VCO with power supply rejection enhancement circuit

An oscillating circuit having a noise reduction circuit is disclosed. The noise reduction circuit is coupled to the current source for the oscillating circuit. The noise reduction circuit reduces a bias noise component from a bias current, and a supply noise component from a supply current. The noise reduction circuit is coupled to the current source at a gate and a supply for the current source. The noise reduction circuit includes a filter coupled to the gate of the current source that reduces the bias noise component. The noise reduction circuit also includes a degeneration circuit coupled to the supply of the current source that reduces the supply noise component. The current source generates an input signal to control the oscillating circuit with reduced noise.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to controlled oscillators, and more particularly, the present invention relates to a controlled oscillator that provides an output frequency controlling signal subject to a noise reduction circuit.

2. Description of the Related Art

The use of wireless communication applications for in-home, in-building networks and direct communications is increasing. These applications modulate data onto at least one carrier and transmit data as a modulated signal via a transmitter. A receiver receives the frequency modulated signal, and demodulates it to recapture the data.

In the various stages of transmitting and receiving signals, local oscillators generate signals that result in an output frequency. Sometimes the desired frequency may vary. Variable frequency oscillators are tunable oscillators that receive a fixed frequency that is output over a selectable wide range of frequencies. To oscillate, a resultant gain should be high, any feedback should be positive, and the coupling from the output to input should be effective. The oscillation frequency is controlled via the frequency tuned circuits. Oscillation should take place at a predictable and stable frequency, without the introduction of excessive noise or degradation of any signals.

The frequency for a variable frequency oscillator can be adjusted via a varactor. A varactor may include a semiconductor diode that works as a variable capacitor when it is reverse-biased. The capacitance of the varactor depends on the reverse-bias voltage. The greater the voltage, the lower the value of the capacitance. The varactor is placed in a parallel with the tuning capacitor, and is isolated for direct current by blocking capacitors. The resulting oscillator may be referred to as a voltage controlled oscillator. Frequency control using the varactor is preferable to the use of mechanically variable capacitors or inductors. Varactors tend to be less bulky and lower in cost.

A voltage controlled oscillator (VCO) circuit may operate at high frequencies, such as in the gigahertz range. Noise within circuits operating at high frequency can impede performance and degrade the control and output signals. For example, noise in the input signal may result in large variations of the resulting output signal. This action, in turn, degrades performance of any circuit, detector, carrier, transmitter device and the like using the VCO circuit. Noise within VCO circuits can come from many sources. Input noise may arrive at the VCO from a coupled power supply or other components, either in the current or voltage. In a fully integrated chip, the power supply, or the chip itself can be noisy.

Noise also arrives at the VCO circuit from the gate, or reference circuit. This noise may occur when the current source is also connected to a current mirror, or bandgap reference. For example, the current mirror is in parallel with the VCO circuit. The current mirror draws current from the current supply coupled to the VCO circuit. This cross connection produces noise that is then input into a gate coupled to the VCO circuit.

Thus, noise is a critical factor in VCO circuits, particularly those that operate at high frequencies. The noise may be amplified to affect output signals correlating to the input signals driving the VCO circuit. Further, supply and bias noise is introduced to the input signals to impede the input signals.

SUMMARY OF THE INVENTION

According to a preferred embodiment of the invention, a noise reduction circuit coupled to an oscillating circuit is disclosed. The noise reduction circuit includes a filter coupled to a gate of a current source for an oscillating circuit to filter a bias noise component into the gate. The noise reduction circuit also includes a degeneration circuit coupled a supply for the current source. The degradation circuit reduces a gain within the current source.

A system for reducing noise in an oscillating circuit also is disclosed. The system includes a filtering device having a first resistance and a capacitance to filter a bias current and coupled to a gate of a current source. The system also includes a degeneration device having a second resistance to reduce a noise component in a supply current at the current source.

