WO2011067776A1

WO2011067776A1 - Circuit for compensating gain variation over operating frequency and/or temperature range

Info

Publication number: WO2011067776A1
Application number: PCT/IN2010/000032
Authority: WO
Inventors: Chandra Bera Subhash; Kumar Jain Virendra; Singh Rajvir; Kumar Garg Vinesh
Original assignee: Indian Space Research Organisation
Priority date: 2009-12-04
Filing date: 2010-01-19
Publication date: 2011-06-09

Abstract

A circuit for compensating gain variation of the RF/microwave system over its operating frequency and/or temperature range comprises a ground line (13), and a main transmission line (12) that is configured with a set of ports at both ends for connecting an input port (10) and an output port (11). Shunt networks (51-54) are connected between the main transmission line and the ground line. Each shunt network is formed by an auxiliary transmission line (14, 24, 34, 44), which is serially connected in between a set of impedances (15, 16, 25, 26, 35, 36, 45, 46). At least one of the impedances is configured as variable impedance in such a way that it provides adjustable impedance for adjusting variable attenuation slope over the operating frequency range depending on voltage or current supplied to the circuit and also for adjusting variable attenuation slope over the operating temperature range depending on temperature dependent voltage or current supplied to the circuit. Such circuit easily optimizes and compensates the gain variation of any RF or microwave systems and is simple in construction.

Description

CIRCUIT FOR COMPENSATING GAIN VARIATION OVER OPERATING FREQUENCY AND/OR TEMPERATURE RANGE

FIELD OF THE INVENTION

The present invention relates to the fields of adjusting an operating frequency and/or temperature range of an radio frequency (RF)/microwave system. The present invention specifically relates to a circuit for compensating gain variation of the RF/microwave system over its operating frequency and/or temperature range.

BACKGROUND OF THE INVENTION

High-speed communication links increase the demand for need of wider bandwidths. In order to meet this demand, monolithic microwave-integrated- circuits (MMICs) are designed to operate over multiple decades of bandwidth. Even though it is small in size and low in cost, one potential disadvantage is the gain slope over the operating frequency range. In general, characteristic is the decreasing gain with increasing frequency. However, gain increases with the increase of frequency near its cutoff frequency for a band limited RF circuit. The line loss increases with frequency in the RF/microwave system such as FET based receiver, channel amplifier, power amplifier and other circuits, and even simple RF cable assembly.

Further, such RF/microwave circuits, particularly active RF/microwave circuits can also have positive and negative gain slope over its operating temperature range. For example, gain of a MESFET and HEMT based RF/microwave amplifiers decreases with the increase of temperature, whereas gain of a BJT based amplifier increases with the increase of temperature. Such variations in frequency and temperature can affect components of the RF/microwave systems, which cause differences in signal strength at different temperatures and frequencies. Thus, equalizers are used for flattening the amplitude-frequency characteristics to compensate the gain variation of the RF/microwave system/subsystem over its operating frequency and temperature range, and it is also necessary to make the equalizer controllable. Such equalizer generates opposite slope with respect to the slopes of the RF/microwave system/subsystem over its operating frequency and temperature range and compensates the slopes by cascading the equalizer with the RF/microwave system/subsystem.

US7289007 describes a passive equalizer for low RF frequency applications. The RF circuitry consists of variable capacitor, adjustable resistors and a thermistor to vary the tilt over the frequency and temperature. Similarly, US7205863 describes a passive equalizer and a thermistor is used at the RF circuitry to achieve variable tilt over the frequency and temperature. Moreover, US 6646519 and US 6107896 also describe a passive equalizer only for frequency variable compensation. Such passive equalizer includes an L type attenuator circuit that is provided with capacitors, inductors and resistors. Such passive equalizers, including those noted above, have a frequency response tilt that is generally directly proportional to frequency. Thus, the conventional equalizers are unable to properly compensate for the frequency response tilt arising from the passive components. Further, US 5274339 describes an active equalizer with separate circuitry for temperature compensation and frequency compensation. This active equalizer is configured with couplers, varactor diodes, inductors and resistors. Such conventional equalizer compensates for temperature changes, but it is usually complicated in design.

