GB2381668A

GB2381668A - Microstrip to coplanar waveguide transition

Info

Publication number: GB2381668A
Application number: GB0126258A
Authority: GB
Inventors: Paul Jonathan Simpson
Original assignee: Marconi Optical Components Ltd; Bookham Technology PLC
Current assignee: Marconi Optical Components Ltd; Lumentum Technology UK Ltd
Priority date: 2001-11-01
Filing date: 2001-11-01
Publication date: 2003-05-07
Also published as: GB0126258D0

Abstract

A transition 122 connects a ground backed coplanar waveguide (GCPW), which includes a hot conductor formed on a ground backed insulating substrate 120 and having parallel top ground planes 130, 131 on either side, to a ground backed microstrip transmission line 121 formed on an insulating substrate 120. The hot conductor of the GCPW and the hot conductor of the microstrip transmission line are electrically connected together, there being vias extending through the insulating substrate and forming an electrical connection between the top ground planes 130, 131 and the ground backing 205. The transition 122 from the GCPW structure to the microstrip structure is such that the characteristic impedance of the feed-line is a substantially constant 50 L along its length within the operating frequency range such that broadband operation is possible. In one aspect, the transition 122 is tapered in width and ends of the ground planes 130, 131 are rounded, and splay outwardly from the hot conductor.

Description

Transition This invention relates to transmission feed-lines and has particular but not necessarily exclusive reference to transmission feed-lines for use in optical modules in which optical signals are converted into electrical signals, particularly for use in optical telecommunication transmission systems.

Background In optical communication systems, there is a need to convert the optical input signal on a fibre optic line into an electrical output signal for further use. The conversion is carried out in a packaged optical receiver module containing essentially a photodiode which converts the optical signal into an electrical signal, a monolithically integrated amplifier (a MMIC) and a transmission feed-line which interconnects the MMIC to a push on connector which fits on to the metal conductor of an RF feedthrough bead in the package wall.

The optical module is hermetically sealed and of very restricted size, typically 12 mm square.

In conventional packaged optical modules the microwave signal is transmitted through the package wall using an RF feed-through bead, which is designed to connect to a microstrip transmission line on a ceramic substrate inside the package. Typically, a microstrip transmission line is utilised to propagate the microwave signal, as this allows direct connection from the transmission feed-line to the RF feed-through bead in the package wall. The high dielectric constant of ceramic substrate

combined with the dispersive nature of microstrip transmission lines, results in high deviation from linear insertion phase being observed, as the optical components approach high data rates, for example 40Gbits/sec. The high degree of phase deviation causes timing jitter problems during data transfer at these high speeds, degrading the optical module's performance.

Alternative technologies include the use of microstrip transmission lines on substrates with a low dielectric constant, which minimises the effect of dispersion. Substrate materials with a low dielectric constant tend to be expensive and difficult to process or have out-gassing problems, for example softboard, that make them undesirable for use in hermetically sealed optical units.

Another alternative is to use coplanar waveguide (CPW) transmission lines, which offers low dispersion characteristics in comparison to microstrip transmission lines. However, problems arise when having to connect to a conventional RF feed-through bead where a microstrip transmission line launch is required (due to the size limitation of the pin on the RF feed-through bead). To overcome this, a CPW to microstrip transmission line transition can be implemented, where the majority of the line is produced in CPW and for the small section near the RF feed-through bead, a microstrip transmission line geometry can be used.

Conventional designs use an abrupt transition that gives rise to a discontinuity in the width of the central hot conductor of the transmission line. It has now been found that such a design results in an inductive reactance in series with the hot conductor and this, combined with the

generation of higher order propagation modes at the discontinuity, results in a significant increase in the phase deviation seen.

It has been found that an abrupt CPW to microstrip transition design is limited in that it permits only usage in narrowband applications due to increased insertion losses at high speeds. Such limitations have only become apparent with the introduction of high speed (40Gbits/sec) devices.

Heretofore using a CPW to microstrip transmission line transition, only a narrowband connection between the MMIC and the RF feedthrough bead has been possible. The present invention overcomes these problems by providing'a broadband transmission feed-line capable of acting as a connection operable over a frequency range from 40 kHz to 40 GHz.