A method for reducing noise in an oscillating circuit also is disclosed. The method includes filtering a bias noise component from a bias current flowing into a gate of a current source for an oscillating circuit. The method also includes reducing a supply noise component from a supply current flowing into a supply of the current source.

A method for reducing noise components also is disclosed. The method includes reducing a bias noise component by passing a bias current through a noise reduction circuit coupled to a gate of a current source to an oscillating circuit. The method also includes reducing a supply noise component by passing a supply current through the noise reduction circuit coupled to a supply of the current source.

A circuit also is disclosed. The circuit includes an oscillating circuit to generate an output signal. The circuit also includes a current source to control the oscillating circuit. The current source receives a signal derived from a reference signal to generate the output signal. The circuit also includes a noise reduction circuit coupled to a gate and a supply of the current source to reduce a noise component within the signal.

It is to be understood that both the foregoing general description and the following detailed description of the preferred embodiments are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the disclosed embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

FIG. 1depicts a block diagram of a phase locked loop (PLL)100having noise reduction circuit110according to the preferred embodiments. PLL100may be a circuit configured to provide an output signal126. PLL100may be used in a variety of communication devices using one or more local oscillators. PLL100may be implemented in a conventional PLL or produce a local oscillation from a reference frequency.

In one example, PLL100includes crystal oscillator102that generates reference frequency signal120. Reference frequency signal120is input into phase detector104, along with feedback signal128. Phase detector104may detect the difference between reference frequency signal120and feedback signal128. A differential signal130is generated by phase detector104to indicate the difference between reference frequency signal120and feedback signal128. Charge pump106can convert differential signal130into either a charged-up signal or a charged-down signal. For example, charge pump106may add an offset current to charge-up and output modified signal132. Filter108filters modified signal132to produce a filtered, or input, signal124. Filter108can be a low pass filter. Controlled oscillator112may receive input signal124.

Bias signal184may be generated by bias reference180. Power supply182may generate power supply signal186. Both bias signal184and supply signal186can include noise components. For example, if bias signal184and supply signal186include currents flowing to noise reduction circuit110, then the current within each signal includes noise that should be reduced or eliminated prior to being inputted to controlled oscillator112. Noise reduction circuit110reduces the noise within signals184and186to input to controlled oscillator112. Input signal124may control controlled oscillator112to generate output signal126having the desired frequency.

PLL100also includes feedback path114that carries feedback signal128. In this example, feedback path114may be used to adjust the frequency of output signal126in accordance with reference frequency signal120. Feedback path114may include a divider and associated components that receive output signal126and divide output signal126by an integer to generate feedback signal128. Preferably, PLL100may adjust the frequency of output signal126until the desired frequency is achieved.

Controlled oscillator112can be a tunable oscillator that can adjust output signal126accordingly. Large noise components within input signal124can cause controlled oscillator112to repeatedly output at undesired frequencies. PLL100may have to adjust repeatedly until the desired frequency is achieved. Further, input signal124may be degraded by the amplification, or gain, provided to noise components in output signal126. Noise reduction circuit110seeks to reduce noise components, therefore, preventing signal degradation and may decrease the gain of noise components input to controlled oscillator112. The noise reduction circuit may include two components, a low pass filter, and a degradation circuit. These components are disclosed in greater detail below.

PLL100is disclosed according to one exemplary embodiment. PLL100also may include additional components or circuits. Further, PLL100may not include all the components disclosed with reference toFIG. 1. For example, PLL100may include only crystal oscillator102, noise reduction circuit110and controlled oscillator112.

FIG. 2depicts a circuit200having a voltage controlled oscillating circuit, or oscillator,202according to the preferred embodiments. Components of oscillator200may correlate to components of PLL100, but the subsequent disclosure is not meant to limit the features disclosed with reference toFIG. 1.

Oscillator202may be any conventional voltage controlled oscillating circuit that receives an input to oscillate accordingly. Oscillator202, can be, for example, a voltage controlled oscillator. Alternatively, oscillator202may be any controlled oscillating circuit, such as a current controlled circuit. Output signal230of oscillator202may be varied by a received input signal. Output signal230may have an output frequency that is a function of the controlling signal of oscillator202. Oscillator202may operate at high frequencies. Preferably, oscillator202may operate in the gigahertz (GHz) range. Oscillator202can be coupled to supply204.