With respect to the conventional circuits, it is more expensive and complex to design the components of such RF/microwave system/subsystem in an effort to stabilize the gain variations over various temperature and frequency ranges. Moreover, the reliability of such equalizer circuits is questionable because they are subject to power failure or other failures due to the nature of the complicated design. Therefore, it is desirable to provide a circuit for compensating gain variation of a RF/microwave system over its operating frequency and/or temperature range, which is capable of overcoming these aforementioned drawbacks.

OBJECT OF THE INVENTION

An object of the present invention is to provide a circuit for compensating gain variation of an RF/microwave system over its operating frequency and/or temperature range, which is simple in construction and easy to optimize gain variation of any RF/microwave systems.

Another object of the present invention is to provide a circuit for compensating gain variation of an RF/microwave system over its operating frequency range as well as over its operating temperature range simultaneously.

Yet another object of the present invention is to provide a circuit for compensating gain variation of an RF/microwave system over its operating frequency range only.

Yet another object of the present invention is to provide a circuit for compensating gain variation of an RF/microwave system over its operating temperature range only.

Yet another object of the present invention is to provide a circuit for compensating gain variation of an RF/microwave system, which ensures that the amplitude tilt over the frequency range is adjustable over the operating temperature range.

Yet another object of the present invention is to provide a circuit for compensating gain variation of an RF/microwave system, which ensures that the amplitude tilt over the temperature range is adjustable over the operating frequency range.

Yet another object of the present invention is to provide a circuit for compensating gain variation of an RF/microwave system, which achieves positive as well as negative amplitude tilt over the operating frequency range.

Yet another object of the present invention is to provide a circuit for compensating gain variation of an RF/microwave system, which achieves positive as well as negative amplitude tilt over the operating temperature range.

Yet another object of the present invention is to provide a circuit for compensating gain variation of an RF/microwave system, which achieves adjustability by varying only DC voltage or current to the active components without physical adjustment of RF components.

SUMMARY OF THE INVENTION

According to one aspect, the present invention, which achieves the objectives, relates to a circuit for compensating gain variation of the RF/microwave system over its operating frequency and/or temperature range comprising a ground line, and a main transmission line that is configured with a set of ports at both ends for connecting an input port and an output port. Shunt networks are connected between the main transmission line and the ground line. Each shunt network is formed by an auxiliary transmission line, which is serially connected in between a set of impedances. At least one of the impedances is configured as variable impedance in such a way that it provides adjustable impedance depending on voltage/current supplied to the circuit for adjusting variable attenuation slope over the operating frequency range. The variable impedance is also configured in such a way that it provides adjustable impedance depending on temperature dependent voltage/current supplied to the circuit for adjusting variable attenuation slope over the operating temperature range. Such circuit easily optimizes and compensates the gain variation of any RF/microwave systems and is simple in construction.

Furthermore, the variable attenuation slope comprises positive attenuation slope and negative attenuation slope. The shunt networks are connected with separation of a quarter wavelength transmission line. Each shunt network is connected with the ground line through capacitors in series. The capacitors are connected between the shunt networks and the ground line for DC voltage/current blocking and for providing RF ground to the shunt networks. The auxiliary transmission lines of each shunt network comprise a length equal to one-eighth wavelength of the main transmission line. The auxiliary transmission lines of each shunt network comprise a length equal to odd multiple of one-eighth wavelength of the main transmission line. The auxiliary transmission lines of each shunt network are configured to adjust the attenuation slope over the operating frequency without affecting insertion loss at a center frequency of operation.

Moreover, the variable impedance is realized by an active device whose impedance is changed by bias voltage/current. The active device comprises a forward biased p-i-n diode, Schottky barrier diode and varactor diode. The active devices are biased by temperature dependent voltage/current generated by a control circuit. The control circuit is connected to the active device for feeding and supplying DC bias voltage/current to the active device through RF chokes or through high impedance RF short-circuited quarter wavelength transmission line. The control circuit comprises a digital control circuit for stepwise variation of the attenuation slope over the operating frequency/temperature range. The circuit is applied for gain slope equalization of a radio frequency (RF)/microwave subsystem/system. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be discussed in greater detail with reference to the accompanying Figures.

