I have 75 Ohm connector connected to 75 Ohm Load trough 50 OHM PCB to PCB connector. What trace impedance should be on PCBs to get optimal Return Loss?
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2\$\begingroup\$ tell something about the frequency or frequency band and how long is the cable. The mismatch can cause substantial reflection or nothing measurable depending on how long is the cable as wavelengths. \$\endgroup\$– user136077Commented Jan 31, 2017 at 22:32
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\$\begingroup\$ in theoretical terms, impedance matching can be done or your can choose the median = 62 Ohms but the goal should be what Return loss can you tolerate? \$\endgroup\$– D.A.S.Commented Jan 31, 2017 at 23:22
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I guess you have
75 Ohm connector -- 50 Ohm PCB connector -- Zo Ohm PCB trace -- 50 Ohm PCB connector -- 75 Ohm connector -- 75 Ohm load
It really depends on your center frequency. You should clarify this. However, assuming a) that your working frequency is low enough so that the connector length is negligible (let's say up to some 100's of MHz) b) AND that the PCB trace is longer than the connectors (longer than some cm) then it would make sense to make the PCB trace 75 Ohm. The small sections of 50 Ohm transmission line would give small reflections.
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\$\begingroup\$ 75 Ohm connector -- 1 inch pcb trace -50 Ohm PCB to PCB connector --1inch PCB trace -- 75 Ohm load, with Return Loss better than 10dB up to 3GHz \$\endgroup\$– AnjayCommented Feb 2, 2017 at 2:37
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\$\begingroup\$ If the first pcb trace is 75 Ohm it plays no role. Then you should compute or measure how the 75 source impedance is seen after the lenght of the 50 Ohm connector. The second PCB trace is not far from lambda/2, depending on the dielectric constant and to a lesser extent on trace width (or impedance). So your load impedance ZL will be seen as ZL irrespective on Zo to a first order. Of course, if you have detailed data, you can simulate this as a function of Zo \$\endgroup\$– PetrusCommented Feb 3, 2017 at 15:35