EP0921588A1

EP0921588A1 - Variable wideband microwave TT-arranged pin diode attenuator

Info

Publication number: EP0921588A1
Application number: EP98440242A
Authority: EP
Inventors: Mario Giovanni Frecassetti
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel CIT SA; Alcatel Lucent SAS
Priority date: 1997-11-05
Filing date: 1998-10-30
Publication date: 1999-06-09
Also published as: IT1295919B1; ITMI972468A1

Abstract

A variable wideband π attenuator is described comprising three PIN diodes, two being part of the shunt arms of the attenuator and one belonging to the series arm. With the use of suitable matching networks (RA1, RA2, RM1, RM2), that vanish the effects produced by the parasitic elements present in the PIN diode network, a considerable amplitude of the operating band is obtained.

Description

The present invention relates to a variable wideband microwave π-arranged PIN diode attenuator including a series arm with at least one PIN diode and two shunt arms each having at least one PIN diode.
The main objectives to be reached in the design of a variable microwave PIN diode attenuator are the followings:

Low insertion loss (i.e. low minimum attenuation);
Good band flatness;
Good matching level at the input and output ports;
Attenuation dynamics (difference between maximum and minimum obtainable attenuation) sufficient for the relevant application.

The known solutions fulfilling the requirements set forth above, used at frequencies greater than 1 GHZ, are substantially of two types:

Solutions consisting in the use of monolithic circuits. Such solutions are expensive or present high insertion loss.
Solutions consisting in the use of hybrid circuits. In general they are realized with ceramic substrates and include:
- Lange couplers to match the attenuator input and output impedances. The Lange couplers are very expensive components and have a restricted band that depends on their geometry;
- at least two PIN diodes, usually known in the art as Beam Leads Diodes. The use of such fragile PIN diodes implies considerable costs, both for their high price and for their mounting.

The optimization of the performance-to-price ratio is hard to get due to the above drawbacks, in particular when the operating frequencies are very high (>3GHz).
In the framework of hybrid solutions, the use of special surface mount packages accomodating the diodes which the attenuator circuit is based on, may allow a remarkable cost reduction and a greater simplicity of implementation as compared with the use of Beam Lead PIN diodes.
In the article: "A Low Cost Surface Mount PIN Diode π Attenuator" by Raymond Waugh, published on Microwave Journal (May 1992, pages 280 to 284), it is disclosed a symmetrical attenuator circuit, with relating bias networks, formed by four PIN diodes and characterized by good electric performances in the frequency band comprised between 300 KHz and 3 GHz. The possible use of surface mount devices, in particular two packages each containing a pair of diodes, can make such a circuit particularly cheap.
But problems remain unsolved when an operating range is required which extends well beyond the 3 GHz value of the solution proposed in said article.
An object of the present invention is to provide a variable attenuator circuit which is capable of overcoming the above prior art drawbacks.
In accordance with the invention, such an object is achieved by a circuit having the characteristics set forth in claim 1.
Further ancillary characteristics are set forth in the dependent claims.
The attached claims are an integral part of this description.
The present invention shall result in being clear from the following detailed description of its structure and embodiments thereof, given by way of a non limiting example, and from the attached drawings, wherein:

Fig. 1 shows a schematic diagram of resistors in π arrangement;
Fig. 2 shows a block diagram of the attenuator in accordance with the present invention;
Fig. 3 shows a non limiting embodiment of the attenuator of Fig. 2;
Fig. 4 shows the equivalent schematic circuit diagram of the package designed as RD in Fig. 3.

The schematic diagram of the disclosed circuit consists of an attenuator having a π (pi) arrangement that, along with the T configuration, is the conventional configuration used in low and medium frequency circuits (typically for frequencies lower than 500MHz).
In Fig. 1 a schematic diagram of resistors in a π arrangement is shown. The series arm comprises resistor Rs and terminals A and B. Connected to said terminals are the input and output of the circuit.
The first shunt arm comprises resistor Rp1 and terminal C, and it is directly connected to the series arm at terminal A.
The second shunt arm comprises resistor Rp2 and terminal D, and it is directly connected to the series arm at terminal B.
Terminals C and D are grounded.
The variation of the attenuation is obtained by controlling the resistance values of the three resistors.
In general, resistors Rp1 and Rp2 are equal. In this circumstance, which is also the case of the present invention, the input and the output of the circuit can be changed with each other, without changing the operating characteristics of the circuit itself.
The need to have variable resistances justifies the use of PIN diodes (at least one in the series arm and at least one in each of the shunt arms) having the function of the resistors of Fig.1: in fact, it is known that a PIN diode, at frequencies higher than the cut-off frequency fc, acts as a variable resistor dependent on the current flowing therethrough.
In accordance with the present invention, it has been discovered that it is possible to make a π-arranged PIN diode attenuator which is very cheap, as compared with standard attenuators, and is characterized by a considerable simplicity of implementation.
Further, such an attenuator, features very good operating characteristics for wide signal-bands all or partly comprised in the microwave band (e.g. for the band comprised between 5 and 10 GHz).
The peculiarity of such attenuator lies in that it allows the use of a single package, accomodating the π-arranged PIN diodes connected to simple matching networks.
Such matching networks perform the important function of nullifying or at least reducing, the undesired effects caused by the parasitic elements of the package which would hinder the use of the attenuator at high frequencies.
It is to be noted that the mere fact of using a single package, significantly reduces the undesired effects as compared with solutions contemplating the use of two or more packages (such as the solution disclosed in the above mentioned article) and which, therefore, entail the presence of more parasitic elements.
In Fig. 2 the block diagram of the attenuator according to the present invention is illustrated. Said diagram includes:

A diode network (block RD)
Four command networks (blocks RC1, RC2, RC3 and RC4)
Two grounding networks (blocks RM1 and RM2)
Two matching networks (blocks RA1 and RA2).

Block RD comprises a set of diodes interconnected according to a π arrangement (at least one diode in the series arm and at least one diode in each of the shunt arms). Terminals A and B are the series arm terminals, while terminals C and D are the remaining terminals, belonging to shunt arms.
The four blocks RC1, RC2, RC3 and RC4 represent four networks, adapted to provide the suitable bias current values to the diodes of block RD, without affecting, because of their presence, the frequence behaviour of the circuit. Such networks connect the circuit with the bias and control circuits of the diodes themselves. Said known circuits, which are not shown in the figure, are directly connected to terminals C1, C2, C3 and C4. Said terminals belong to blocks RC1, RC2, RC3 and RC4, respectively.
Block RC2 is directly connected, by way of its terminal C5, to terminal D and to the first terminal of block RM1. Block RM1 has a second terminal connected to ground.
Block RC4 is directly connected, through its terminal C6, to terminal C and to the first terminal of block RM2. Block RM2 has a second terminal connected to ground.
The purpose of the two blocks RM1 and RM2 is to eliminate, as far as the radio-frequency signal is concerned, the undesired effects produced by the reactive elements present in the shunt arms of block RD. Blocks RM1 and RM2 allow, at the same time, the bias currents to transit in the block RD itself, without affecting said currents. The features of blocks RM1 and RM2 affect the performances of the attenuator in a significant manner.
Block RC1 is directly connected, by way of its terminal C7, to the first terminal of block RA1 which, through its second terminal, is directly connected to terminal A.
Terminal C7 is directly connected to IO1, which is the first input or output terminal of the attenuator for the frequency signal.
Block RC3 is directly connected, by way of its terminal C8, to the first terminal of block RA2 which, by way of its second terminal, is directly connected to terminal B.
Terminal C8 is directly connected to IO2, which is the second input or output terminal of the attenuator for the frequency signal.
The purpose of the two blocks RA1 and RA2 is to eliminate, as far as the radio-frequency signal is concerned, the undesired effects produced by the reactive elements present in the series arm of block RD.
The two blocks RA1 and RA2, along with blocks RM1 and RM2, allow the achievement of low insertion loss, a good band flatness and a good impedance matching.
Blocks RA1, RA2, RM1 and RM2 form the matching networtks the importance of which has already been pointed up.
In the case where the command networks are made equal to each other, as well as the grounding networks and the matching networks, the circuit can be used without modifying its operating characteristics by changing the input terminals with the output ones.
The circuit of Fig. 3 features particular structures (1, 2, 3, 4, RM₁ and RM₂), known in the art as the so called radial stub structures.
A radial stub structure is a wideband connection to ground: it behaves exactly as a short circuit at a well-defined frequency, in relation to which the radial stub is dimensioned, and it approximates the short circuit in a neighborhood of that frequency.
A radial stub behaves inductively at frequencies higher than the dimensioning frequency and behaves capacitively at lower frequencies.
With respect to the bias currents it behaves as an open circuit.
As to block RD in Fig. 2, it can be made by means of a package accomodating the diodes in a π arrangement. As a package, the SMT (Surface Mount Technology) plastic package SOT143 can be chosen, for istance.
In Fig. 4 the equivalent schematic circuit diagram of the STO143 package in relation to the frequency signal is shown. Such an equivalent circuit is characterized by the presence of the parasitic inductors L1, L2, L3 and L4.
Inductors L1 and L2 are respectively in series with the diodes of the first and of the second shunt arms of the π-arranged diode network.
Inductor L3 is connected between the terminal A (Fig. 1) and that terminal, belonging to the first shunt arm, connected to the series arm of the diode network.
Inductor L4 is connected between the terminal B (Fig. 1) and the terminal, belonging to the second shunt arm, connected to the series arm of the diode network.
These parasitic elements do not allow an effective use of the SOT143 as a high frequency attenuator since, as compared with the ideal case, worsenings occurs in insertion loss, in band flatness and in matching.
According to the present invention, in order to vanish the effects of the parasitic elements, suitable matching networks (grounding networks and matching networks in Fig. 2) are used.
The grounding networks (blocks RM1 and RM2 in Fig. 2) have the purpose of compensating the undesired effects produced by the parasitic inductors L1 and L2 (Fig. 4).
Each of the grounding networks is comprised of a matched radial stub (RM1 and RM2 structures in Fig. 3), i.e. it is dimensioned for a higher frequency than the midband frequency, for example at a frequency equal to about the upper band limit, directly connected to the respective terminal C or D of RD.
A good virtual ground directly connected to the diodes of the shunt arms is thus obtained.
The matching networks (blocks RA1 and RA2 in Fig. 2) compensate for the undesired effects produced by the parasitic inductors L3 and L4 (Fig. 4).
Each of the matching networks comprises a transmission line, respectively LT1 and LT2 (Fig. 3), with a characteristic impedance slightly greater than the circuit impedance and a length nearly equal to λ/4 (λ corresponding to the midband frequency), i.e. with wavelength evaluated at about the lower limit of the band of interest. This is obtainable with a suitable narrowing of the input and output lines, realized at suitable points thereof (Fig. 3).
Each of the command networks (blocks RC1, RC2, RC3 and RC4 in Fig. 2) comprises:

a high impedance line which is λ/4 long (at the midband frequency of the relevant band), AL1, AL2, AL3, AL4, respectively, whose terminations are the terminals of the network itself;
at least one radial stub ( structures 1, 2, 3 and 4 in Fig. 3), dimensioned at the midband frequency and directly connected to said high impedance line.

The purpose of each of the structures 1, 2, 3 and 4 (Fig. 3) is to decouple the diode bias and control circuits from the frequency signal attenuation circuit.
The circuit also features two elements (capacitors COND1 and COND2 in Fig. 3) which are open circuits for the bias currents and are inserted at the input and at the output of the attenuator.
The advantages arising from the present invention result in being clear from the above description.
The basic advantages of the novel circuit consist in the substantial cheapness as compared with the standard arrangements, in the facility of implementation and in the possibility of use for wide signal bands all or partially contained in the "microwave" band.
It is also to be noted that it is not necessay to use Lange couplers at the input and at the output of the circuit.
Finally, it is evident how the circuit features very good characteristics in terms of overall dimensions and simplicity of layout.

Claims

Variable wideband microwave π-arranged PIN diode attenuator, including a series arm (Rs) with at least one PIN diode, two shunt arms (Rpl, Rp2) each with at least one PIN diode, characterized in that it comprises matching means (RA1, RA2, RM1, RM2) directly connected to said series and shunt arms and designed to reduce or nullify the undesired effects produced by the parasitic reactive elements (L1, L2, L3, L4) present in the π-arranged diode network.
Attenuator according to claim 1, characterized in that said matching means (RM1, RM2) connected to said shunt arms comprise at least one first radial stub directly connected to one of said shunt arms.
Attenuator according to claim 2, characterized in that said first radial stub (RM1, RM2) is dimensioned dependent on the frequency band and of the reactive characteristics of said parasitic elements (L1, L2) present in said shunt arm.
Attenuator according to claim 3, characterized in that said parasitic reactive elements (L1, L2) present in said shunt arm are inductive, and in that said first radial stub (RM1, RM2) is dimensioned for a frequency which is higher than the midband frequency at a frequency equal to about the upper band limit.
Attenuator according to claim 1, characterized in that said matching means (RA1, RA2) connected to said series arm, comprises at least a first transmission line (LT1, LT2), directly connected to one of the terminals of said series arm.
Attenuator according to claim 5, characterized in that said first transmission line (LT1, LT2) is dimensioned dependent on the frequency band of the characteristic impedance of the attenuator, and of the reactive characteristics of said parasitic elements (L3, L4) present in said series arm.
Attenuator according to claim 6, characterized in that said parasitic reactive elements (L3, L4) present in said series arm are inductive, and in that said first transmission line (LT, LT2) is dimensioned with characteristic impedance slightly greater than that of the attenuator, i.e. with a length nearly equal to λ/4 (λ corresponding to the midband frequency), that is to say with a wavelength evaluated at about the lower limit of the band of interest.
Attenuator according to any of the claims 1 to 7, characterized in that it further comprises four command networks (RC1, RC2, RC3 and RC4) designed to provide suitable bias current values to said diode network, without affecting the frequency behaviour of said attenuator.
Attenuator according to claim 8, characterized in that each of said four networks comprises a second transmission line (AL1, AL2, AL3, AL4) one-quarter wave length (λ/4) long, to which at least a second radial stub (1, 2, 3, 4) is directly connected, dimensioned at the midband frequency.
Attenuator according to any of claims 1 to 9, characterized in that said π-arranged diodes are accomodated in a single package.
Attenuator according to claim 10, characterized in that said package is of SMT (Surface Mount Technology) type.
Attenuator according to any of claims 1 to 11 characterized in that it is used in a frequency band higher than 3GHz.