FIG. 1 shows a schematic diagram of an equalizer circuit for compensating gain variation of an RF/microwave system over its operating frequency and/or temperature range, in accordance with an exemplary embodiment of the present invention;

FIG. 2 illustrates an implementation diagram of the equalizer circuit for compensating gain variation of the RF/microwave system over its operating frequency and/or temperature range, in accordance with an exemplary embodiment of the present invention;

FIG. 3 illustrates a typical amplitude (attenuation) characteristic of the equalizer circuit over its operating frequency range, in accordance with an exemplary embodiment of the present invention;

FIG. 4 illustrates a typical amplitude (attenuation) characteristic of the equalizer circuit over its operating temperature range, in accordance with an exemplary embodiment of the present invention; and

FIG. 5 illustrates a typical amplitude (attenuation) characteristic of the equalizer circuit over its operating frequency at three different operating temperatures, in accordance with an exemplary embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 , a schematic diagram of an equalizer circuit for compensating gain variation of an RF/microwave system over its operating frequency and/or temperature range is illustrated, in accordance with an exemplary embodiment of the present invention. The circuit is a variable tilt active equalizer that is configured to achieve adjustable amount of amplitude tilt by changing voltage/current of the equalizer over its operating frequency and temperature range to compensate any amount of amplitude variation of the RF/Microwave systems/subsystems/circuits. Hereafter, the equalizer circuit is being referred as variable tilt active equalizer only for the purpose of explanation.

The variable tilt active equalizer comprises a RF input port (10) and a RF output port (11) connected at the ends of a transmission line (12). The equalizer also includes four shunt networks (51 , 52, 53, 54) connected in between the transmission line (12) and a RF ground line (13). Each shunt network comprises two impedances and a transmission line connected in series. For example, the shunt network (51) is configured with two impedances (15, 16) and a transmission line (14) connected in series, the shunt network (52) is configured with two impedances (25, 26) and a transmission line (24) connected in series, the shunt network (53) is configured with two impedances (35, 36) and a transmission line (34) connected in series and the shunt network (54) is configured with two impedances (45, 46) and a transmission line (44) connected in series. The length of the transmission lines (14, 24, 34, 44) can be equal/more/less than a one eighth wavelength transmission line. However, the length of the transmission lines (14, 24, 34, 44) is preferably equal to one eighth wavelength transmission line. At least one of these impedances should be voltage/current variable impedance to provide variable attenuation slope over the operating frequency and/or temperature range of the RF/microwave system. By increasing the number of shunt networks (51 , 52, 53, 54), the available range of amplitude slope over the operating frequency and/or temperature of the RF/microwave system can be increased with good input and output impedance matching.

Referring to FIG. 2, an implementation diagram of the equalizer circuit for compensating gain variation of the RF/microwave system over its operating frequency and/or temperature range is illustrated, in accordance with an exemplary embodiment of the present invention. The RF input port (10) and the RF output port (11) are connected at the ends of the transmission line (12). The shunt networks (51 , 52, 53, 54) are connected in between the transmission line

(12) and the RF ground line (13) through series capacitors (17, 27, 37, 47), respectively. Each shunt network is configured with two p-i-n diodes and a transmission line connected in series, where the diodes act as impedance. For example, the shunt network (51) is configured with two diodes (15, 16) and a transmission line (14) connected in series, the shunt network (52) is configured with two diodes (25, 26) and a transmission line (24) connected in series, the shunt network (53) is configured with two diodes (35, 36) and a transmission line (34) connected in series and the shunt network (54) is configured with two diodes (45, 46) and a transmission line (44) connected in series.

