WO2008118046A1

WO2008118046A1 - Coupled line step attenuator

Info

Publication number: WO2008118046A1
Application number: PCT/SE2007/000300
Authority: WO
Inventors: Peter Karlsson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-03-28
Filing date: 2007-03-28
Publication date: 2008-10-02

Abstract

Step attenuator circuit and a method for attenuation of electrical signals comprising a biasing circuit (202) for placing the step attenuator circuit (200) in one of on or off modes, at least one biased switching element (D1, D2, D3) for providing high impedance in the 'off-mode and low impedance in the 'on'-mode, where the step attenuator circuit further comprises: at least one pair of transmission line elements (210, 240; 220, 230) connected to the at least one biased switching elements (D1, D2, D3), and at least one directional coupler (250) comprising at least an input and an output port (PORT1, PORT2) connected to the at least one pair of transmission line elements.

Description

COUPLED LINE STEP ATTENUATOR

TECHNICAL FIELD

The present invention is related to step attenuators such as they, for example, are used in microwave circuits.

BACKGROUND OF THE INVENTION

Step attenuators are used in many applications for adjusting gain and power levels. The need for gain adjustment arises in many areas, such as control of gain both for transmitted and received signals or when a certain maximum power level for a signal should not be exceeded. Normally, fixed attenuators are built according to two schemes referred to as T-type or Pi-type attenuators.

A desirable step attenuator should be able to attenuate the signal by a predefined amount, be able to handle the power dissipated in it and be frequency independent.

An example of an ordinary step attenuator consisting of two switches and one fixed attenuator is shown in Fig. 1.

Standard step attenuators, including the one shown in Fig. 1, suffer from frequency dependence and often substantial loss in the switches at higher frequencies. This translates to more insertion loss when the switches are turned off and a non-flat frequency response both in the case where the switches are turned on and in the case they are turned off.

One known variable attenuator is described in the US patent 5,477,200. According to US 5,477,200, a variable reflection attenuator is disclosed, where PIN-diodes are used as variable resistance elements and where a 4-port coupler is constructed so as to assure that when one of the diodes is biased by a certain voltage, the same current flows through both PIN-diodes. Since the resistance of a PIN-diode is dependent on the magnitude of the current flowing through it, the resistance of both PIN-diodes will be equal. In this fashion the effect of stray inductances in PIN-diodes is minimized.

One other variable attenuator is disclosed in US patent US 6,414,565, where the variable attenuation of a signal can be varied continuously through electromagnetic coupling of two comb lines. Applying a voltage to one of the terminals of the attenuator controls the magnitude of the current flowing through the comb line, and also the amount of electromagnetic coupling between the two lines. This in turn, has an effect on the overall impedance of the attenuator.

The object of the present invention is however to present an alternative solution for a step attenuator, which among others, is frequency independent or at least much more frequency independent than step attenuators according to known technology.

SUMMARY OF THE INVENTION

The object of this invention is achieved by a step attenuator circuit for attenuation of electrical signals where the step attenuator circuit comprises

- a biasing circuit for placing the step attenuator circuit in one of on or off modes, - at least one biased switching element for providing high impedance in the "off-mode and low impedance in the "on"-mode,

- at least one pair of transmission line elements connected to the at least one biased switching elements and;

- at least one directional coupler comprising at least an input and an output port connected to the at least one pair of transmission line elements.

In one embodiment of the step attenuator circuit according to the present invention the step attenuator circuit additionally comprises a DC feed choke connected to the biasing circuit and the at least one pair of transmission line elements. The additional DC feed choke has the advantage of supplying DC voltage to the rest of the step attenuator circuit, while at the same time acting as an open circuit at high frequencies and therefore isolating the rest of the step attenuation circuit from the biasing circuit.

According to one variant of the present invention, the biasing circuit mentioned earlier may be a control voltage source circuit.

It may be mentioned that the at least one biased switching element may comprise one of the elements from the group of PIN diodes, transistors, microelectromechanic switches and other components able to perform a switching function. Also, the abovementioned pair of transmission line elements may comprise one of the elements from the group of striplines, microstrip lines, coaxial lines, and other elements suitable for use as transmission lines.