Brief Description of the Invention By the present invention there is provided a transition from a ground backed coplanar waveguide (GCPW) including a hot conductor formed on a ground backed insulating substrate and having parallel top ground planes on either side, to a ground backed microstrip transmission line formed on an insulating substrate, the hot conductor of the GCPW and the hot conductor of the microstrip transmission line being connected together, there being vias extending through the insulating substrate and forming an electrical connection between the top ground planes and the

ground backing, characterised in that the transition from the GCPW structure to the microstrip structure is such that the characteristic impedance of the feed-line is a substantially constant 50Q along it's length within the operating frequency range such that broadband operation is possible.

The present invention also provides a transition from a ground backed coplanar waveguide (GCPW) including a hot conductor formed on a ground backed insulating substrate and having parallel top ground planes on either side, to a ground backed microstrip transmission line formed on an insulating substrate, the hot conductor of the GCPW and the hot conductor of the microstrip transmission line being connected together, there being vias extending through the insulating substrate and forming an electrical connection between the top ground planes and the ground backing, characterised in that the ends of the parallel top ground planes are rounded so that the transition from the GCPW structure to the microstrip structure is such that broadband operation is possible.

The present invention yet further provides a transition from a ground backed coplanar waveguide (GCPW) including a hot conductor formed on a ground backed insulating substrate and having parallel top ground planes on either side, to a ground backed microstrip transmission line formed on an insulating substrate, the hot conductor of the GCPW and the hot conductor of the microstrip transmission line being connected together, there being vias extending through the insulating substrate and forming an electrical connection between the top ground planes and the ground backing, characterised in that the connection between the hot conductor of the microstrip transmission line and the hot conductor of the GCPW is tapered from a narrower width in the coplanar structure to a

wider width in the microstrip structure and in that the ground planes of the coplanar waveguides are splayed outwardly away from the hot conductor in the region of the taper, the separation increasing as the width of the taper increases so as to maintain a characteristic impedance of 50Q across the operational bandwidth of the device so that the transition from the GCPW structure to the microstrip structure is such that broadband operation is possible.

The coplanar waveguide may also have a ground backing.

The connection between the hot conductor of the microstrip transmission line and the hot conductor of the GCPW may be tapered from a narrower width in the GCPW structure to a wider hot conductor in the microstrip transmission line structure The coplanar ground planes may be splayed outwardly away from the hot conductor in the region of the taper, the separation increasing as the width of the taper increases.

The hot conductor of the GCPW and the hot conductor of the microstrip transmission line may be integral.

The coplanar waveguides may terminate in rounded ends and the rounded ends may be located over and electrically connected to vias. The rounded ends may follow the plan outline of the vias as closely as can be fabricated.

The transition may be incorporated in a circuit and be located between an MMIC and an RF feed through bead. The MMIC and the transition may be hermetically sealed in a box. The box may be of metal and the ground backings may be electrically connected to the box.

There may additionally be a photoelectric device within the box and the MMIC may be an amplifier for the photoelectric device.

Description of the Preferred Embodiments of the Invention The invention will now be described with reference to the accompanying drawings of which :- Figure 1. plan cross section of an optical module Figure 2. is a vertical cross section of the module of Fig 1, Figure 3. is a cross section of a feed through showing the external connector, Figure 4. is cross section along the line IV-IV of fig 3, Figure 5. is a schematic representation of a push-on fitting Figure 6 is a perspective view of a transition, Figure 7. is an enlarged scrap view of an alternative end of an MMIC.

Figure 8. is plan view of an alternative form of transition, Figure 9. is sectional view of the transition of Fig 8 along the line IX-IX, Figure 10 is a plan view of a transition in accordance with the invention, Fig 11 is a sectional view of Fig 10 along the line XI-XI, Fig 12 is a plan view of a connection between the transition of Fig 10 and a MMIC, and Fig 13 is a perspective view of the transition shown in plan view in Fig 12.

Referring to Figs 1 and 2 these show a general view of an optical module 1 in which modulated light enters on an optical fibre 2 and the light is converted into an electrical signal on line output 3 that is connected to an external RF connector 3 1.

Within the metal box 4, which is hermetically sealed by a lid 5, there is essentially a lens 6 which focuses the light onto a photo-diode 7. This generates a signal responsive to the incoming light and this signal is amplified by a monolithically integrated amplifier (MMIC) 8. The output from the MMIC can vary in frequency from 40kHz to 40GHZ and this signal has to be fed to the output line 3. On either side of the MMIC are dc bias control circuits 9,10 and the output of the MMIC is conventionally connected to the external connector 31 via an RF feed through bead 30 by a transmission line illustrated generally by 11.

