However I have a new problem now... looks like my Metrix Nixie counter has a problem with its LF / 1Meg input ! Signal is only 10MHz so it can take it no problem. However it would not register / count anything at all, only zeros on the Nixies, even though I have good signal amplitude, 5Vpp unloaded, and no matter how I fiddled with the input "sensitivty" pot/control. Plus, when I connect the counter, I can see on the scope that the signal goes haywire. Big drop in amplitude + it becomes completely out of shape, looking like a wiggly triangle, no more a rectangular pulse at all ! If I use the HF / 50R input instead, all is well.
So, the LF front-end has a problem.... some more repairs in the queue !
This counter is not easy to work on I fear, very compact in there. It's not going to be fun I think...
I would try another signal source, as maybe it can't detect the narrow pulse on the low frequency input? , I know some of the old counters I've got can be a bit fussy on the input signal.
David
OK I spent a few minutes testing the counter.
Result : the /F input CAN go up to 10MHz no problem... IF the conditions are right.
Let me explain... First I connected the pulse generator only to the scope, 10MHz again. Got a fair bit of ringing. Switched the scope to 50R input... much better, looks decent now (see pic)
Have my baseline now, so I can now connect the counter as well as see what happens.
1) if I connect the HF / 50R input of the counter, it adds a little bit of ringing but minimal, still looks decent and can count just fine. (see pic)
2) Then I switch the counter to its LF / 1Meg input. Oh oh... it adds A LOT of ringing, so much that if the pulse is too narrow, it becomes so distorted that it hardly looks like a square anymore. That's when the counter fails to count. But if you increase the pulse width a little bit, at some point the ripple becomes low enough that the counter manages to pick up the pulses.. but it still looks far from a square (see pic).
So the question is why does the LF input distort the generator output SO much ?!
Of course it adds more capacitance, but one would thing it would dampen the signal not make it "ring" ?
Or... the rsie time is too high, too much high-frequency harmonics, and the capacitive load of the 1Meg input low-passes the signal so much that it kills too much of the spectrum / harmonics... and the little that remains... is not enough to make it look anywhere near a square wave.
SO.... I could make one more experiment : keep the same frequency and pulse wigth, but progressively decrese rise time and see if that make the signal look more and more like a square.
Yes that would be interesting !