Wave transmission lines and networks – Coupling networks – Balanced to unbalanced circuits
Reexamination Certificate
1998-04-30
2001-05-15
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Balanced to unbalanced circuits
C333S033000
Reexamination Certificate
active
06232849
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a waveguide to microstrip transition where a waveguide probe is passed from the waveguide through a circuit board before connecting to the microstrip line. The invention also relates to a microstrip to microstrip transition where a signal carried on the microstrip requires to pass through a multilayer circuit board. The invention is particularly related to transitions for use with radio frequency signals.
DESCRIPTION OF THE RELEVANT PRIOR ART
In a double-sided or multilayer circuit board which is connected to a waveguide, one way in which signals may be taken from the waveguide to the board requires a probe to pass through the waveguide wall and the board so that when the probe protrudes into the waveguide it will pick up signals propagating in the waveguide. In order for such an arrangement to work properly it is necessary to connect the probe to a microstrip conductor. This is typically done by drilling a hole in the circuit board prior to etching the board then fitting a probe in the drilled hole. This arrangement would provide a low loss transition as long as there is no or little loss of signal into the board's materials, such as the (polytetrafluoroethylene) and FR
4
(fiberglass resin material) layers, and the probe passing through the board's materials does not present too great a mismatch. However, with this arrangement, the transition is lossy, thereby resulting in over
1
dB. of loss. This is unacceptable for a low-noise receiver. It is believed that the ground planes on each side of the FR
4
act as a parallel plate waveguide into which signals can couple resulting in extra losses and also changing the impedance of the transition causing a mismatch to the microstrip line. Any mismatch will also result in loss of signal.
An object of the present invention is to provide an improved transition which obviates or mitigates at least one of the aforementioned disadvantages.
SUMMARY OF THE INVENTION
In its broadest aspect this is achieved by providing a plurality of plated holes in a circuit board and disposing these plated holes around the RF waveguide or microstrip to microstrip transition and connecting these plated holes to ground. This results in a transition with a substantially improved performance which has much less loss than the prior art arrangement.
In a preferred arrangement all the holes, including the hole receiving the probe are plated through-holes for reasons of simplicity and cost. In an alternative arrangement blind holes can be used The plurality of holes can be provided in a multilayer board or in a double-sided board.
Conveniently, four of these plated through-holes are disposed around the waveguide or microstrip to microstrip probe and are ideally placed on the circumference of a circle centered on the probe. The holes are in close proximity to the feedthrough.
According to one aspect of the present invention there is provided an RF waveguide to circuit transition wherein a waveguide probe passes through a circuit board and is connected to a signal conductor on the circuit board, the circuit board having a ground plane, a plurality of plated holes disposed in proximity to and spaced around the waveguide probe such that the transition impedance is as close as possible to the characteristic impedance of the microstrip line, the plated holes being coupled to the ground plane to minimise leakage of signals carried by the probe into the board. Preferably, the holes are plated through-holes. Alternatively, the holes are plated blind holes. Preferably also the probe hole is plated.
Preferably also, four plated holes are disposed around the periphery of the waveguide probe. Conveniently the plated holes being placed on the circumference of a circle centred on the probe. Alternatively, three, five or any suitable number of holes may be used to achieve satisfactory performance.
It has been found that four such plated through-holes provide satisfactory results, but the spacing of the holes is important in relation to the feed-through, holes requiring to be close in proximity to the feed-through. The required proximity and spacing of the holes is frequency dependent. It is has been found that if the holes are too close, then a transition impedance may be low with respect of the commonly-used characteristic impedance of 50 ohms and if too far away would allow unacceptable leakage into the board. In addition, the spacing of holes in relation to each other is also important as the size of the gap between the plated holes and the frequency of the RF signal dictates how much RF signal can pass between the holes. While various examples of hole size, hole diameter and spacing are given, it will be understood that these parameters may be obtained in particular cases by routine experimentation.
According to another aspect of the present invention there is provided a method of minimising leakage of RF signals from a waveguide to conductor transition passing through a circuit board adjacent the waveguide and in which a probe extends through the circuit board and waveguide wall into the waveguide, such method comprising the steps of,
disposing a plurality of holes in the circuit koard around the periphery of the probe,
plating or otherwise covering the side of each hole with a conductive material, and
connecting the plated holes to a ground plane.
Preferably, said method involves providing through-holes in the circuit board. Alternatively, the method involves providing blind holes in the circuit board.
Conveniently, the circuit board is a multilayer board.
According to another aspect of the invention there is provided a circuit board for receiving a probe for insertion into a waveguide to receive signals therefrom and transfer these signals to the circuit board, the circuit board comprising, at least one conductor for connection to the probe, a ground plane,
a through-hole for receiving the probe,
a plurality of plated holes disposed in the circuit board around the periphery of the plated through hole, the plated holes being connected to said ground, said holes being proportioned and dimensioned so that leakage of signals into the circuit board from the probe is minimised.
Preferably, the through-hole for receiving the probe is plated.
According to another aspect of the present invention there is provided a circuit board transition for minimising leakage of signals carried on microstrip conductors, which signals pass through the circuit boards, said circuit board transition comprising a first microstrip location on one side of the circuit board for carrying a signal, a second microstrip location on the other side of the board for receiving the signal, signal conducting means passing through the board and connected to the first and the second microstrip conductors, a ground plane disposed in the circuit board, a plurality of plated through holes or blind holes disposed in the circuit board about the periphery of the signal conducting means, the plated through holes or blind holes being coupled to said ground plane to minimise leakage of the signal passing through said board between the first and second microstrip.
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Baird Andrew Patrick
Flynn Stephen John
Gifford, Krass, Groh Sprinkle, Anderson & Citkowski, P.C.
Lee Benny
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