor (compare902inFIG. 9) changes from 0.1 pf to 4 pf, the oscillating frequency changes from about 2.45 GHz to about 1.39 GHz, with the drain voltage at about 1.5V and the gate voltage at about 0.7V.

The VCO can again be modulated by feeding a modulating signal to a Patterned Ground Shield (PGS) layer of the substrate which affects the transformer, thus forming a VCO with modulation.FIG. 11shows a schematic cross-sectional view of the transformer1102, illustrating the location of the PGS layer1100underneath the transformer1102. The modulation signal1104is provided to the PGS layer1100via a transistor structure1106. The PGS layer1100can be set to floating or grounding to modulate the capacitance of the transformer1102to the substrate1108. The effective inductance of the transformer1102is modulated by the modulating signal1104and the carrier frequency of the VCO900is thus modulated by the modulating signal.FIG. 12shows a frequency modulated (FM) signal spectrum1200with a modulation signal frequency of about 200 Hz and amplitude about 1V with an offset of about 0.5V, while the drain voltage is about 1.5V and the gate voltage is about 0.7V.

In another arrangement illustrated inFIG. 13, an additional metal layer1300may be provided in conjunction with a PGS layer1302for providing electrical isolation, since the PGS layer1300is being used for the modulating signal1306. This arrangement is suitable for applications where isolation of the oscillator or VCO circuit, represented by transformer1308, is important. As shown inFIG. 14, in another arrangement, an oscillator or VCO circuit, represented by transformer1400, can be modulated by utilizing a deep N-well (DNVV)1402within a substrate1404to control the oscillator or VCO. In this embodiment, a modulation signal1406is directly provided to the deep N-well1402, which operates as a P-N junction. The modulation signal1406modulates the P-N junction capacitance and resistance, which in turn modulates the substrate1404capacitance and resistance and thus the transformer1400to substrate1404capacitance and resistance. The P-N junction is formed between N-well1402and the P-type substrate1404, with the P-type substrate providing grounding.

FIG. 15shows a flowchart1500illustrating a method of frequency modulating a CMOS power oscillator. At step1502, a transformer-based feedback CMOS power oscillator circuit formed on a chip-substrate is provided, the oscillator circuit including a transformer coupled to a transistor. At step1504, the capacitance of the transformer to the chip-substrate is modulated for frequency modulating an output of the power oscillator.

The combination of a CMOS oscillator or VCO with a method of modulating the oscillator or VCO in the described arrangements can result in a device that is suitable for small applications due to fewer components being used compared to existing devices, with high power, high efficiency and low phase noise. The overall size is reduced due to utilising the CMOS-based combination of a power amplifier, oscillator and modulator. The device is cost effective and can be used in e.g. simple transceiver applications as well as remote controls and Bluetooth applications.