Printed circuit integrated broadband directional bridge

There are provided methods and devices for realizing a broadband directional bridge, and a network analyzer test set based on the directional bridge structure. In some embodiments the directional bridge comprises a resistive bridge and a balun. The balun further comprises a transmission line which is surrounded by ferrite material and is implemented on a main printed circuit board (PCB).

INCORPORATION BY REFERENCE

FIELD OF THE INVENTION

The present invention relates to the field of test bridges for network analyzers, more particularly to a broadband directional bridge for vector network analyzers.

BACKGROUND

Directional couplers are passive microwave components used for power division between transmission lines in a network. The directional property enables to couple power propagating in a certain direction. Directional couplers are a common building block in microwave systems.

There are numerous designs of directional couplers, such as coupled line sections (i.e. ¼ wavelength, multi-section or tapered), resistive bridges, Bethe hole couplers and many more. Implementation of such directional couplers may be on PCB (Printed Circuit Board), ceramic, metal structures etc. These designs are not amenable to broadband operation encompassing very low frequencies in view of the long wavelength, resulting in large dimensions and high loss at the high frequencies.

Another well-known structure for directional couplers is based on transformers. Transformers with magnetic core allow broadband operation over a range of 1:100-1:1000 in frequency, however it does not perform adequately in the microwave range in view of the deterioration in performance of magnetic components.

A directional bridge is a specific circuit implementing directional coupler functionality, used in extremely broadband applications, such as in Vector Network Analyzers test sets. A well-known design of a directional bridge is a combination of a resistive bridge and a balun structure composed of a coaxial transmission line surrounded by a ferrite material (usually the ferrite material takes the form of ferrite beads). Such structure is described in an article by Joel Dunsmore of Hewlett-Packard in “RF Design” journal of Nov. 1991, pp. 105-108, “Simple SMT Bridge Circuit Mimics Ultra-Broadband Coupler”. In VNA applications, such ferrite-based directional bridges are typically used in pairs, back-to-back, one taking a sample of the transmitted source signals, and one taking a sample of reflected or incoming signal. An example of such a dual directional bridge scheme is illustrated in U.S. Pat. No. 4,962,359 to Dunsmore entitled “Dual directional bridge and balun used as reflectometer test set”. In this patent, the two directional bridges share a common structure of a coaxial transmission line surrounded by ferrite material.

The main difficulty in realizing this design is connecting the coaxial transmission line segment (the balun) to the carrier (typically a PCB) of the resistive bridge circuit. The area needed for soldering to the PCB both the center conductor of the coaxial transmission line and the shield of the coaxial transmission line induces parasitic impedance to the ground, resulting in difficulty achieving good impedance matching at higher RF frequency (which influences the directional bridge performance).

Moreover, the use of coaxial transmission line soldered to a carrier PCB poses complexity in the production process.

It would therefore be desirable to provide an improved, cheap and easily producible device not hinging on coaxial transmission lines.

It would be further desirable to provide improved, and low cost integrated PCB-based directional bridge that overcomes at least some of the aforementioned problems with the prior art.

SUMMARY OF INVENTION

Prior to the summary of the invention being set forth, it may be helpful to set forth definitions of certain terms that will be used hereinafter.

As used herein the term “directional bridge” is used with respect to a combination of a resistive bridge and a broadband balun, to describe a device that is designed to pass radio frequency signals in both directions, while having an additional port in which the output signal is proportional to the signal flowing in one direction only.

As used herein the term “main PCB” refers to a printed circuit board on which the main system (e.g., primarily, a network analyzer or a network analyzer test set) is implemented.

As used herein the term “dedicated PCB” is an additional printed circuit board implementing primarily the transmission line of the directional bridge, so that it is mounted on the “main PCB”

As used herein the term “common PCB based transmission line” refers to the case that there are two back-to-back directional bridges and the transmission line segments are a continuation of each other and they are implemented by a single common transmission line.

The term “Vector Network analyzer (VNA)” as used herein and through the specification and claims should be understood to encompass an electrical device used to generate and transmit RF signals and to measure the ratios between the received RF signals and the transmitted one. Those ratios represent the reflection and transmission coefficients of the tested port.

The term “resistive bridge” as used herein and through the specification and claims should be understood to encompass an electrical circuit with 4 nodes ordered in a circle, with four resistances (impedances) between adjacent nodes, and sources/sinks connected to opposing nodes, such as in Wheatstone bridge. This type of electrical circuit is used to characterize electrical impedances.

The term “balun” as used herein and through the specification and claims should be understood to encompass a device that joins a balanced line (one that has two conductors, with equal currents in opposite directions, such as a twisted pair cable) to an unbalanced line (one that has just one conductor and a ground, such as a coaxial cable). In the context of the use of a balun in a directional bridge it is further desired that the balanced end has a high common-mode impedance with respect to ground (being “floating”).