It is not a simple matter to interconnect the MMIC and the output line 3. The overall dimensions of the module must be kept as small as possible, typically 12 mm square, because the module is mounted onto a circuit board comprising the optical telecommunication system and as a result there is little room in the module. Due to the need to keep the module hermetically sealed, the electrical feed into the module is via standard RF feed-through means as shown in Figs 3 and 4. The RF feedthrough has a metal conductor 12 sealed into a glass ring 13, which in turn has a metal outside sheath 14.

Connected, as a sliding fit into the conductor 12, is a sleeve 15 carrying a tab 16, which is typically soldered or epoxied to the hot conductor of a microstrip transmission line 17. The term hot conductor as

used herein refers to the conductor within a transmission line system to which the RF signal is applied to. The microstrip transmission line 17 is laid down on an alumina substrate 18 and below the substrate 18 is a lower ground plane 20, which in turn is electrically connected to the metal box 4 to complete the circuit.

These types of modules are used in son characteristic impedance circuits and it has now been found that for maximum broadband usage the impedance of the transition in the transmission line should be maintained at 50n along its length at all times.

In prior art transition systems this was not appreciated and not provided.

With a ground backed microstrip connector as shown in Fig 6, where there is a microstrip transmission line consisting of a hot conductor 50 on an insulating alumina substrate 51 and a lower ground plane electrode 52. the width of the hot conductor of the microstrip transmission line determines the impedance of the transmission line. To provide a characteristic impedance of 50Q when used on a 254um thick alumina substrate, which has a relative dielectric constant Er-10, the hot conductor has to be 240um wide. In Fig 5 the tab 16 is typically 150um wide and the conductor 12 is cm in diameter, thus it is possible to mechanically connect to the microstrip transmission line to the RF feedthrough.

Typically the connection terminals on the output of the MMIC are in the form of ground backed coplanar waveguide (GCPW) pads. These pads are conventionally only used for the testing of the MMIC and not for

it's operational electrical connection. This is most clearly seen in Figs 6 and 7 which show a GCPW and MMIC respectively. In Fig 6 the MMIC has a central, hot conductor pad 53 between a pair of parallel top ground plane conductors 54 and 55. These ground plane conductors are connected to the lower ground plane 63 by means of vias 60 and 61. The ground planes 52 and 63 are commonly connected to ground via the package 4.

In Fig 7 the top ground plane pads 54 and 55 are connected through the semi-insulating layer 64 by electrically conducting vias 60 and 61 which extend through the semi-insulating layer 64 and connect the lower ground plane 63 with the top ground plane conductors.

When located on a common substrate a ground backed coplanar waveguide has better performance than a microstrip transmission line in that it has impedance which is less frequency dependant. The GCPW structure results in less interpenetration of the substrate by the electric fields and thus at high frequencies electric field crowding effects under the hot conductor are reduced thus reducing dispersive effects.

As a consequence there has been developed a transmission feed-line structure in which the majority of the conductor region is a GCPW. This is shown in Figs 8 and 9. In these Figures the microstrip transmission line 80 extends over only a relatively short part of the transmission line and the portion 90 of the transmission line is a GCPW. The width of the hot conductor 91 in the GCPW has to be less than the width of the hot conductor microstrip transmission line 80.

The characteristic line impedance of the GCPW is determined by the ratio of the spacing of the ground plane conductors to the width of the hot conductor. For an alumina substrate with Er - 10 and a thickness of 254um, to maintain a characteristic line impedance of 50Q, a ratio of typically 1.86 is required. Hence for a hot conductor width of lOOum to 120um such as is suitable for wire bonding to the MMIC electrode pads,

whilst still maintaining a compact design, the spacing of the ground plane conductors will be in the range 186um to 223um.

There thus has to be a connection between the two hot conductors 80 and 91 (even if in practise the two conductors are actually integral one with the other). The best form of which was, before the present invention, as shown at 96 and 98. The top ground conductors are formed with laterally extending wings 96a and 98a and connected with vias 100 along the length of the conductors and through the wings 96a and 98a.