According to another embodiment of the present invention, the at least one directional coupler in the step attenuator circuit mentioned earlier may comprise a four-port directional coupler.

According to one other aspect of the present invention, the object of the invention is achieved by a method of attenuation of electrical signals by means of an attenuation circuit comprising the steps: a) switching a bias switching circuit in said attenuation circuit by means of a biasing voltage b) coupling a signal to an input port of a directional coupler having the input port and an output port in the attenuation circuit c) coupling the signal from the input port to the output port of said directional coupler.

The method steps are especially suitable to be implemented by the step attenuator circuit above.

According to yet another aspect of the present invention, the object of the invention is achieved by a mobile station for communication in a wireless communication network comprising an attenuation circuit for attenuation of electrical signals, where the attenuation circuit further comprises: - a biasing circuit for placing the attenuation circuit in one of on or off modes,

- at least one biased switching element for providing high impedance in the "off -mode and low impedance in the "on"-mode,

- at least one pair of transmission line elements connected to the at least one biased switching elements , and - at least one directional coupler comprising at least an input and an output port connected to the at least one pair of transmission line elements.

According to yet another aspect of the present invention, the object of the invention is achieved by a base station for communication in a wireless communication network comprising an attenuation circuit for attenuation of electrical signals, where the attenuation circuit further comprises:

- a biasing circuit for placing the attenuation circuit in one of on or off modes,

- at least one biased switching element for providing high impedance in the "off-mode and low impedance in the "on"-mode, - at least one pair of transmission line elements connected to the at least one biased switching elements, and

Similar to the step attenuation circuit mentioned earlier, the method steps of the method according to the present invention are especially suited to be executed by the attenuation circuit in the mobile station or the base station according to the present invention.

These and other advantages will become more apparent when reading the following detailed description together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS (OPTIONAL)

Fig. 1 is a schematic representation of a step attenuator according to known technology.

Fig. 2 is a schematic representation of an embodiment of a step attenuator according to the present invention.

Fig. 3 is an idealized schematic representation of the embodiment in Fig. 2, when the step attenuator is in the "off-state.

Fig. 4 is an idealized schematic representation of the embodiment in Fig. 2, when the step attenuator is in the "on"-state.

Fig. 5 is an equivalent schematic representation of the circuit in Fig. 4 when the step attenuator is in the "on"-state and the transmission line sections represent quarter-wave transformers.

Fig. 6 illustrates a graph over the simulated input reflection of the step attenuator as a function of frequency. Finally, Fig. 7 illustrates a graph over the simulated forward transmission of the step attenuator as a function of frequency.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 is a representation of a typical step attenuator circuit 100 according to known technology.

Here, a first transmission line port PORT1 is connected to a first switch T1 , while another transmission line port PORT2 is connected to a second switch T2. Both switches, in turn, are connected to the attenuator 110 via a transmission line 120.

The transmission lines and the transmission line ports typically have a characteristic impedance of 50 Ω (not shown).

Now, when both switches T1 and T2 are in the up position (the position shown in Fig. 1), the attenuator is engaged into the circuit. The total attenuation in the circuit will then comprise the insertion loss in the switches T1 and T2 plus the fixed attenuator value. A typical attenuator known to the skilled person will most probably be resistive where the arrangement of the resistors will resemble the letter T or π (Greek pi). When the switches T1 and T2 are in the down position, the fixed attenuator will be disengaged and the attenuation in the circuit will consist solely of the insertion loss for the two switches and the connecting transmission line 130.

Commercially available high frequency (frequencies above 5 GHz) step attenuators usually have an insertion loss of 1 dB, while the input/output return loss is often both frequency and batch dependent.

Fig. 2 illustrates an electric circuit according to one possible embodiment of the present invention.