Supply204supplies current to oscillator202. Alternatively, supply204may include components that result in supplying a current to oscillator202. Coupled between supply204and oscillator202may be noise reduction circuit206. Noise reduction circuit206seeks to reduce, or eliminate, the amount of noise input to oscillator202. Noise entering oscillator202may degrade the generation of output signal130. Alternatively, other circuit elements may be coupled between supply204and oscillator202besides noise reduction circuit206. Preferably, however, noise reduction circuit206can be coupled to the input of oscillator202.

Supply204also may be connected to other circuit elements, such as element228. Element228may be a gate or diode coupled to a bandgap reference236. Bandgap reference circuit236may draw current from supply204, in addition to oscillator202. Current216may be referred to as a bias current. Element228and bandgap reference circuit236may act as a current mirror used in an integrated circuit for biasing. Bias current216can be generated by drawing some of the supply current from supply204to element228to oscillator202.

Bias current216may include a noise component that also flows to oscillator202. Noise reduction circuit206may reduce the bias noise component in bias current216before it enters oscillator202.

Supply204also generates supply current226to be input to oscillator202. Noise reduction circuit206also may reduce the noise component of supply current226. In addition, noise reduction circuit206may provide an increased impedance into the input of oscillator202.

Current source224can be coupled to the input of oscillator202. Current source224also is coupled to noise reduction circuit206. Current source224also may be known as a varactor. Preferably, current source224acts as a common source amplifier. Further, current source.224may comprise a first semiconductor material. For example, current source224may comprise a p-channel metal oxide semiconductor (PMOS) diode having its drain coupled to oscillator202and its source coupled to supply204via noise reduction circuit206. The gate of current source224is coupled to noise reduction circuit206and subject to bias current216generated by bandgap reference circuit236and element228. Current source224may have a gain that is applicable to a noise component within supply current226. Noise reduction circuit206seeks to reduce the gain of current supply224and reduce the bias current216flowing into its gate.

Noise reduction circuit206includes low pass filter210and degeneration circuit220. Low pass filter210filters the noise in bias current216. Low pass filter210may be any conventional low pass filter. Preferably, low pass filter210includes resistance212and capacitance214. Resistance212may include a resistor. Resistance212may include a resistor having a value of approximately 200 kilohms. Capacitance214may include a capacitor. Capacitance214may include a capacitor having, for example, a value of 1 picofarad.

Low pass filter210may allow one or more frequency bands to be transmitted while rejecting signals outside of these bands. For example, in low pass filter210, a transmitted band may extend from zero to some maximum frequency. Low pass filter210can be a passive filter. Low pass filter210may reject, or filter, noise at frequencies outside a range of interest for oscillator202. The resistance and capacitance values of low pass filter210may be varied in accordance with the desired frequency range.

Thus, noise reduction circuit206may be configured to have low pass filter210between a current mirror such as element228and bandgap reference circuit236, and a current source224coupled to an input of oscillator202. Bias current216can be filtered accordingly, and the bias noise component is reduced before reaching current supply224. This feature, in turn, may reduce the noise component of input signal208.

Noise reduction circuit206also includes degeneration circuit220. Degeneration circuit220is coupled to the input, or source, of current source224. Current source224, in turn, provides input signal224to VCO202.

Current source224may be a varactor to tune VCO202. A varactor diode can use a p-n junction and a reverse bias to control VCO202. A varactor may have a structure to allow the capacitance of a diode within current source224to vary with a reverse voltage. A voltage controlled capacitance may be useful for tuning applications, such as tuning VCO202.

