WO2014086385A1

WO2014086385A1 - An i/q network

Info

Publication number: WO2014086385A1
Application number: PCT/EP2012/074213
Authority: WO
Inventors: Mingquan Bao
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2012-12-03
Filing date: 2012-12-03
Publication date: 2014-06-12

Abstract

An l/Q network (700) comprising an all-pass filter (401) which is adapted to receive differential input signals at a differential input port (120) with first (131) and second (132) terminals and to output differential I and Q voltage signals at differential I and Q output ports with respective first (111, 122) and second (112, 121) terminals. The all-pass filter (401) is adapted to receive the differential input signals as current signals, and in the all-pass filter the differential input port (130) is also arranged to be one of the differential I and Q output ports.

Description

AN l/Q NETWORK

TECHNICAL FIELD

The present invention discloses an improved l/Q network.

BACKGROUND

l/Q networks are used in a large variety of radio and radar applications. The role of an l/Q network is to take an input signal and use it to generate two output signals which are ninety degrees phase shifted with respect to each other, i.e. one of the output signals is an "In phase" signal, and the other output signal is a "Quadrature phase" signal; hence the name l/Q-network.

Known kinds of l/Q- networks include so called 90° hybrids which are based on distributed transmission lines, for example so called coupled line quadrature hybrids and branch-line hybrids.

Other kinds of known l/Q networks include so-called polyphase filters and quadrature all-pass filters, QAFs, which are based on lumped passive components, e.g., resistors, capacitors, and inductors.

Both the polyphase filter and the QAF are driven by differential input voltage signals which have the same amplitude but with a 180-phase difference between them. Consequently, such l/Q networks have two differential output signals (±V_I and ±V_Q) which have the same amplitude but with a 90 degree phase difference between them, so that the phase difference between +V_I and +V_Q is 90 degrees, as is also the case with the phase difference between -V_I and -V_Q. Between +V_I and -V_I there is a 180 phase difference, as is also the case between +V_Q and -V_Q. The 90 hybrids are built using several transmission lines, the lengths of which are at least a quarter of an operational wavelength of the 90° hybrid. The 90° hybrids' physical size makes them impractical in many kinds of implementations, for example in monolithic microwave integrated circuits, MMIC, especially at frequencies below approximately 20GHz. In contrast, the lumped components l/Q networks (i.e. polyphase filters and QAFs) are more compact than 90° hybrids.

Both polyphase filters and QAFs above are driven by voltage signals. In many l/Q networks which are based on polyphase filters and QAFs, a buffer amplifier is used between the input signals and the l/Q network, to perform the functions both of amplification and impedance transformation. The buffer amplifier usually comprises transistors, and resistors and/or inductors at the collector(s), in order to transfer AC current signals into AC voltage signals. However, this transformation reduces the output signals' amplitude, and also cost either DC power if resistors are used, or chip area if inductors are used.

SUMMARY

It is an object of the invention to obviate at least some of the disadvantages mentioned above, and to provide an improved l/Q network. This object is achieved by means of an l/Q network which comprises an all-pass filter that is adapted to receive differential input signals at a differential input port which has first and second terminals, and to output differential I and Q voltage signals at differential I and Q output ports with respective first and second terminals.

The all-pass filter is adapted to receive the differential input signals as current signals, and in the all-pass filter the differential input port is also arranged to be one of the differential I and Q output ports. By means of this design, a number of advantages are gained over previously known l/Q networks, as will become evident from the following detailed description.

In embodiments of the l/Q network, in the all-pass filter there is arranged a passive component of a first kind between the first terminal of the differential I output port and the first terminal of the differential Q output port as well as between the second terminal of the differential I output port and the second terminal of the differential Q output port. There is also arranged a passive component of a second kind between the first terminal of the differential I output port and the second terminal of the differential Q output port as well as between the second terminal of the differential I output port and the first terminal of the differential Q output port. There is also at least one resistance connected between the terminals of the differential I and Q output port which is not arranged to also be the differential input port.

