6602 Impedance Matching

[otheredward]

Dear All,

I have a Perken Elmer single photon counter (spcm-arq) which has two BNC connection. The first is the output connection, each time a photon reaches the detector a TTL pulse is output on this connection. The second is the gate. When logic low is applied the output of the detector stops. When high is applied it behaves normally. Perken Elmer lists this device as 50 ohm. When I connect the gate BNC to a high impedance function generator it has no affect on the output. When I connect the gate to the function generator with a 50 ohm terminator everything works as expected. Now I'd like to connect the gate connection to one of the PFI lines on my 6602 board. I'm using the Measurement and Automation test panel to raise and lower the level of my PFI line. When I connect a PFI line to the gate and raise and lower the level nothing happens. I've found a discussion about termination and the 6602 board here http://forums.ni.com/t5/Counter-Timer/impedance-maching/m-p/679861#M6024 which confirms that the board is 75 ohm and, as recommended in the manual, a 68 ohm resistor along with a 150pf cap can be used to impedance match when using the board for output. I've tried using the resistor and cap but it still doesn't work. Can anyone tell me exactly how I should hook the resistor and capacitor up (series or parallel and does order matter if series) and should I terminate my photon counter or the board? Thanks very much for any help.

Best,

Ed

[Josh_L1]

Hi Edward,

I wonder if it's possible for you to check the length of your TTL pulse. If the high pulse is too short, the card may not be able to see the signal at all.

More importantly, can you variable the output from your function generator? This may not be a termination issue if the output signal is off.

I hope this helps.

Josh L.