Supply current226may include a noise component that is reduced by noise reduction circuit206. Specifically, degeneration circuit220may reduce the noise component in supply current226. Degeneration circuit220may reduce a gain from current source224to input signal208. In this instance, a gain would apply to a noise component within input current208versus a noise component of supply current226. Degeneration circuit220also may provide an output impedance to prevent noise from creeping out of oscillator202back to current supply224.

As disclosed above, current source224may comprise a PMOS diode. Current source224also may be any semiconductor-type, such as an n-channel metal oxide semiconductor.

Degeneration circuit220includes resistance222, and is coupled to the source of current source224. Preferably, resistance222may be coupled in series with supply current224.

For example, resistance222may have a value of 40 ohms. Using resistance222, the entire resistance of degeneration circuit220may be increased, which in turn may directly reduce noise from supply204. Output impedance also may be increased by a factor of resistance222.

When a resistor is added to the source of current source224, the effective gain of current source224may be reduced. Thus, degeneration circuit220also may reduce the gain of the noise component in supply current226and degeneration circuit220. Current source224may act as a common source amplifier may and reduce the gain of the noise component within current source224.

Degeneration circuit220can also provide an increased output impedance to the input of oscillator202. Degeneration circuit220may increase the output resistance (Rout) of current source224. An increased Routmay reduce the coupling from supply204to current source224. Reduction of coupling may be important if supply204is noisy. The entire resistance may be increased by a factor of the value of resistance222of degeneration circuit220.

Thus, noise reduction circuit206is coupled between supply204and current supply224for oscillator202. Noise reduction circuit206may include low pass filter210coupled to the gate of current supply224, and degeneration circuit220coupled to the source of current supply224. Low pass filter210may reduce the noise component of bias current216flowing into current supply224. Degeneration circuit220may reduce the noise component of supply current226from supply204.

FIG. 3depicts a flowchart for reducing noise using a noise reduction circuit coupled to an oscillating circuit according to the preferred embodiments. For example only, reference may be made to features or elements disclosed inFIGS. 1 and 2in disclosing the preferred embodiments.

Step300executes by generating a bias current, such as bias current216. The generated bias current includes a noise component that is input to a gate of a current source for an oscillating circuit. For example, bias current216flows into the gate of current source224. The bias current may be generated by a current mirror, such as band gap reference circuit236coupled to supply204. Step302executes by filtering the bias current to remove the bias noise component using a low pass filter, such as low pass filter210. The low pass filter includes a capacitance and a resistance that removes the noise that is not within an allowable range. Step304executes by reducing the noise component in the bias current by removing the noise component.

Step306executes by generating a supply current, such as supply current226, from a supply coupled to the oscillating circuit. Preferably, the supply may be coupled to a current source for the oscillating circuit. For example, supply204is coupled to current source224. The supply current may be supplied directly to the current supply through a degeneration circuit, such as degeneration circuit220. Step308executes by reducing the supply noise component within the supply current using the degeneration circuit. The supply noise may be reduced using a resistance in the degeneration circuit. Step310executes by impeding noise from the supply side of the current source for the oscillating circuit. The impedance may prevent noise components from leaking out of the supply for current source. Further, the oscillating circuit may not increase the gain of a noise component from the current source to the input of the oscillating circuit.

Steps300,302and304may be executed simultaneously with step306,308and310. Alternatively, these steps may be executed in any order. Further, the steps may be executed without the execution of the other steps. For example, steps300,302and304may be executed and steps306,308and310may not be executed.

Step312executes by inputting the supply current and the bias current into the current source. The current source, such as current source224, then may supply an input signal to control the oscillating circuit, such as VCO202.

One having ordinary skill in the art will readily understand that the invention as discussed above may be practiced with steps in a different order, and/or with hardware elements in configurations that are different than those disclosed. Therefore, although the invention has been disclosed based upon these preferred embodiments, it would be apparent to those of skill in the art that certain modifications, variations, and alternative constructions are apparent, while remaining within the spirit and scope of the claimed invention. In order to determine the metes and bounds of the invention, therefore, reference should be made to the appended claims and equivalents thereof.