The step attenuator circuit 200 comprises a biasing circuit 202, which, in turn, comprises a DC voltage source V1 connected to a biasing network, consisting of a resistor R3 and a capacitor C3. The resistor part R3 of the biasing network in Fig. 2 is further connected to a DC feed choke L in the form of an inductor. The function of the DC feed choke L is to supply DC voltage to the rest of the electric circuit, while at the same time to isolate the biasing circuit 202 (comprising the DC voltage source and the biasing network) from the rest of the circuit at high frequencies.

Of course, the DC feed choke may be realized by any other means which ensures a DC input current through it and isolation of the biasing circuit 202 at high frequencies. 5

In a simulation of the embodiment of Fig. 2, the DC voltage source V1 was chosen to deliver a voltage with an amplitude of 5V with a delay time td = 10 ms, while the rise time for the square wave generated by the voltage source V1 was chosen to be tr = 1 ms. However, these values should be regarded as example values only and may be adjusted 10 depending on the application.

It may be mentioned here, that the delay and rise times are defined according to the standard known to those skilled in the art.

Furthermore, the electric circuit in Fig. 2 comprises four transmission line sections 210, 15 220, 230, 240, where the first and second transmission lines 210 and 220 are connected to the DC feed choke L, the second and third transmission lines 220 and 230 to the third diode D3, the first transmission line 210 to the first diode D1 and the directional coupler 250, and finally, the fourth transmission line 240 to the second diode D2 and the directional coupler 250.

20 It should be mentioned here, that the electrical lengths of the transmission line sections 210-240 are chosen such that the combined length of transmission line sections 210 and 220 and the combined length of transmission line sections 230 and 240 both constitute a quarter of a wavelength at the center frequency, in order to act as quarter-wave transformers at these frequencies. Apart from that, it may be mentioned that the 25 transmission lines 210-240 may be constructed as striplines, microstrip lines, coaxial lines, or any other components suitable for performing the function of a transmission line in the attenuator circuit in Fig. 2.

Also, the characteristic impedance for the transmission lines 210-240 is chosen to 30 be 50 Ω, but, of course may be chosen to have other values in order to match the input and output impedances in the electric circuit in Fig. 2, thereby avoiding signal reflections in the circuit. For the skilled person it will be apparent that the characteristic impedance may be adjusted through the right choice of materials or geometrical dimensions of the transmission lines.

35 The function of the three diodes D1 , D2, D3 is to emulate the function of a switch, such as, for example the switches T1 and T2 from Fig. 1. In this example, the diodes are chosen to be PIN-diodes. However, the switch function may be performed by any other suitable component, such as different kinds of transistors, microelectromechanical switches (MEMS), and other components performing the switching function.

As far as the remainder of the electric circuit in Fig. 2 is concerned, it consists of a four- port directional coupler 250 having its through-port connected to a first diode D1 , a first resistor R1, and the transmission line section 210, its isolated port connected to a second diode D2, a second resistor R2 and the transmission line section 240. The input port PORT 1 of the circuit is connected to the directional coupler input port through a first DC blocking capacitor C1 , and the output port PORT 2 is connected to the coupled port of the directional coupler through a second DC blocking capacitor C2.

Now, when the step attenuator according to the embodiment in Fig. 2 is in the "off- position the diodes D1 , D2 and D3 are not biased. The electric circuit in Fig. 2 may then be simplified to the idealized circuit illustrated in Fig. 3.

In the "off-state, the diodes have relatively high impedance compared to the 50Ω impedance of the transmission line sections 210-240, and act therefore as open circuits. Hence, they may be omitted as shown in Fig. 3.

It should be noted that the biasing circuit has also been omitted from Fig. 3, since it is not involved in the RF signal processing in the attenuator. This is due to the high isolation at high frequencies (RF frequencies and higher) provided by the DC feed choke L. Also, the first and second DC blocking capacitors C1 and C2 may be omitted due to their short-circuit behavior at high frequencies.

What remains of the circuit in Fig. 2 are the directional coupler 250 and the four transmission line sections 210-240, where the combined electrical length of said transmission line sections is half a wavelength.