The passive components of the first kind are chosen from one of the component types inductors and capacitors, and the passive components of the second kind are chosen from the other of the component types inductors and capacitors.

In embodiments of the l/Q network, in the all-pass filter, the passive component of the first kind can also be chosen either as a serial combination of an inductor and a capacitor or as a parallel combination of an inductor and a capacitor, and the passive component of the second kind is chosen from the other of said combinations.

In embodiments of the l/Q network, the l/Q network additionally comprises a current source connected to the all-pass filter, arranged to generate the differential input current signals to the all-pass filter by means of an input voltage or current signal.

In embodiments of the l/Q network, the current source comprises emitter coupled first and second bipolar junction transistors, with the collector of the first of the transistors being connected to the second terminal of the differential input port of the all-pass filter, and the collector of the second of said transistors being connected to the first terminal of the differential input port of the all pass filter. The bases of the transistors are arranged to be used as first and second terminals in an input port of the l/Q network, and in the l/Q network the emitter coupling of the first and second transistors is also connected to a means for applying a biasing signal to the transistors. In embodiments of the l/Q network, the means for applying a biasing signal to the transistors comprises a third bipolar junction transistor whose collector is connected to the emitter coupling of the first and second transistors, and whose base is arranged to receive a biasing signal, and the emitter of the third bipolar junction transistor is grounded.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, with reference to the appended drawings, in which

Fig 1 shows a first schematic embodiment of an l/Q network, and

Figs 2-5 show various embodiment of the l/Q network of fig 1 , and

Figs 6 and 7 show an l/Q network with a current source.

DETAILED DESCRIPTION

Embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Like numbers in the drawings refer to like elements throughout. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the invention.

Fig 1 shows a first embodiment of an l/Q network 100. In this embodiment, the l/Q network 100 comprises an all-pass filter ("APF") 101 , which, as shown in fig 1 , is arranged to output so called differential I and Q voltage signals, i.e. the all-pass filter 101 is arranged to output two I voltage signals, which have a phase difference of 180 degrees between them; in fig 1 , one of the two output I voltage signals is denoted -V_I and the other is denoted +V_I. Similarly, the all pass filter 101 is arranged to output two Q voltage signals, which have a phase difference of 180 degrees between them, and in fig 1 , one of the two output Q voltage signals is denoted -V_Q and the other is denoted +V_Q.

The all pass filter 101 has an I output port 1 10 with two terminals, one terminal 1 1 1 for the signal +V_I and one terminal 1 12 for the signal -V_I. In addition, the all pass filter 101 has a Q output port 120 which also has two terminals, one terminal 121 for the signal +V_Q and one terminal 122 for the signal -V_Q. As shown in fig 1 , the all-pass filter 101 is arranged to receive differential input signals at a differential input port 130, which port, since it is differential, exhibits two terminals, 131 , 132. The differential input signals which the all- pass filter is arranged to receive are current signals, denoted as +l_c at the terminal 132 and -l_c at the terminal 131. The differential input port 130 is arranged to also be one of the all pass filter's differential I and Q output ports, 1 10, 120. In fig 1 , it is the differential I port 1 10 which is also used as the differential input port 130, but the differential Q output port 120 could also have been used for this purpose. Fig 2 shows a more detailed block diagram of an embodiment of an all pass filter 201 . In fig 2, components or parts which are shown in fig 1 have retained their reference numbers from fig 1.

In the embodiment shown in fig 2, two passive components of a first kind, (marked as 'A' in fig 2), and two passive components of a second kind, (marked as 'B' in fig 2), are used between the terminals of the I and Q differential output ports, as follows:

Between the first terminal 1 1 1 of the differential I output port and the first terminal 121 of the differential Q output port, there is arranged a passive component 230 of a first kind ("A"), as is also the case between the second terminal 1 12 of the differential I output port and the second terminal 122 of the differential Q output port, where there is also arranged a passive component 220 of the first kind.

