Abstract:
Coaxial RF test probe for trouble-shooting microstrip circuits while minimizing power losses and increasing reliability. The present probes include a coaxial cable, and a grounding pin and a circuit pin supported in isolated, closely-spaced relation to each other, communicating with the outer jacket contact and the central contact, respectively, of the coaxial cable. The pins communicate through a DC block with a testing meter to evaluate predetermined areas of a microstrip circuit while providing fixed-distance grounding to closely-spaced plane areas.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improved coaxial RF probe for testing or trouble-shooting microstrip circuits while avoiding the need for adding conventional coupled test ports or resistive network test ports, which test ports degrade the performance of microstrip circuits, require space and increase the cost and weight of the circuit board. 
     2. Discussion of the State of the Art 
     Manufacture and maintenance of a microstrip circuit require that functionality of various stages of the circuit be measurable. In RF circuitry it is conventional to incorporate coupled test ports on resistive network test ports at predetermined critical locations along the microstrip circuit in order to test, measure or trouble-shoot the performance of the circuit at these specific locations. 
     Such test ports have several disadvantages including being localized, expensive, requiring space and adding weight to the circuit board. More importantly, such test ports degrade the operation or performance of the circuit even when no tests are being performed. 
     It is also known to test the operation or performance of a microstrip circuit by means of a coaxial RF probe or sniffer probe having a central circuit contact and an outer coaxial jacket contact having a flexible ground wire designed to be clipped to a ground plane area spaced from the microstrip circuit. A DC-blocking device is associated with the central contact in order to provide a contact which protects the sensitive measurement device, such as a spectrum analyzer, power meter or other similar instrument against direct current damage. 
     While such conventional coaxial RF probes enable testing at any point along a microstrip circuit and avoid the need for test ports on the microstrip circuit, they do not have a reliable grounding system or provide a fixed-distance ground point and therefore their performance is erratic and unreliable. Any coaxial RF probe exhibits high electrical losses, and such losses are unpredictable and variable when the spacing between the microstrip contact and the ground contact is varied. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel, fixed-contact coaxial RF probe which avoids the problems of prior coaxial RF probes by fixing the distance between the circuit contact and the ground plane contact in order to provide reliable and repeatable measurements. The need for test ports, which degrade the performance of the microstrip circuit, is avoided. Also variation in the electrical losses normally encountered with the use of conventional coaxial RF probes having a flexible ground wire, depending upon the arbitrary distance between the ground contact and the microstrip contact, are eliminated so that the losses are predictable and balance each other with repeated measurements. Moreover, the present test probes are easier to use than the prior-known test probes having a flexible wire ground lead which must be clipped or otherwise grounded to a ground plane, in a first operation, prior to the steps of pressing the central test contact against the area of the microstrip circuit being evaluated. 
    
    
     THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a diagrammatic view of a coaxial radio frequency test probe according to a preferred embodiment of the present invention, illustrating the probe in position between a microstrip/ground plane of a printed circuit board and a power measuring device such as a spectrum analyzer, and 
     FIG. 2 is a magnified view of the probe of FIG. 1 taken along the line  2 — 2  thereof. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, the coaxial RF test probe  10  thereof is similar in most respects to a conventional coaxial RF test probe except that a conductive receptacle plate  11  supporting a grounding contact pin  12  is soldered in fixed spacing relative to the coaxial circuit contact section  13  of the probe, in electrical connection with the outer conductive jacket  14  of the semi-rigid coaxial cable  15  of the probe and insulated against electrical communication with the central test contact pin  16  of the coaxial cable  15 . 
     The plate  11  and grounding contact pin  12  replace the flexible grounding wire and clip used on prior known coaxial RF test probes on which the grounding wire is soldered or otherwise fixed in electrical connection with the outer conductive jacket of the coaxial cable, and a terminal clip is soldered to the end of the grounding wire for grounding attachment to variable locations of the ground plane of the microstrip circuit board. 
     The test probe  10  of FIGS. 1 and 2 comprises the semi-rigid coaxial cable section  15  connected by means of an upper jack connector  17  to a conventional coaxial DC block fitting  18  having inside/outside capacitance in series with both the outer conductive jacket  14  and the inner central conductive contact  16  of the cable  15 . This prevents or blocks the flow of DC current to the sensitive meter or analyzer  19  while permitting RF power to flow uninterrupted through a length of flexible coaxial cable  20  to the analyzer  19 . Cable  20  is connected to the probe  10  by means of an outlet jack connector  21 . The DC block fitting preferably is a MDC 1182-A-S18 device commercially-available from MIDISCO, Commack, N.Y. It operates over a frequency range of 0.1 to 18 GHz. 
     The lower end of the semi-rigid coaxial cable section  15  is connected to the circuit contact section  13  of the probe  10  by means of a lower inlet jack connector  22 . The conductive receptacle plate  11  is provided with spaced bores  23  and  24 . Bore  23  is slightly larger in diameter than the diameter of the coaxial circuit contact section  13 , and the receptacle plate  11  is soldered to the underside of the inlet jack connector  22  and to the outer conductive jacket  14  of the coaxial cable, insulated against electrical connection with the central test contact pin  16  of the coaxial cable. Plate  11  preferably is a passivated stainless steel and may be plated, such as with gold, to improve soldering properties. 
     The smaller bore  24  of the receptacle plate  11  has a diameter slightly larger than the diameter of the conductive tubular receptical or housing  25  containing an internal spring  26  for spring-loading the pin  12  in adjustable extension from the housing  25 . The conductive housing  25  preferably is soldered to the plate  11 , within the bore  24 , for grounding the contact pin  12  in electrical connection with the plate  11  and with the outer conductive jacket  14  of the coaxial cable  15 , insulated from the central conductor or pin  16  of the cable  15 . Alternatively, the grounding pin can be soldered to the plate  11 , within the bore  24 , or may be vertically adjustably engaged therewithin, such as threadably-engaged, for vertical adjustment relative to the length of the contact section  13  and the ground plane height. However, the preferred embodiment involves spring-loading the contact pin  12  within the housing  25 , which makes the pin self-adjusting and assures good contact with the grounding plane  27 . 
     The essential novelty of the present test probes is that the circuit contact pin  16  and the grounding contact pin  12  are fixed to the probe in closely spaced relation to each other, whereby electrical losses encountered during use of the probe are minimized and rendered uniform and predictable. Therefore the probe can be calibrated against a standard to allow for the uniform electrical losses. 
     As illustrated by FIG. 1, the close spacing between the circuit contact pin  16  and the grounding pin  12  requires that the ground plane  27  and the microstrip circuit  28  to be evaluated must be closely spaced on the surface of the printed circuit board (PCB)  29 . For example, in a test probe or sniffer for testing 2.0 to 2.5 GH and 50 MHz to 18 GH signals, a preferred spacing between the tips or centers of the pins  12  and  16  is between about 0.2 and 0.3 inch, most preferably about 0.24 inch, which requires that the ground plane and the microstrip are spaced no more than about 0.22 inch. Such probes have been found to encounter power losses between about 10 and 12 dB. 
     It will be apparent to those skilled in the art that the fixed spacing between the grounding pin  12  and the circuit pin  16  can be greater than 0.3 inch, up to about 0.5 inch, but that the greater spacing produces reduced reliability during testing. For example, the receptical plate  11  can be slightly longer in the horizontal direction to increase the spacing between the bores  23  and  24 , or two or more horizontally-spaced alternative bores  24  may be provided in the plate  11  to adapt the probe for alternative use in testing microstrip circuits in which the ground plane is spaced from the microstrip by more than about 0.3 inch, up to about 0.5 inch. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.