Can't get a tank circuit to ring

[guitchess]

Hello,

Just got an Oscope and was trying to measure some inductors using the ringing frequency technique.  Despite trying various values of caps and sizes of inductors, I'm unable to get the tank circuit to ring. 

I think I have several issues working against me.  Just thought I would see if some of you might have some helpful input.
Here are my ideas for why the test isn't working:
1.  My pulse gen doesn't have a fast enough rise time.  The best I've come up with so far is to program a PIC with a pulse.  This only gives me a rise time of 36nS.  Is the slow speed of my pulse washing out the ring?
2.  The output current of the PIC is only about 20mA.  Is this low amperage not sufficient to saturate the cap, inductor, or both therefore not giving the tank enough energy to ring?

I have plans to build the pulse gen discussed by W2AEW with the 74ac16 which will give me a faster edge.  It will only be a slight increase in current, however. 

I know there are other ways to measure inductance with an Oscope, but most/all of them require  a precision function gen.  I just got the scope so that will be down the road a bit.  Also, the frequencies I would need to test the inductors I'm using would make the function gen cost more than my scope. 
I also know that my measurement of inductors really isn't that important.  Just a learning experiment.

Thanks.

[fourfathom]

What are the LC values for your tank?  How are you coupling the impulse into the tank?  What frequency are you trying for?  If you are directly connecting a low-impedance driving impulse to a high-impedance tank then you are very likely swamping the circuit -- lowering the "Q" until it won't ring enough to detect.  Show us a schematic.

[guitchess]

What are the LC values for your tank?  How are you coupling the impulse into the tank?  What frequency are you trying for?  If you are directly connecting a low-impedance driving impulse to a high-impedance tank then you are very likely swamping the circuit -- lowering the "Q" until it won't ring enough to detect.  Show us a schematic.

1.  LC values - I have varied them wildly.  From 150pf - 100nf caps and  from approx 57nH (5T air core) - ?dozens of turns on a toroid mH?

2.  Low/High impedance mismatch   -   An example of why I'm in the beginner section.  I'm not sure of how to classify these.  The pulse I started experimenting with was a 1k square wave from an LM358 wave generator(rise time measured about 65nS).  When I couldn't get the expected results, I switched to using a square wave generated from a PIC at about 400khz (rise time of about 38nS). My rudimentary understanding of impedance makes me think that I used both high/low impedance tank circuit values for high and low freqs tried.

3. Circuits  -  Too many to timely list.   Pulse>tank>gnd     ,   Pulse>R>tank>gnd   ,   Pulse>C>tank>gnd  ,   Pulse>tank>R>gnd  , etc.

One experiment session I thought I was starting to get results, but it was a mislead.  I tried to connect the inductor with test leads instead of inserting into the breadboard.  I was picking up noise from the leads which appeared to be minuscule ringing. 

Thanks.

[fourfathom]

Here's a very simple circuit, with a 3V 400 KHz square wave, rise/fall times 40ns, feeding a tank circuit through a 1K resistor.  There is a 1Meg load resistor representing a scope probe, but actually that 1K resistor swamps out any probe effects.  The capacitor is 100pF, the inductor 1uH.  The inductor Q is also not critical, this also being dominated by the 1K resistor.

You can see the ringing, just over 1V pk-pk and lasting a few cycles before that 1K resistor damps it out.  The rise and fall time of the driving signal will have some effect on the amplitude of the ringing.

[alsetalokin4017]

Heh... usually the problem is to get a circuit to _stop_ ringing....


1. 74AC14, not 74AC16 perhaps?

2. I didn't have an AC version on hand but I built the Fast Edge Oscillator using 74HC14 and it works fine. Faster rise/fall times unloaded than my Tek 2213a can display.

3. I have no trouble ringing a small tank circuit with this oscillator.