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I have some accelerometer vibration sensors that pass into a cabinet. They use BNC type bulkheads.

Several of the bulkheads got damaged. The original bulkheads are 50 ohm and the coax cables on either side are also 50 ohm. I can only readily get 75 ohm replacements. The signal coming from the accelerometer is less than 2 kHz.

What is the effect of this mismatch?

My thought is that, because the signal is solidly below Radio Frequencies, the coax should behave as wire and not as a transmission line. Because of this, the mismatch is irrelevant.

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  • \$\begingroup\$ the signal is less than 2kHz in all its frequency components? (to clarify with an extreme example: a 2kHz square wave will show you ringing like there is no tomorrow) \$\endgroup\$
    – Sredni Vashtar
    Commented Nov 21, 2020 at 14:05
  • \$\begingroup\$ @SredniVashtar It is possible that there are higher harmonics that I'm not aware of. The signal is filtered with a low pass in the DAQ with a cutoff 2kHz. \$\endgroup\$
    – vini_i
    Commented Nov 21, 2020 at 14:07
  • \$\begingroup\$ Be aware that while the RF mismatch IS insignificant the pin diameter is different and (if too small) may make intermittent contact or (if too large) may force the socket damaging it. I can never remember which way round causes damage. \$\endgroup\$
    – user16324
    Commented Nov 21, 2020 at 14:10
  • \$\begingroup\$ if you have an oscilloscope and a reasonably long piece of cable, you can test the setup feeding a sample signal when the cable is terminated with 50 ohm and then when it's terminated with 75 ohms. (edited: the fast rise oscillator is required to measure the characteristic impedance - here you just need to check what happens with your sample signal) \$\endgroup\$
    – Sredni Vashtar
    Commented Nov 21, 2020 at 14:13
  • \$\begingroup\$ I just realized you just changed the connectors, not the cable... I misread the question. \$\endgroup\$
    – Sredni Vashtar
    Commented Nov 21, 2020 at 14:21

1 Answer 1

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What is the effect of this mismatch?

At 2 kHz there will be no problem. At 20 MHz I doubt there would be much of a problem either.

My thought is that, because the signal is solidly below Radio Frequencies, the coax should behave as wire and not as a transmission line. Because of this, the mismatch is irrelevant.

I agree.

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  • \$\begingroup\$ Thanks for the confirmation. \$\endgroup\$
    – vini_i
    Commented Nov 21, 2020 at 14:05
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    \$\begingroup\$ Note that 75 Ω BNC and 50 Ω BNC connectors are physically different, and while you can force the connectors together it may cause damage to them. \$\endgroup\$
    – Hearth
    Commented Nov 21, 2020 at 14:05
  • \$\begingroup\$ @Hearth well said! \$\endgroup\$
    – Andy aka
    Commented Nov 21, 2020 at 14:07
  • \$\begingroup\$ @Hearth Good note, will check. \$\endgroup\$
    – vini_i
    Commented Nov 21, 2020 at 14:08

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