Hi,
Like Tim said, I think the pulse from your pulse generator is too short. Here is a simulation of a filter being tested with a short pulse and a long pulse:
You would think the short pulse indicates higher BW, but the filters are identical.
Try adding about 18 inches of 50 Ohm coax in parallel with the capacitor in your pulse generator. This will length the pulse to about 4ns.
Regards,
Jay_Diddy_B
Jay_Diddy_B, Thanks.
Ok, these questions are going to officially get me labeled as a complete rookie....
(I didn't build the pulse generator - I'm just using it. Except for the transistor pretty much everything on the board looks like SMD; so it's kind of small stuff and I'd prefer not to wreck an otherwise nice pulse generator.
)
I see a 1M Ohm resistor (connected to the Collector?) and a 10k resistor (connected to the Base) near the transistor; instead of one 50 Ohm resistor connected to the Emitter I see two 50 Ohm resistors next to each other - not sure why there are two there.
There are only two capacitors that I see near the transistor (and also reasonably near the BNC connector) - one that is considerably larger and further from the transistor and another that is relatively smaller and a little closer to the transistor. The one that is larger is pretty much the same physical size as two other capacitors also on the board - all three measure about 420nF. The one that is smaller and closer to the transistor and BNC is difficult to measure; sometimes I measured it as being about 3-5pF and sometimes around 60pf?? Any chance it is this smaller capacitor that is the right capacitor to connect to the coax?
Assuming I have the right capacitor (the one that measures 3-5pF and occasionally 60pF), any chance you could post a diagram showing where to add the 18" of 50 Ohm coax (from where to where)?
Thanks
Howard, I saw your suggestion: "You can extend the pulse with a foot of RG174 open ended stub in parallel with the cap." I wasn't sure if "open ended stub" meant just solder the length of the coax onto one side (either side?) of the capacitor and let the other end of the coax dangle free, or something else. Any clarifications or suggestions on how not to mess up a working generator would be very welcome. Thx! EF