In one aspect, a directional bridge comprising: a resistive bridge and a balun, wherein the balun comprises a transmission line, said transmission line is surrounded by ferrite material, and wherein the transmission line is implemented on a main printed circuit board (PCB).

In many embodiments, said transmission line is implemented on a dedicated PCB and wherein the dedicated PCB is mounted or assembled on said main PCB.

In many embodiments, a resistive bridge is mounted or embedded on said dedicated PCB.

In many embodiments, said main PCB comprises at least one cutout.

In many embodiments, at least one ferrite material surrounds said transmission line through the said at least one cutout.

In many embodiments, said transmission line is part of at least one coupled-line directional coupler.

In many embodiments, the directional bridge is part of a network analyzer test set.

In another aspect, a dual directional bridge comprising two directional bridges wherein said two directional bridges are connected back-to-back and wherein said two directional bridges comprise: a resistive bridge and a balun, the balun comprises a transmission line, wherein said transmission line is surrounded by ferrite material, and wherein the transmission line is implemented on a printed circuit board (PCB).

In many embodiments, the two back-to-back directional bridges share a common PCB-based transmission line.

In many embodiments, the shared common PCB-based transmission line is a dedicated PCB that is mounted on a main PCB.

In another aspect, a network analyzer test set comprising a dual directional bridge, said dual directional bridge comprises: two directional bridges wherein said two directional bridges are connected back-to-back and wherein said two directional bridges comprise: a resistive bridge and a balun, the balun comprises a transmission line, wherein said transmission line is surrounded by ferrite material, and wherein the transmission line is implemented on a printed circuit board (PCB).

In many embodiments, the two back-to-back directional bridges share a common PCB-based transmission line.

In many embodiments, the shared common PCB-based transmission line is a dedicated PCB that is mounted on a main PCB.

DETAILED DESCRIPTION

The present invention relates to the field of directional bridges, which are specific circuits implementing directional coupler functionality and are typically used in extremely broadband applications, such as in Vector Network Analyzers test sets.

In accordance with some embodiments there is provided a directional bridge, structured or mounted on a circuit board such as a multilayer PCB. The directional bridge comprises a resistive bridge and a balun. The balun further comprises a transmission line, such as a stripline which is embedded in at least one of the layers of the multilayer PCB.

In some cases, the directional bridge may be completely implemented on a PCB which may contain additional circuits (e.g. a main PCB), or it may be a standalone dedicated PCB connected (e.g. soldered) to a main PCB, using for example standard assembly techniques such as surface mount technology (SMT). In either case, the directional bridge may be a single directional bridge, or a dual back-to-back directional bridge structure sharing for example the same balun's transmission line.

In accordance with configurations there are provided methods and a directional bridge comprising significant improvement in matching (e.g. return loss and insertion loss), cost and yield.

Reference is made toFIG. 1A-1Eillustrating a directional bridge100in accordance with some embodiments.FIG. 1Ashows an isometric view of a main PCB carrying a dedicated PCB on which a balun and a resistive bridge are implemented, surrounded by ferrite material and covered by metallic cover fixtures.FIGS. 1B-1Dshow cross section views in the X-Z plane, Y-Z plane, X-Y plane respectively of the main PCB carrying the dedicated PCB.

FIG. 1Eshows a schematic block diagram of the directional bridge100as implemented on a dedicated PCB, installed on a main PCB.

Specifically, the bridge100comprises a dedicated circuit board such as dedicated PCB101mounted (e.g. soldered) on a main circuit board such as main PCB103. A balun's108transmission line (e.g. shielded stripline) is implemented on or is part of the dedicated PCB101. A resistive bridge102is mounted on or part of the dedicated PCB101, thus avoiding the distance (and the parasitic effects) between the transmission line's end and the resistive bridge's components. Ferrite material, such as one or more ferrite beads104, surround the dedicated PCB101providing the high common-mode impedance at the balun's balanced end. A cut-out106of appropriate size in the main PCB103provides the place required for accommodating the ferrite material surrounding the dedicated PCB101.

A cover structure105may be attached to the main PCB103covering the components of the bridge100, e.g. the dedicated PCB101and the ferrite beads104. The cover structure105is used for affixing the ferrite material104and preventing their relative movements during shock and vibration. Additionally, the cover structure105is used to provide electrical shielding for avoiding undesired coupling of electromagnetic signals to external components and structures.

The dedicated PCB101is connected (e.g. soldered) to the main PCB103using standard assembly techniques such as surface mount technology (SMT).

Due to the use of stripline technology, this mounting no longer depends on the soldering workmanship, as was the case in prior art embodiments using coaxial transmission line, resulting in consistent accuracy and performance based only on EM simulation and PCB manufacturing process tolerances.

The physical dimension of the directional bridge is frequency dependent. In some embodiments, a broadband directional bridge covering 3-10 GHz may be about 5 cm long, and requiring about 3-8 ferrite beads. In some embodiments the resistive bridge102may be assembled on the main PCB, instead of on the dedicated PCB.