Thus, in the "off -state, the circuit is reduced to form a loop with small influence from the coupling between the input and output lines. The loss between the input and output ports PORT 1 and PORT 2 in the attenuator circuit in Fig. 3 then mainly consists of transmission line loss which is usually low. This transmission line loss may be lowered further by choosing suitable material characteristics for the transmission line.

Turning now to Fig. 4, the electric circuit from Fig. 2 is represented in the "on"-state. Here, the three diodes D1 , D2 and D3 are biased by the biasing part of the electric circuit mentioned earlier, where this time the diodes act as closed switches.

In this position, the forward biased diodes D1-D3 have a low impedance compared to the 50 Ω transmission lines in Fig. 2. Therefore, the PIN diodes may be substituted by short- circuits in the circuit, and it may be simplified to the idealized form of the circuit in Fig. 4. Also, the first and second DC blocking capacitors C1 and C2 have been omitted due to their short-circuit behavior at high frequencies.

Let now in this simplified electric circuit in Fig. 4 the electrical lengths of the transmission line sections 210-240 be such that the combined length of transmission line sections 210 and 220 and the combined length of transmission line sections 230 and 240 both constitute a quarter of a wavelength. The short-circuited point between the transmission line sections 220 and 230 will then be transformed to open circuits (high impedance) at the points where the coupler 250 connects to the transmission line sections 210 and 240, respectively. The electric circuit may then be simplified to the equivalent one represented in Fig. 5.

The simplified circuit in Fig. 5 represents a directional coupler 250 with the through port (upper left arm of the coupler) terminated into R1 and the isolated port (upper right arm) terminated into R2, respectively, assuming that PORT1 is connected to the input port (lower left arm) and PORT2 to the coupled port (lower right). R1 and R2 are chosen to have a resistance value matching the line and port impedances. The present embodiment utilizes a coupled microstrip line directional coupler, but the use of other types of directional couplers with appropriate port connections is obvious to those skilled in the art.

A fraction of the RF signal at the attenuator input port (PORT1) will thus be coupled to the attenuator output port (PORT 2), the remainder being terminated in the resistors R1 and θ

R2. The coupling factor of the directional coupler 250 is thus essentially the only parameter that determines the "on" state attenuation in the step attenuator. Since the circuit in Fig. 5 is symmetric and reciprocal, PORT 1 (normally the input port) may as well function as the output port, while PORT 2 may function as the input port. The signal to be attenuated may therefore enter the circuit either from PORT 1 or from PORT 2.

Typical simulation results for the circuit in Fig. 2 with a coupling factor of -14 dB are shown in the form of graphs in Figs. 6 and 7.

The graph in Fig. 6 shows the simulated input port (PORT1) reflection expressed in dB as a function of the signal frequency in GHz. The upper curve 610 represents the input reflection in the circuit when the attenuator is in the "off-state. It is clearly visible that the input reflection is substantially constant at about -14 dB for frequencies between 5-8 GHz. Marked with the reference number 620, the lower curve represents the input reflection when the attenuator is the "on" state. One observes clearly that the attenuation has a dip of about -32 dB at around 6,9 GHz. For the rest of the frequency span, the attenuation is well below the -14 dB line. The input reflection will thus typically be better than the corresponding coupling factor level for both states.

The graph in Fig. 7 illustrates the simulated forward transmission (PORT1 to PORT2) of the attenuator circuit expressed in dB as a function of the signal frequency in GHz, using the same frequency range as in Fig. 6. In this graph, the six marker symbols in Fig. 7, represented by the reference numbers m1-m6, are chosen to evaluate the attenuation of the circuit in Fig. 2 for different frequency values of interest.

Referred to as 710 in Fig. 7, the upper line represents the forward transmission in dB during the "off '-state of the attenuator. The attenuation in the "off' state is, as stated earlier, essentially only limited by the insertion loss of the directional coupler 250 and the transmission line sections 210 - 240. In contrast, the influence of the PIN diodes D1-D3 on the attenuation in the "off-state is negligible.

One can clearly see the essentially flat frequency response of the attenuator according to the present invention in Fig. 7.

