High-Q integrated RF filters

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit of priority from a co-pending U.S Provisional application entitled “HIGH-Q INTEGRATED RF FILTERS” having Ser. No. 60/564,016 and filed on Apr. 21, 2004, the disclosure of which is incorporated herein by reference for all purposes.

FIELD

The present invention relates generally to integrated filters, and more particularly to high-Q integrated filters for operation at radio frequencies (RF) and their associated tuning.

BACKGROUND

Filters find widespread use in radio transceivers. FIG. 1 shows a radio transceiver that employs filters in both receive and transmit channels. These filters limit noise while attenuating potential interfering signals as well as spurious signals. Most communication systems require RF filters with sharp frequency responses that make monolithic integration difficult. As a result, RF filters typically use bulky technologies, such as surface acoustic wave (SAW) or ceramic resonators. It would therefore be desirable to find a way to integrate these RF filters to reduce their size and cost.

SUMMARY

In one or more embodiments, a system for high-Q integrated RF filters is provided. In one embodiment, the system comprises novel LC filters and a Q-enhancement circuit that can be integrated to overcome problems associated with conventional filters. The LC filters provide a sharp frequency notch while the Q-enhancement circuit creates negative resistance to improve the quality factor (Q) of these and other LC resonators. Because the filters and Q-enhancement circuit can be integrated, they are suitable for use in a variety of radio transceiver applications where conventional circuits are too bulky or expensive.

In one embodiment, a filter system is provided that comprises a resonate LC filter, and a Q-enhancement circuit coupled to the resonate LC filter, wherein the Q-enhancement circuit operates to improve a quality factor of the filter system.

In one embodiment, a communication device is provided that includes an amplifier and a filter system. The filter system comprises a resonate LC filter, and a Q-enhancement circuit coupled to the resonate LC filter, wherein the Q-enhancement circuit operates to improve a quality factor of the filter system.

BRIEF DESCRIPTION OF THE DRAWINGS

The forgoing aspects and the attendant advantages of the described embodiments will become more readily apparent by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:

FIG. 1 shows a diagram of a standard radio transceiver;

FIG. 2 shows several circuits used to models losses in resonators;

FIGS. 3a-b show a diagrams of two simple LC resonators;

FIGS. 4a-b shows graphs that illustrate the impedance and amplitude as a function of frequency for the resonators shown in FIGS. 3a-b, respectively;

FIG. 5 shows a detailed diagram of one embodiment of an LC resonator network;

FIG. 6 shows a detailed diagram of one embodiment of an LC resonator network;

FIGS. 7a-b show graphs that illustrate the impedance transfer functions for the filter networks shown in FIGS. 5 and 6;

FIG. 8 shows a detailed diagram of one embodiment of an RF amplifier comprising one embodiment of an LC filter network;

FIG. 9 shows a detailed diagram of one embodiment of an RF amplifier comprising one embodiment of an LC filter network;

FIG. 10 shows a detailed diagram of one embodiment of a Q-enhancement circuit;

FIG. 11 shows detailed diagram of one embodiment of a Q-enhancement circuit coupled to one embodiments of the LC filter network shown in FIG. 5;

FIG. 12 shows detailed diagram of one embodiment of a Q-enhancement circuit coupled to one embodiments of the LC filter network shown in FIG. 6;

FIG. 13 shows one embodiment of a frequency adjustment circuit for use with one or more embodiments of the LC filter networks; and

FIG. 14 shows one embodiment of an RF amplifier, LC filter network, Q-enhancement circuit, and frequency adjustment circuit for use in a radio transceiver.

DETAILED DESCRIPTION

In one or more embodiments, a system for high-Q integrated RF filters is provided. In a radio transceiver, the two candidates for RF filter integration are the transmit band filter (TXF) and the image reject filter (IRF) shown in FIG. 1. The transmit band filter removes spurious signals produced by the upconversion mixers in the radio transmitter and limits noise in the receive band that would otherwise leak through the duplex filter and desensitize the radio receiver. The receive band noise poses a problem in full duplex communication systems, where the transmitter and receiver operate simultaneously.

The location of the image signal in a radio receiver depends on the architecture and frequency plan of the system. A heterodyne radio receiver uses two or more downconverting mixers to translate the RF signal to baseband. As such, the image frequency of the first downconverting mixer is separated from the receive signal by twice the IF frequency. The image frequency problem becomes especially challenging in low-IF receiver architectures. A direct conversion receiver avoids this problem but may be subject to strong leakage from the transmitter in full duplex systems. In this situation, the image reject filter (IRF) acts as either a receive band filter or a transmit band notch filter.

A typical filter is formed using resonators. In the case of electrical filters, these resonators are comprised of inductors and capacitors. Practical values for integrated inductors are a few to several nanohenries, while integrated capacitors are limited to tens of picofarads. These components exhibit losses—characterized by a parameter known as quality factor (Q)—which makes them appear non-ideal.