The transmission feed-line of the invention is shown in Figs 10 to 13. It is fabricated on a ceramic alumina substrate 120 using conventional techniques and utilises a very short length of microstrip transmission line 121 allowing connection to the feed-through bead in the package wall. A smooth tapered transition 122 to a narrow geometry ground backed coplanar waveguide (GCPW) 123 then follows and this minimises the introduction of parasitic reactances occurring in series with the hot conductor of the transmission feed line and furthermore prevents any higher order modes from being generated. The low dispersion GCPW then provides a high frequency transmission line to the MMIC where bond wires 200, 201, and 202 provide connection to the MMIC's RF output pads 53,54 and 55. Although the output pads 54 and 55 are connected with vias to the ground plane 63 and the ground plane 63 is

connected to the ground plane 205 through the package 4, the bond wires 201 and 202 provide additional electrical integrity to the connection. The design has minimal deviation from linear insertion phase and a low insertion loss, over a wide frequency range.

The top pattern of the transmision feed-line showing the plated via

holes used is shown at 130 and 131. The substrate is standard 99. 6% pure alumina and is typically 254pm thick. The metalisation of the top and bottom conductors uses a NiCr/TiW/Au scheme with thicknesses lOnm, 20nm and 5um respectively. The via holes provide connection from the bottom ground plane to the top ground plane and are all 300u. m diameter plated vias to provide a low inductive path.

The transmission line is designed for a 50n system and therefore the connection to the MMIC is made using the ground backed coplanar waveguide (GCPW) with characteristic impedance of 50n. The geometry used to provide the 50Q GCPW line is displayed in Figure 10.

The ground to ground spacing 140, is made less than one tenth of the wavelength of the RF signal at 40GHz propagating along the GCPW.

This ensures that only a single quasi-Transverse Electro-Magnetic (TEM) mode propagates on the GCPW across the frequency range of interest and has approximately zero dispersion.

Continuous plated vias 141 to 145 and 141 a to 145a exist along the length of GCPW to provide the connection between the top metal GCPW ground planes and the lower ground plane. These are closely spaced to maintain a continuous ground potential along the length of the line and to ensure that resonances do not occur in the frequency range of interest.

The continuous grounding combined with the narrow GCPW geometry

makes the top ground planes the main ground coupling strips for the transverse electric field and hence minimal grounding occurs to the lower ground plane on the substrate.

The GCPW undergoes a transition to the microstrip transmission line which allows connection to the RF feed-through bead. The

microstrip transmission line is 240um wide to produce a characteristic transmission line impedance of 50Q, so to match the impedance of the 50Q RF package feed-through bead to the 500. external RF connector.

The transition between the GCPW and microstrip transmission line is performed by using a symmetrical linear transition for the central hot conductor, where the width changes from that required for the GCPW to that required for the microstrip transmission line. The GCPW top ground planes are curved away from the central hot conductor to maintain a constant line impedance of 50Q along the length of the transition.

When the central conductor width is that required for a son microstrip transmission line the GCPW ground planes are sufficiently spaced from the central hot conductor to prevent any interaction with the electric fields associated with the microstrip transmission line.

The top GCPW ground planes have vias 145 and 145a at the end of the transition to ensure that the ground plane signals from the top ground plane of the GCPW are returned to the lower ground plane. The vias are aligned with the end of the linear transition from GCPW to microstrip transmission line to ensure that the path to ground is inline with the change to a microstrip transmission line. Furthermore the ground planes are narrowed at the ends and curved round the vias to guide the ground signals down into the vias. These features ensure that the ground signals

are directed to the lower ground plane, minimising any back reflections that could result in the propagation of other waveguide modes, that could cause increased loss and possible resonances occurring.

The length of the transition is kept short (approximately a quarter of the wavelength of the RF signal at 40GHz propagating along the GCPW) to minimise the formation of higher order propagation modes. Transitions occurring in less than a quarter of the wavelength of the RF signal at 40GHz propagating along the GCPW will act as discontinuities.

The invention provides a low loss (-0. 55dB at 40GHz), and low dispersion (minimal deviation from linear insertion phase is 3. 5 at 40GHz), broadband (40kHz-40GHz) transmission feed-line for the routing of electrical microwave signals to the outside of 40Gbit/sec optical module packages. By removing the conventional abrupt transitions it minimises any discontinuities in the conductors that give rise to parasitic inductances, radiation loss (increase loss) and surface-wave loss/propagation that will cause higher order modes to be generated that give rise to a reduced insertion phase performance.