For example, the marker point m4 represents the forward transmission in dB at the frequency of 5 GHz. According to the simulation the forward transmission at m4 was calculated to be -0,36 dB. At m5, i.e. at the center frequency of 6.5 GHz, the forward transmission was calculated to be -0,39 dB, and at m6, i.e. at 8 GHz, the corresponding value was -0,38 dB.

The second line with reference number 720 represents the forward transmission in dB when the attenuator is in the "on"-state. Even in this case, the frequency response of the attenuator with respect to forward transmission was essentially flat. By way of example, the forward transmission at the marker point ml , i.e. at 5 GHz, was calculated to be -14,3 dB. At the marker point m2, i.e. at 6.5 GHz, the forward transmission was -13,8 dB, while at m3, i.e. at 8 GHz, the corresponding value was -14,3 dB. The attenuation in the "on" -state is thus essentially given by the coupling factor level for the directional coupler.

It should be noted that the example embodiment presented in this disclosure should only be construed as for illustration only and that there are many other possible embodiments of the attenuator according to the present invention which will be obvious to the skilled person after having read the foregoing detailed description and consulted the accompanying drawings.

Therefore, in its widest sense, the present invention is only limited by the scope of protection defined by the accompanying claims.

Claims

1. A step attenuator circuit (200) for attenuation of electrical signals, the circuit comprising

- a biasing circuit (202) for placing the step attenuator circuit (200) in one of on or off modes,

- at least one biased switching element (D1 , D2, D3) for providing high impedance in the "off-mode and low impedance in the "on"-mode,

characterized by

- at least one pair of transmission line elements (210, 240; 220, 230) connected to the at least one biased switching elements (D1, D2, D3), and

- at least one directional coupler (250) comprising at least an input and an output port (PORT1, PORT2) connected to the at least one pair of transmission line elements.

2. Step attenuator circuit according to claim 1

characterized by

a DC feed choke (L) connected to said biasing circuit (202) and said at least one pair of transmission line elements (210, 240; 220, 230).

3. Step attenuator circuit according to claims 1 or 2

characterized by

that said biasing circuit (202) is a control voltage source circuit.

4. Step attenuator circuit according to one of the preceding claims 1-3

characterized by

that said at least one biased switching element (D1, D2, D3) comprises one of the elements from the group of PIN diodes, transistors, microelectromechanic switches and other components able to perform a switching function.

5. Step attenuator circuit according to one of the preceding claims 1-4

characterized by

that said at least one pair of transmission line elements (210, 240; 220, 230) comprises one of the elements from the group of striplines, microstrip lines, coaxial lines, and other elements suitable for use as transmission lines.

6. Step attenuator according to one of the preceding claims 1-5

characterized by

that said at least one directional coupler (250) comprises a four-port directional coupler.

7. A method of attenuation of electrical signals by means of an attenuation circuit comprising the steps: a) switching a bias switching circuit in said attenuation circuit by means of a biasing voltage b) coupling a signal to an input port of a directional coupler having said input port and an output port in said attenuation circuit c) coupling said signal from the input port to the output port of said directional coupler.

8. Mobile station for communication in a wireless communication network comprising an attenuation circuit (200) for attenuation of electrical signals, the attenuation circuit (200) further comprising:

- a biasing circuit (202) for placing the attenuation circuit (200) in one of on or off modes,

- at least one biased switching element (D1 , D2, D3) for providing high impedance in the "off -mode and low impedance in the "on"-mode, where said attenuation circuit (200) is characterized by

- at least one directional coupler (250) comprising at least an input and an output port connected (PORT1 , PORT2) to the at least one pair of transmission line elements (210, 240; 220, 230).

Base station for communication in a wireless communication network comprising an attenuation circuit (200) for attenuation of electrical signals, the attenuation circuit (200) further comprising:

- at least one biased switching element (D1, D2, D3) for providing high impedance in the "off'-mode and low impedance in the "on"-mode,

where said attenuation circuit (200) is characterized by »

- at feast one directional coupler (250) comprising at least an input and an output port (PORT1 , PORT2) connected to the at least one pair of transmission line elements (210, 240; 220, 230).