I have a detector with a preamp that has an output impedance of \$50\Omega\$. My oscilloscope has an input impedance of \$1\rm{M}\Omega\$ and capacitance of 16pF. The signals I'm interested in are on the order of 10MHz. What is the effect of my oscilloscope not being matched to the preamp?

  • \$\begingroup\$ What will happen? Higher frequencies will have noticeable reflections, use a 50 Ohm resistor from core to shield and measure across it. If you need this often, you can get 50 ohm termination adapters or use an op-amp with 50 ohm input impedance. \$\endgroup\$ – C. Towne Springer Jan 12 '14 at 3:59
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    \$\begingroup\$ I would be very surprised if the oscilloscope didn't have as setting where the port impedance can be switched to 50R. Most oscilloscopes have this facility even ones dating back to the 1970's. Use of a 1:1 probe and the 50R setting should provide a suitable measurement match. \$\endgroup\$ – Martin Jan 12 '14 at 8:50
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    \$\begingroup\$ @Martin Many (most?) entry-level oscilloscopes do not have a 50Ω setting even today. \$\endgroup\$ – AndrejaKo Jan 12 '14 at 8:51
  • \$\begingroup\$ In that case a pi network will bridge the gap however the transformation is quite extreme. Best to simply terminate it in 50R and measure across it. 16pF will have a negligible effect on impedance mismatch at 10MHz. \$\endgroup\$ – Martin Jan 12 '14 at 8:56
  • \$\begingroup\$ So I guess it also depends on the length of the cable from the amplifier to the scope? If a short cable is being used, then there are still reflections but they might not matter? \$\endgroup\$ – cpc333 Jan 12 '14 at 16:15

As mentioned in one of the comments if the cable is short there will be very little effect at 10MHz - short means that the cable is much less than one wavelength. For example the delay through 1 meter of cable will be ~5ns, the reflection will get back to the amplifier in another ins giving a 10s round trip. This is 10% of the 10MHz upper limit that you have.

The gain of the amplifier will be double the calibrated value (i.e. a 20dB amplifier will have a voltage gain of 26dB in this condition).

The rolloff of 16pF and 50Ohms will be 3dB @ 200MHz so nothing to worry about.

If the scope does not have a 50 ohm input selection and you want to terminate the cable correctly I would use a BNC T connector plugged into the scope with a 50 Ohm terminator in one of the ports and the input signal into the other.


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