The first terminal 1 1 1 of the differential I output port and the second terminal 122 of the differential Q output port are connected by a passive component 240 of a second kind ("B"), as is also the case with the second terminal 1 12 of the differential I output port and the second terminal 122 of the differential Q output port, where there is also arranged a passive component 210 of the second kind.

In addition to the passive components described above, there is also comprised a resistor 250 between the terminals 121 , 122, of the differential Q output port. Naturally, this resistor can be replaced by one or more serially connected resistors with the same total resistance. As shown in fig 2, in the embodiment 201 of an all-pass filter, it is the differential I output port with its terminals 1 1 1 , 1 12 which is also used as the differential input port 130. In embodiments in which the differential Q output port is also used as the differential input port 130, the resistor 250 should instead be arranged between the terminals 1 1 1 , 1 12 of the differential I output port.

As will be shown in more detail below, the passive components of both the first and the second kind can be, for example, capacitors or inductances.

Fig 3 shows a more detailed example of an embodiment 301 of an all pass filter 301 : in this embodiment, the passive components of a first kind are inductors 330, 320, and the passive components of a second kind are capacitors 340, 310. In this embodiment, due to the way the passive components of the first and second kinds are chosen, the I and Q output ports with their respective terminals "switch places" with respect to the differential input port 130 as compared to the embodiments shown in figs 1 and 2. As is shown in fig 4 in another example of an embodiment 401 , the opposite choice of passive components from fig 3 can also be the case, i.e. in this embodiment the passive components of a first kind are capacitors 430, 420, and the passive components of a second kind are inductors 440, 410. In this embodiment, due to the way the passive components of the first and second kinds are chosen, the I and Q output ports with their respective terminals have the places shown in figs 1 and 2 with respect to the differential input port 130.

As is shown in fig 5 in another example of an embodiment 501 of an all pass filter, each of the passive components of the first and second kind can also comprise one or more passive "sub-component", connected either in serial or in parallel. In the embodiment 501 shown in fig. 5, there are two passive components 530 and 520 of the first kind, each of which comprise an inductor L₂ in series with a capacitor C₁ . In addition to the two passive components 530 and 520 of the first kind, the embodiment 501 also comprises two passive components 540 and 510 of the second kind, each of which comprises an inductor L₁ in parallel with a capacitor C₂. Naturally, in each parallel connection there can be comprised additional passive "subcomponents", which is also the case with the serial connections. It should be mentioned that the passive components of the first and second kind can also be chosen in the opposite manner to that shown in fig 5, i.e. the parallel combinations 510, 540 can be chosen as the first component and the serial combinations 510, 540 can be chosen as the second component. If the passive components of the first and second kind are chosen in the opposite manner to that shown in fig 5, the I and Q output ports with their respective terminals will "switch places" with respect to the differential input port 130, as was the case between the embodiments of figs 3 and 4.

Fig 6 shows a further embodiment 600 of the l/Q network. In this embodiment, the l/Q network comprises a current source 601 as well as an all-pass filter 101. As shown in fig 6, the current source 601 is a differential current source, i.e. it has a first 621 and a second 622 current output terminal, which terminals are also labeled as -l_c and +l_c in fig 6. The current source 601 is arranged to create the differential current outputs -l_c and +l_c by means of an input P_IN which the current source 601 is arranged to receive at an input port 610.

In various embodiments, the input PIN to the current source 601 can be in the shape of an input current or an input voltage, and the input port 610 can be arranged to be a differential or "single-ended" input port, meaning that P_IN can be arranged to be a differential or "single-ended" input signal. The first 621 and the second 622 current output terminals of the current source 601 can either, as shown in fig 6, be connected to the "I input" terminals 131 , 132, or to the "Q input" terminals 121 , 122.

Fig 7 shows the l/Q network of fig 6, but shows an example of a more detailed embodiment of the current source 601. The all-pass filter which is used together with the current source 601 can be chosen from any of the embodiments shown in figs 2-5, but as an example, the embodiment 401 of fig 4 is chosen in fig 7, although with a modification which will be pointed out below.