The use of GCPW to feed the signal along the majority of the length, offers the advantage over conventional microstrip lines. Not only due to the reduction in phase deviation, but in addition the ground planes shield any radiation loss from the central conductor (which on GCPW is thinner again reducing the radiation loss) therefore minimising the possibility of box resonances occurring within the package.

It will be appreciated that although the transition from one transmission line structure to another is shown as part of an optical module, it could be

used to make a connection to a MMIC which was not connected to an optical component. For example the transition could be used to connect a hermetically sealed MMIC to an RF line where no optical components are incorporated in the hermetically sealed container. The container itself may not be hermetically sealed if that is not required, and the transition could then be used to make an RF connection to a MMIC or other similarly functioning component.

Claims

1. A transition from a ground backed coplanar waveguide (GCPW) including a hot conductor formed on a ground backed insulating substrate and having parallel top ground planes on either side, to a ground backed microstrip transmission line formed on an insulating substrate, the hot conductor of the GCPW and the hot conductor of the microstrip transmission line being connected together, there being vias extending through the insulating substrate and forming an electrical connection between the top ground planes and the ground backing, characterised in that the transition from the GCPW structure to the microstrip structure is such that the characteristic impedance of the feed-line is a substantially constant 500 along it's length within the operating frequency range such that broadband operation is possible.

2. A transition from a ground backed coplanar waveguide (GCPW) including a hot conductor formed on a ground backed insulating substrate and having parallel top ground planes on either side, to a ground backed microstrip transmission line formed on an insulating substrate, the hot conductor of the GCPW and the hot conductor of the microstrip transmission line being connected together, there being vias extending through the insulating substrate and forming an electrical connection between the top ground planes and the ground backing, characterised in that the ends of the parallel top ground planes are rounded so that the transition from the GCPW structure to the microstrip structure is such that broadband operation is possible.

3. A transition from a ground backed coplanar waveguide (GCPW) including a hot conductor formed on a ground backed insulating substrate and having parallel top ground planes on either side, to a ground backed microstrip transmission line formed on an insulating substrate, the hot conductor of the GCPW and the hot conductor of the microstrip transmission line being connected together, there being vias extending through the insulating substrate and forming an electrical connection between the top ground planes and the ground backing, characterised in that the connection between the hot conductor of the microstrip transmission line and the hot conductor of the GCPW is tapered from a narrower width in the coplanar structure to a wider width in the microstrip structure and in that the ground planes of the coplanar waveguides are splayed outwardly away from the hot conductor in the region of the taper, the separation increasing as the width of the taper increases so as to maintain a characteristic impedance of 500 across the operational bandwidth of the device so that the transition from the GCPW structure to the microstrip structure is such that broadband operation is possible.

4. A transition as claimed in claim 1 or 2, further characterised in that the connection between the hot conductor of the microstrip transmission line and the hot conductor of the GCPW is tapered from a narrower width in the GCPW structure to a wider hot conductor in the microstrip transmission line structure.

5. A transition as claimed in claim 4 further characterised in that the coplanar ground planes are splayed outwardly away from the hot

conductor in the region of the taper, the separation increasing as the width of the taper increases.

6. A transition as claimed in any one of the preceding claims further characterised in that the hot conductor of the GCPW and hot conductor of the the microstrip transmission line are integral.

7. A transition as claimed in claim 2,3 or 4 further characterised in that coplanar waveguides terminate in rounded ends.

8. A transition as claimed in claim 7 further characterised in that the rounded ends are located over and electrically connected to vias.

9. A transition as claimed in claim 7 or 8 further characterised in that the rounded ends follow the plan outline of the vias as closely as can be fabricated.

10. An electronic circuit incorporating a transition as claimed in any one of claims I to 9 located between an MMIC and an RF feed through bead.

11. A circuit as claimed in claim 10 in which the MMIC and the transition are hermetically sealed in a box.

12. A circuit as claimed in claim 11 in which the box is a metal box and the ground backings are electrically connected to the box

13. A circuit as claimed in claim 11 or 12 in which the MMIC is an amplifier amplifying an electrical output of a photo-electric device also contained within the box.

14. A transition from a ground backed coplanar waveguide including a hot conductor formed on an insulating substrate and having parallel top ground planes on either side, to a ground backed microstrip transmission line formed on an insulating substrate substantially as herein described with reference to and as illustrated by Figs 10 to 13 of the accompanying drawings.