Switching input BNC signal to two output BNC signals using BJTs?

Question

^{simulate this circuit – Schematic created using CircuitLab}

Hi everyone, I recently completed the construction of a spectrometer that studies electron transfer events in inorganic molecules. I am trying to further improve my design however, mainly one major flaw.

I'd like to control where the output of my photomultiplier (PMT) tube is going using a set of BJT transistors as we currently have to manually plug the BNC into the appropriate device. Given the complexity of the setup, it's not feasible to expect workers in the future to be able to do this.

I am somewhat unsure as to how BNC/coax cables operate and would like to ensure that my circuit will work before I invest the effort into building the switch box.

The TTL logic to each gate is to come from an NI USB 6008 IO box.

Answer 1

It will not work (reliably) like this.

The transistors turn on when their base voltage is below their emitter voltage. But here the emitters are DC coupled to the PMT output. So for this to work the PMT output needs to have a DC-component at the TTL high level.

I would consider:

1. Looking for a relay/switch with BNC connectors
2. Seeing if CMOS switches like CD4066 will suffice
3. Redesign

Answer 2

You should bear in mind that the BNC connectors and the coax cables used in these kind of applications generally have impedances of either 50 Ohms or 75 Ohms, so the apparatus you're working with is likely to have input and output impedances to match. CMOS switches like the 4066 and 4051 typically have ON resistances in excess of 100 Ohms, which are therefore likely to have quite a significant effect the signals you're measuring.

As well as the logic signals, the 6008 has a +5V output capable of giving 200mA, so I would use this to provide power for a small signal relay with a changeover contact and use a simple NPN common emitter switch to drive the coil from a digital control line. Don't forget the protection diode.