As shown in fig 7, the current source 601 comprises first 720 and second 710 emitter coupled bipolar junction transistors. The collector of the first transistor 720 is connected to the first input terminal 132 of the differential input port of the all-pass filter 401 , and the collector of the second 710 transistor is connected to the second terminal 131 of the differential input port of the all-pass filter 401 . The base of the first transistor 710 is arranged to be used as a first terminal 706 in an input port 210 of the l/Q network 700, and the base of the second transistor 720 is arranged to be used as a second terminal 707 in an input port 210 of the l/Q network 700. As shown in fig 7, the emitter coupling of the first and second transistors is connected to a means 730 for applying a biasing signal, here in the form of a current, to the transistors 710 and 720 in 601 , said means in this case comprising a third bipolar junction transistor 730, whose collector is connected to the emitter coupling of the first and second transistors, and whose base is arranged to receive a biasing signal 708. The emitter of the third bipolar junction transistor 730 is grounded.

Turning now to the embodiment of an all-pass filter 401 which is comprised in the l/Q network 700, we see that there are two resistors 750, 760, connected in series between the "Q output" terminals 121 , 122. In a previous embodiment, there was one resistor connected between these two terminals. Naturally, the one resistor shown in other embodiments can be replaced by a number of other resistors, connected in series or in parallel, as long as a desired total resistance is obtained.

As shown in fig 7, a point 770 between the two resistors 750, 760 is used as input port for a supply voltage V_c to the transistors 710, 720, of the current source 601.

As those skilled in the art will realize, the bipolar junction transistors shown in fig 7 can be replaced by other kinds of transistors, for example FET transistors. If the bipolar junction transistors are replaced by FET transistors, the ports of the FET transistors should be substituted for the ports of the bipolar junction transistors as follows:

Bipolar junction transistor FET

Base Gate

Collector Drain

Emitter Source

Turning now to the sizes of the inductors and capacitors used in the embodiments of figs 3 and 4, i.e. embodiments in which each of the passive components comprise only one single component, as opposed an embodiment such as the one in fig 5, where one or more of the passive components comprise two or more sub-components connected in series or in parallel, we find the following:

According to Kirchhoffs current law, and with V| denoting the differential signal +V_l and V_Q denoting the differential signal +V_Q, and letting ω denote the angular center frequency of the l/Q output signal of the l/Q network, we obtain:

From equations 1 a and 1 b, we obtain:

We see that if:

then equations 2a and 2b become

Equations 4a and 4b show that the voltages of V_Q and V_l have the same amplitude with phase difference of 90°. We also see that the components denoted as C and L, i.e. capacitors and inductors, can be exchanged in position, which will lead to the corresponding output signals' V_l and V_Q's positions being exchanged as well. With references to the equations above, suitable ranges for the parameters ω, L, C and R are as follows:

Turning now to embodiments such as the one in fig 5, i.e. an embodiment in which one or more of the passive components comprise two or more subcomponents connected in series or in parallel, it can be shown that if either the first or the second kind of passive component comprises subcomponents connected in parallel and the either kind of passive component comprises sub-components connected in series, the l/Q network can in fact be used as a dual band l/Q network. In such embodiments, suitable ranges between the inductance L₁ and the capacitance C₂ of the passive component of the first kind, and between the inductance L₂ and the capacitance C₁ of the passive component of the second kind are as follows:

where ω₁ and ω₂ are first and second angular center frequencies of the l/Q output signal of the dual frequency l/Q network. In the drawings and specification, there have been disclosed exemplary embodiments of the invention. However, many variations and modifications can be made to these embodiments without substantially departing from the principles of the present invention. Accordingly, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.

The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims.