Reference is made toFIG. 2illustrating PCB200, in accordance with embodiments. PCB200may be manufactured according to a method where the main PCB203and the dedicated PCB201are produced on the same (e.g. single) PCB200, in accordance with configurations. Following production, the dedicated PCB201is separated into a standalone part along line207and mounted (e.g. soldered) to the main PCB203. Concurrently, a cutout section206required to accommodate the ferrite material surrounding the dedicated PCB is removed/separated as well from the main PCB203.

Reference is made toFIG. 3showing a bridge300in accordance with embodiments. The balun's transmission line301is implemented using stripline technology on a main circuit board such as PCB303together with the resistive bridge302, therefore eliminating the need for soldering two separate PCBs. Cut-outs306of the PCB on both sides of the balun's transmission line make it a “floating” stripline, enabling mounting of split ferrite cores304. Split ferrite cores are used here as the “floating” stripline does not have a “free end” to allow insertion of ferrite beads.

The methods described herein can be used to implement a dual back-to-back directional bridge sharing (as part of their baluns) a common stripline transmission line.

Reference is made toFIG. 4Ashowing a cut-out in the X-Y plane of a dual directional bridge400and toFIG. 4Bshowing a schematic block diagram of a dual directional bridge with input port410, output port411, reflected port412and coupled port413, in accordance with some embodiments. A stripline transmission line may be implemented on or is part of a dedicated PCB401and shared by one or more baluns, such as two baluns408of the two directional bridges. Two resistive bridges402, one for each directional bridge, are mounted on either side of the common stripline transmission line. Ferrite material, such as one or more ferrite beads404, surrounds the dedicated PCB401. The dedicated PCB is mounted (e.g. soldered) to a main PCB403. A cut-out406of appropriate size in the main PCB403enables accommodation of the ferrite material surrounding the dedicated PCB401.

Reference is madeFIG. 5illustrating an assembled dual directional bridge500, shown schematically inFIG. 4, in accordance with embodiments. The balun's transmission line is implemented using stripline technology on a dedicated PCB501, on which the two resistive bridges (not shown) are mounted on either side. The dedicated PCB501is surrounded by ferrite beads504and connected to a main PCB503. An appropriate cut-out506in the main PCB is provided to accommodate the ferrite beads. The complete assembly performs as a dual back-to-back directional bridge: input port510, output port511, reflected port512and coupled port513.

In some embodiments, a dual back-to-back directional bridge sharing (as part of their baluns) a common stripline transmission can be implemented using a single PCB with “floating” stripline.

Reference is madeFIG. 6illustrating an assembled dual back-to-back directional bridge600implemented using a single PCB with “floating” stripline on which split ferrite cores604are mounted in accordance with embodiments. The directional bridge600comprises a balun601and two resistive bridges (not shown) implemented on a PCB603. Cut-outs606of the PCB on both sides of the balun's transmission line make it a “floating” stripline, enabling mounting of split ferrite cores604. The complete assembly performs as a dual back-to back directional bridge: input port610, output port611, reflected port612and coupled port613.

In some embodiments, a directional bridge may be further combined with a coupled-line directional coupler. The concatenation of both devices exhibits lower insertion loss at higher frequencies due to the fact that the coupled-line directional coupler is more effective than the directional bridge at these frequencies. As example, a stand-alone directional bridge with 16 dB coupling ratio may have insertion loss of about 1.5 dB, while a coupled-line directional coupler may have insertion loss of about 0.15 dB. The concatenation of both devices exhibits about 0.25-0.5 dB insertion loss at high frequencies. The coupled-line directional coupler can be incorporated within the same multilayer PCB as the directional bridge balun's transmission line. Further, the use of a stripline technology ensures matching of the odd and even modes phase velocity, resulting in good directivity. Moreover, the transmission line used in the balun of the directional bridge can serve as the through-arm of the coupled-line directional coupler.

Back-to-back directional bridges are typically required in network analyzer test set applications. These can be implemented using two single directional bridges connected back-to-back as per the various embodiments of this invention. Moreover, the techniques described herein can be used to implement a dual back-to-back directional bridge sharing a common stripline transmission line as part of their baluns.

Reference is made toFIG. 7showing a system700comprising of a two-port VNA703and dual back-to-back directional bridges702connected to each port.

The use of directional bridges and VNA may be, for instance, as disclosed in the present invention's applicant US Patent Application Publications US-2015-0212129 entitled ‘VECTOR NETWORK ANALYZER’ and US-2015-0323577 entitled ‘BALANCED BRIDGE’, which are incorporated herein by reference in its entirety.

It is to be understood that the phraseology and terminology employed herein is not to be construed as limiting and are for descriptive purpose only. The principles and uses of the teachings of the present invention may be better understood with reference to the accompanying description, figures and examples.