GND
Signal
GND
Fig. 1: Cross-section of a trace of the test board
Mini Coax connector integrated in test boards
Transmission characteristics of a connector can only be measured within its typical environment. The environment for a board-to-board connector is generally a pcb (printed circuit board). The approach to determine the transmission characteristics of the connector itself is to measure the complete arrangement with special calibration techniques and to remove the parameters of the connector. The performance of the test board is responsible for the accuracy of the parameters on the connector itself.
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GND |
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Fig. 1: Cross-section of a trace of the test board |
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The signal integrity of a test board is mainly influenced by the characteristic impedance, losses of pcb traces and the design of micro vias. It's obvious that the pin in hole of the connector and the vias have to be considered as one. This means that the thickness, material and the amount of layers of the pcb have to be taken into consideration for the characterisation of a board-to-board connector.
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Fig. 2 shows the test boards with the integrated Mini Coax connector. The test boards are executed in stripline technology with a characteristic impedance of 50 Ω. To connect the test boards with the cables of the measurement instrument, SMA connectors are used. |
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Fig. 2: Test boards with implemented Mini Coax connector |
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For high speed digital applications the main parameters are defined in the time-domain, like reflection loss, rise-time degradation, eye-opening etc. For RF-applications like in radio base stations or broadcasting services the frequency-domain is the preferred point-of view in order to characterise the systems and the components in regards to return loss, insertion loss, isolation, linearity etc.
Fig. 3 shows the signal path given by the Mini Coax connector and the components on the evaluation board. Significant areas in the signal path are indicated with the characters A...G and T for traces on the board.
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Single |
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SMA
Mini
Mini |
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Fig. 3: Signal path of a board to board Mini Coax connector (SMA connectors used for measurement device connection) |
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The signal path was measured with a Time Domain Reflectometer (TDR) to determine the discontinuities of the arrangement.
Fig. 4: TDR profile
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Legend |
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A |
SMA connector |
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T1 |
50 Ω trace of the backplane |
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B |
Pin in hole of the straight Mini Coax module |
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C |
Section of straight contact behaves capacitive |
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D |
Transition of the straight module to the angled module behaves inductive |
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E |
Section of angled contact behaves capacitive |
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F |
Pin in hole of the angled Mini Coax module |
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T2 |
50 Ω trace of the daughtercard |
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G |
SMA connector |
Figure 5 shows the equivalent circuit of the signal path with grouped elements for the Mini Coax connector. The parameters had been extracted from the measured TDR profile.
Fig. 5: Equivalent circuit with grouped elements
Insertion and return loss
Fig. 6: Insertion loss according to Fig. 2
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gated |
Fig. 7: Return loss according to Fig. 2
To extract the parameters of the connector itself the method of gating was selected. This eliminates the effects of SMA launchers. The test launchers reduce the performance of transmission characteristics due to their discontinuities.
Near-end crosstalk of the Mini Coax modules
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source line: 2, affected line: 1, contact
distance x = 4.40 mm |
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Even for the shortest distance of 4.40 mm crosstalk-values better than 30 dB can be achieved for frequencies of 2.5 GHz and above. |
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Fig. 9: Measured configurations [mm] |
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