Claims

1. An l/Q network (100, 600, 700) comprising an all-pass filter (101 , 201 , 301 , 401 , 501 ) adapted to receive differential input signals at a differential input port (130) with first (131 ) and second (132) terminals, and to output differential I and Q voltage signals at differential I and Q output ports (1 10, 120) with respective first (1 1 1 , 121 ) and second (1 12, 122) terminals, which all-pass filter (101 , 201 , 301 , 401 , 501 ) is adapted to receive said differential input signals as current signals, and in which all-pass filter the differential input port (130) is also arranged to be one of said differential I and Q output ports (1 10, 120).

2. The l/Q network (100) of claim 1 , where, in the all-pass filter (201 , 301 , 401 ), there is arranged a passive component (230, 330, 430) of a first kind between the first terminal (1 1 1 ) of the differential I output port and the first terminal (121 ) of the differential Q output port as well as (220, 320, 420) between the second terminal (1 12) of the differential I output port and the second terminal (122) of the differential Q output port, and a passive component (240, 340, 440) of a second kind between the first terminal (1 1 1 ) of the differential I output port and the second terminal (122) of the differential Q output port as well as (210, 310, 410) between the second terminal (1 12) of the differential I output port and the first terminal (121 ) of the differential Q output port, and with at least one resistance (250) connected between the terminals of the differential I and Q output port which is not arranged to also be the differential input port, with the passive components of the first kind being chosen from one of the component types inductors and capacitors and the passive components of the second kind being chosen from the other of the component types inductors and capacitors.

3. The l/Q network (100) of claim 2, where, in the all-pass filter (501 ), the passive component of the first kind can also be chosen either as a serial combination (520, 530) of an inductor and a capacitor or as a parallel combination (510, 540) of an inductor and a capacitor, and the passive component of the second kind is chosen from the other of said combinations.

4. The l/Q network (600, 700) of any of claims 1 -3, additionally comprising a current source (601 ) connected to the all-pass filter and arranged to generate said differential input current signals to the all-pass filter by means of an input voltage or current signal.

5. The l/Q network (700) of claim 4, in which the current source (601 ) comprises emitter coupled first (720) and second (710) bipolar junction transistors, with the collector of the first (720) of said transistors being connected to the second terminal (132) of the differential input port of the all pass filter (401 ), and the collector of the second (710) of said transistors being connected to the first terminal (131 ) of the differential input port of the all pass filter (401 ), and the bases of said transistors being arranged to be used as first (706) and second (707) terminals in an input port (210) of the l/Q network, in which l/Q network the emitter coupling of the first and second transistors is also connected to a means (730) for applying a biasing signal to the transistors.

6. The l/Q network (700) of claim 5, in which the means for applying a biasing signal to the current source comprises a third bipolar junction transistor (730), whose collector is connected to the emitter coupling of the first and second transistors, and whose base is arranged to receive a biasing signal (708), with the emitter of the third bipolar junction transistor being grounded.

7. The l/Q network (700) of claim 5 or 6, in which the bipolar junction transistors are replaced by FET transistors, with the following substitutions of ports of the transistors:

Bipolar junction FET

Base Gate

Collector Drain

Emitter Source

8. The l/Q network (100, 600, 700) of any of claims 5-7, comprising a first (750) and a second (760) resistance connected in series between the terminals of the differential I and Q output port which is not arranged to also be the differential input port, with a point (770) between said resistances being arranged to be used as input port for a supply voltage for said transistors.

9. The l/Q network of claim 2, wherein the relationship in the all-pass filter (201 , 301 , 401 ) between the inductances L, the capacitances C and the resistances R satisfy:

where ω is the angular center frequency of the l/Q output signal of the l/Q network.

10. The l/Q network of claim 3, wherein the relationship in the all-pass filter (501 ), between the inductance L₁ and the capacitance C₂ in the passive component of the first kind, and between the inductance L₂ and the capacitance C₁ in the passive component of the second kind satisfy:

where ω₁ and ω₂ are first and second angular center frequencies of the l/Q output signal of the l/Q network.