FIG. 2 shows circuits that model the losses in resonators. The losses can be modeled by the resistances (R_S and R_P) as shown in the models provided in FIG. 2. The quality factor Q is then defined as;

Q = X L R s = X C R s ⁢ ⁢ and ⁢ ⁢ Q = R p X L = R p X C
for series and parallel resistances, respectively. In practice, the quality factor Q for integrated components is usually less than fifty.

Another definition for the quality factor Q indicates the sharpness of the frequency response, with;

Q = ω o 2 ⁢ Δ ⁢ ⁢ ω
where Δω is the one-sided 3 dB bandwidth.

FIGS. 3a-b show simple LC resonators that are series and parallel connected, respectively. The series LC resonator is described by the following equation;
Z_in=s²+ω_o²
where s equals jω and ω_o represents the resonance frequency;

ω o = 1 LC

The impedance of this network dips at the resonance frequency (i.e., 1880 MHz) as shown in the impedance and amplitude graphs shown in FIG. 4a. The quality factor Q of the series LC resonator shown in FIG. 3a depends on the notch impedance, which should be minimized for maximum effect. Unfortunately, this makes the impedance at the offset frequency very low and impractical. In contrast, the parallel LC resonator shown in FIG. 3b obeys the transfer function;
Z_in=(s²+ω_o²)⁻¹
which peaks at the resonance frequency (i.e., 1960 MHz). This response is also shown in the impedance and amplitude graphs provided in FIG. 4b. Here, the quality factor tracks the resonant impedance, which should be maximized. This creates a different problem as the resonance impedance becomes too high for RF circuits.

FIGS. 5 and 6 show embodiments of two LC resonator networks. They combine both parallel and series resonators to provide the notch responses shown in FIGS. 7a and 7b, respectively.

The LC resonator shown in FIG. 5 comprises an inductor (L₁) in parallel with a resistor (R₁) forming a first parallel combination that is coupled between a positive supply (V₊) and an input current (i_in). The resonator also comprises a second parallel combination comprising a capacitor (C₂) and an inductor (L₂) coupled between the input current (i_in) and a capacitor (C₁). The resonator further comprises a capacitor (C₃) coupled to the first and second parallel combinations at an output terminal (V_out).

The network shown in FIG. 5 creates a low-side notch suitable for use as an image reject filter (IRF). At lower frequencies, the admittance of inductor L₂ exceeds that of capacitor C₂ (Y_L2>Y_C2), meaning the current flowing through this inductor sees the parallel combination of capacitors C₁ and C₂. With the admittance of capacitor C₂ much higher than that of capacitor C₁ (Y_C2>Y_C1), most of the current flows to the output, phase-shifted so as to lower the effective impedance. This creates a resonance and notch at;
ω_notch=(√{square root over (L₁(C₁+C₂))})⁻¹

At higher frequencies, Y_C2>Y_L2 and capacitor C₂ appears in series with capacitor C₁. This forms a simple parallel LC resonator with inductor L1 that resonates at;

ω pass = ( L 1 ⁢ C 1 ⁢ C 2 C 1 + C 2 ) - 1 ⁢ • ⁡ ( L 1 ⁢ C 1 ) - 1

In practice, the values of inductor L₁ and capacitor C₂ are much bigger than inductor L₂ and capacitor C₁, respectively. Capacitor C₃ is needed for large values of resistor R₁.

The LC resonator network shown in FIG. 6 comprises a parallel combination that comprises an inductor (L₂) and a capacitor (C₂). The parallel combination is coupled to an inductor (L₁) and an input current (i_in). The inductor (L₁) is further coupled to a positive supply (V₊). A resistor (R₁) is coupled between the positive supply (V₊) and the input current (i_in). A capacitor (C₁) is coupled to the parallel combination and the input current (i_in) at an output terminal (V_out).

In the network shown in FIG. 6, the notch occurs above the center of the passband. This is the location for the transmit band filter (TXF). At lower frequencies, Y_L2>Y_C2 and inductor L₂ appears in series with inductor L₁, creating a parallel resonator with;
ω_pass=(√{square root over ((L₁+L₂)C₁)})⁻¹

At higher frequencies, Y_C2>Y_L2 and inductor L₂ appears in parallel with inductor L₁. Since the admittance of inductor L₂ is much higher than inductor L₁, a low impedance path is formed at;

ω notch = ( L 1 ⁢ L 2 L 1 + L 2 ⁢ C 1 ) - 1 ≈ ( L 1 ⁢ C 1 ) - 1
and a notch is produced. For this filter, the values of inductor L₁ and capacitor C₂ are also much bigger than inductor L₂ and capacitor C₁.