Furthermore, the equalizer comprises a control circuit (22) is connected via a RF choke (21) for supplying bias currents ( , l₂) to all the diodes (15, 16, 25, 26, 35, 36, 45, 46) through bias points (19, 20). RF chokes (18, 28, 38, 48) are connected with the transmission lines (14, 24, 34, 44) to feed DC voltage/currents to the diodes (15, 16, 25, 26, 35, 36, 45, 46) without affecting RF performance of the equalizer. These RF chokes exhibit a high value inductance. RF short-circuited high impedance quarter wavelength transmission line may also be used to realize the RF choke (21). The capacitors (17, 27, 37, 47) are connected between the shunt networks (51 , 52, 53, 54) and the RF ground line

(13) for DC voltage/current blocking and to provide RF ground to the shunt networks (51, 52, 53, 54). Moreover, the length of the transmission lines (14, 24, 34, 44) is preferably odd multiple of one-eighth wavelength (λ/8, 3λ/8,.....) of transmission line. By varying/adjusting bias currents ( , ) impedance of the diodes can be varied/ adjusted. Variable impedances of the diodes provide variable/adjustable amplitude slope over the operating frequency range. Controlling bias currents ( , ) with the temperature, amplitude slope can be achieved over the operating temperature range. By increasing the number of shunt networks (51 , 52, 53, 54), the available range of amplitude slope over the operating frequency and/or temperature can be increased with good input and output impedance matching. The control circuit (22) provides temperature-controlled voltage/current to the diodes to provide variable amplitude slope over the operating frequency and/or temperature of the RF/microwave system.

Referring to FIG. 3, a typical amplitude (attenuation) characteristic of the equalizer circuit over its operating frequency range is illustrated, in accordance with an exemplary embodiment of the present invention. The attenuation characteristic of the equalizer over its operating frequency range is mapped at a particular temperature of Tc- As shown in FIG. 3, the attenuation of the equalizer changes with the operating frequency range of the RF/microwave system from FL (say 3 GHz) to FH (say 5 GHz). It also shows that depending upon the combination of diode currents and , the attenuation may increase or decrease with the increase of operating frequency. Further, it shows that slope of the attenuation over its operating frequency can be adjusted by adjusting the currents of the diodes.

Referring to FIG. 4, a typical amplitude (attenuation) characteristic of the equalizer circuit over its operating temperature range is illustrated, in accordance with an exemplary embodiment of the present invention. The amplitude, i.e. attenuation, characteristic of the equalizer over its operating temperature range is mapped at a particular operating frequency of Fc. It shows that the attenuation of the equalizer changes over its operating temperature range from TL (say -20°C) to TH (say +70°C). It also shows that depending upon the combination of temperature controlled diode currents and b, the attenuation may increase or decrease with the increase of operating temperature. Further, it shows that slope of the attenuation over its operating temperature can be adjusted by adjusting the currents of the diodes.

Referring to FIG. 5, a typical amplitude (attenuation) characteristic of the equalizer circuit over its operating frequency at three different operating temperatures is illustrated, in accordance with an exemplary embodiment of the present invention. The attenuation characteristic of the equalizer over its operating frequency range is mapped at three different operating temperatures. It shows the attenuation characteristic for a particular setting of diode currents ( , ) at temperature T_c. Such equalizer can be applied for gain slope equalization of the RF/microwave circuits/systems by cascading the equalizer circuit with any wideband RF/microwave circuits/systems to improve gain-versus-frequency response over a wide bandwidth. The equalizer can be applied for gain slope equalization of MMIC amplifiers that are designed to operate over multiple decades of bandwidth. While they are small in size and low in cost, one potential disadvantage is the gain slope over the frequency of operation. Thus, the equalizer circuit is added to the MMIC amplifiers, which enables flatter gain- versus-frequency response over a wide bandwidth.

