# Photomultiplier BNC cable impedance matching

I'm not sure how to connect the 50 ohm resistors to match the BNC cable's 50 ohm impedance. I draw something but it could be different.

• What's the thingy with the blue squiggly lines on with the words "bottom view" written underneath? – Andy aka Nov 7 '15 at 20:59
• How long is your 50 ohm coax? What is your max pulse rate ? What is the width of your pulses ? All these figures determine what the best approach for you is . – Autistic Nov 8 '15 at 8:08
• Autistic, Coax cable (RG58) with BNC-BNC connectors is 1 meter long, max pulse rate is 100k/sec, width is 300 to 400ns (that's with a NaI(Tl) crystal's decay time). – Neutrino Nov 8 '15 at 10:07
• Andy, this is a voltage divider. Each line is a simple resistor, same as can be seen above, but with a resistance between 0.3M and 10M. (Could be e.g. 11x1M. Depends on application and PMT.) – Neutrino Nov 8 '15 at 10:39

Why do you want to amplify the PMTs output? Aren't PMTs supposed to drive 50 ohms at a reasonable level?

Anyways, if you just want to terminate a 50 ohm coax to a recording device (high impedance), just place the resistor to ground and place the recorder in parallel. Also, if your pmt is designed to drive 50 ohms, you shouldn't need the resistor in the beginning (internally matched). That said, I'm a RF guy so I don't use PMTS much. If you give me more info about the device and what you want to do I can comment more.

Otherwise, I would just take the resistor to ground, couple the signal into the non-inverting input, and do the feedback on the inverting input.

EDIT: in other words, you show resistors in series, they need to be in parallel. If you do not have a PMT internally matched to 50 ohms, drive the output into a 50 ohm resistor to ground (or whatever resistance will give you total resistance of 50 ohms - the PMT may not have high impedance).

Connect coax to PMT output and resistor. At the receiving end, you still need 50 ohms to ground. Connect the noninverting opamp input to the coax and resistor. Create the gain feedback on the inverting input of opamp (resistor looped back from output and connected to resistor to ground).

Also review this stackexchange discussion What is the importance of the $50\Omega$ resistors in this RF op-amp circuit?

Something like this but resistors may need to be tailored for actual impedance

• Yes, I want to amplify the PMT's output. PMT is a raw R6094, don't know if it is already matched. If not matched a effect called "ringing" (part of the output is reflected back) will occur. This is basically a impedance matching problem. "just place the resistor to ground and place the recorder in parallel" There are two resistors drawn, do I only need the one on the right then? – Neutrino Nov 8 '15 at 10:16
• Observe figure 36 in this [Hamamatsu document] (google.com/url?q=http://www.bo.infn.it/ams/…). – johnnymopo Nov 8 '15 at 16:43
• Their documentation did not specify internally matched so follow the image's suggestion – johnnymopo Nov 8 '15 at 16:44
• Answer edited, I could not create image – johnnymopo Nov 8 '15 at 16:55
• Thanks for the image, that clarifies it. The only thing that should be different is the input of the transimpedance amplifier opamp should be a inverting one. en.wikipedia.org/wiki/Transimpedance_amplifier – Neutrino Nov 9 '15 at 10:44