FIGS. 7a-b show graphs that illustrate the impedance transfer functions for the filter networks shown in FIGS. 5 and 6, respectively. The first graphs illustrate the real and imaginary parts of the impedance presented by the notch filters. Notice that both the real and imaginary parts of the impedance approach zero at the notch frequency. The second graphs illustrate the frequency responses of the notch filters.

FIGS. 8-9 show how embodiments of the LC filters shown in FIGS. 5 and 6 readily connect to standard RF amplifiers found in radio transceivers. In fact, part of the LC notch filter actually forms the output load of the amplifiers shown in FIGS. 8-9. Although the amplifiers of FIGS. 8-9 are shown with bipolar transistors, field effect transistors are suitable for use in other embodiments.

The quality factor Q for integrated components is insufficient to realize the high-Q filters needed at the front-end of the radio transceiver. To address these applications, the quality factor Q must be improved, which is possible by introducing negative resistance (to reduce the loss modeled by resistance R_P. In fact, an infinite quality factor Q is developed when the negative resistance exactly cancels R_P.

FIG. 10 shows one embodiment of a novel Q-enhancement circuit that operates to simulate a negative resistance. The Q-enhancement circuit simply senses the input voltage V_in and generates an output current that reacts opposite to any change in V_in. It comprises a translinear loop that operates as follows. The input voltage V_in establishes a current governed by equation;
V_in−I₁R₁−V_be1+V_be2+I₂R₂=V₊

The base-emitter voltages of transistors Q₁ and Q₂ are approximately equal if the input voltage difference (V_in−V₊) is less than the product I_T1R₁. (Note that the linearity of the circuit depends on this product.) This allows the above equation to be rewritten as;
V_in−(I₁−I₂)R=V₊
when R=R₁=R₂. With

I 1 = I T ⁢ ⁢ 1 2 + Δ ⁢ ⁢ I
and

I 2 = I T ⁢ ⁢ 2 2 - Δ ⁢ ⁢ I ,
the input difference current becomes;

Δ ⁢ ⁢ I = V in - V + 2 ⁢ R

Transistors Q₁ through Q₄ form a differential current mirror governed by the following equality;

I 1 I 2 = I 3 I 4

Using I₂=I_T1−I₁ plus I₃=I_T2−I₄ and then simplifying, the resulting equation yields;
I₄=k(I_T2−I₄)
where

k = I T ⁢ ⁢ 1 - I 1 I 1 .
Finally, solving for I₄ provides;
I₄=M(I_T1−I₁)
where M is the ratio of bias currents

I T ⁢ ⁢ 2 I T ⁢ ⁢ 1 .
This means that the output current I_out is a scaled version of current I₂, which varies oppositely to input current I₁ and input voltage V_in. By definition, this provides an adjustable negative resistance.

It's important that the LC filter networks and the Q-enhancement circuit discussed above introduce as little noise and distortion as possible. This aspect is aided by the fact that the negative resistance circuit does not connect directly to the output of the filter.

FIGS. 11 and 12 illustrate embodiments of a Q-enhancement circuit coupled to the filters circuits provided in FIGS. 5 and 6, respectively. Note that the negative resistance realized is not entirely real (resistive) and any imaginary component (reactive) will need to be absorbed by the LC filter network.

Lastly, tuning of the resonant frequency and quality factor for a high-Q filter is especially important. FIG. 13 shows one embodiment of an adjustment circuit that operates to adjust the resonant frequency. The adjustment circuit comprises a variable capacitor (C_2b), or varactor, and capacitor (C_2a) that are in parallel with an inductor (L₂). Note that capacitor C_2a is needed to allow the control voltage (V_c) from being developed across the varactor since the inductor L₂ is a short at dc. The frequency tuning should occur first, followed by any adjustments to the negative resistance circuit used to control the quality factor. It may also be necessary to re-center the resonant frequency after adjusting the quality factor (negative resistance).

FIG. 14 shows one embodiment of an RF amplifier, LC filter, Q-enhancement circuit, and frequency adjustment circuit for use in a radio transceiver.

The present invention includes a novel LC filter network and Q-enhancement circuit used to provide a narrowband notch filter response. The circuits enable monolithic integration and thereby eliminate bulky and expensive SAW and ceramic filters. The embodiments described above are illustrative and are not intended to limit the scope of the invention to the particular embodiments described. It should be noted that embodiments of the system are suitable for use in a variety of communication devices, including but not limited to, mobile telephone, PDAs, notebook computers, pagers, email devices and any other type of device that could benefit by the use of one or more embodiments of the system for high-Q integrated RF filters.

Accordingly, while one or more embodiments of a system for high-Q integrated RF filters have been illustrated and described, it will be appreciated that various changes can be made to the embodiments without departing from their spirit or essential characteristics. Therefore, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.