Moreover, the equalizer can be added with MMIC attenuators and also p-i- n diode based discrete attenuators for gain slope equalization of attenuators, which exhibits attenuation slopes over its frequency ranges and enables flatter gain-versus-frequency response over its wide operating frequency range. Further, the equalizer circuit is added with any RF/microwave subsystems such as SSPAs, Receivers, channel amplifiers, for gain slope equalization even in a full transponder chain, which enables to achieve flatter gain-versus-frequency response over a wide bandwidth. The equalizer exhibits the characteristic of slope adjustment by adjusting bias voltage/current for adjustable gain slope equalization of the RF/microwave circuits/systems. Thus, the equalizer is capable to equalize various amounts of slope just by adjusting the supply voltage/current. The equalizer circuit can be added to a step attenuator circuit with poor band attenuation slope, which provides broadband step attenuation. The command provided to the step attenuator can be linked with the equalizer to provide different required slopes at different attenuation values in order to achieve broadband high attenuation range step attenuator.

In case of broadband linearizer, the distortion generator circuits in the linearizer circuit exhibit a problem with narrow frequency band performance. Therefore, gain and phase non-linearity is different over its operating frequency range. Moreover, gain flatness at small signal and large signal operating condition is different over its operating frequency range. Hence, two numbers of equalizers with opposite frequency slope characteristic are utilized to achieve broad frequency band performance of the linearizer circuit. The equalizer circuit exhibits another characteristic of adjustment of insertion loss without affecting the frequency slope of the loss characteristic. Therefore, in addition to the frequency compensation, it is capable for temperature compensation of any microwave circuit/system.

It is evident to those skilled in the art that although the invention herein is described in terms of specific embodiments thereof, there exist numerous alternatives, modifications and variations of the invention. Hence all variations, modifications and alternatives that falls within the broad scope of the appended claims comes under the gamut of the invention.

Claims

WE CLAIM:

1. A circuit for compensating gain variation over operating frequency range and/or operating temperature range, comprising:

a ground line;

a main transmission line configured with a plurality of ports at both ends for connecting an input port and an output port; and

one or more shunt networks connected between said main transmission line and said ground line, each shunt network is formed by an auxiliary transmission line that is serially connected in between a plurality of impedances, wherein at least one of said plurality of impedances is configured as variable impedance in such a way that it provides adjustable impedance depending on voltage or current supplied to the circuit for adjusting variable attenuation slope over the operating frequency and/or temperature range.

2. The circuit as claimed in claim 1 , wherein the variable attenuation slope comprises positive attenuation slope and negative attenuation slope over the operating frequency and/or temperature range.

3. The circuit as claimed in claim 1 , wherein said shunt networks are connected with separation of a quarter wavelength transmission line.

4. The circuit as claimed in claim 1 , wherein each shunt network is connected with said ground line through at least one capacitor in series.

5. The circuit as claimed in claim 4, wherein said capacitor is connected between said shunt networks and said ground line for DC voltage or current blocking and for providing RF ground to said shunt networks.

6. The circuit as claimed in claim 1 , wherein said auxiliary transmission lines of each shunt network comprise a length equal to one-eighth wavelength of said main transmission line.

7. The circuit as claimed in claim 1 , wherein said auxiliary transmission lines of each shunt network comprise a length equal to odd multiple of one-eighth wavelength of said main transmission line.

8. The circuit as claimed in claim 1 , wherein said auxiliary transmission lines of each shunt network are configured to adjust the attenuation slope over the operating frequency without affecting insertion loss at a center frequency of operation.

9. The circuit as claimed in claim 1 , wherein the variable impedance is realized by an active device whose impedance is changed by bias voltage or current.

10. The circuit as claimed in claim 9, wherein said active device comprises a forward biased p-i-n diode, Schottky barrier diode and varactor diode.

11. The circuit as claimed in claim 9, wherein said active device is biased by temperature dependent voltage or current generated by a control circuit.

12. The circuit as claimed in claim 11 , wherein said control circuit is connected to said active device through a plurality of RF chokes or through a high impedance RF short-circuited quarter wavelength transmission line for supplying bias voltage or current to said active device.

13. The circuit as claimed in claim 11 , wherein said control circuit comprises a digital control circuit for stepwise variation of the attenuation slope over the operating frequency or temperature range.

14. The circuit as claimed in claim 1, wherein the circuit is applied for gain slope equalization of a radio frequency (RF) or microwave subsystem or system.