New Video: Measure unknown L and C values with your scope - Page 1
Forum section: electronics / beginners

author: w2aew
author_rank: Super Contributor
timestamp: May 04, 2013, 03:05:15 am
content: New video I posted tonight, showing how to use the fast-edge pulse generator circuit (from the TDR video) and your oscilloscope to measure the value of unknown capacitors and inductors.

author: JuiceKing
author_rank: Regular Contributor
timestamp: May 13, 2013, 11:36:01 am
content: Hi Alan,Thanks very much for this and all your instructional videos!I do try them out when I can, and usually it's easy to reproduce the results you show. This time, I'm getting a strange result and hope you might see why.I'm using an HP 215A as my pulse source. It has a 50-ohm output impedance. I adjusted it for a 1 nsec pulse, directed that through the 10 pf cap, into the 1 nF/1 uH tank circuit, which went to ground. More precisely, the tank circuit cap measured 0.970 nF and the choke measured 1.42 uH, so I expected a 4.29 MHz resonance.Instead, I saw a 184 MHz ring on the pulse, which damped out after maybe 5 cycles.Any ideas what I'm doing different/wrong?Thanks!- Ken

author: w2aew
author_rank: Super Contributor
timestamp: May 13, 2013, 04:24:10 pm
content: You may be exciting some other parasitic resonance, but it's tough to say without seeing what you are doing. You may be seeing ringing from your probe's ground lead length. Have you watched my video on probe ground lead length and it's affect on signal integrity, ringing, etc.? Also, you probably don't want to use a 1ns wide pulse, since the generator has a 1ns risetime at best. You can use a MUCH wider pulse (just a couple of kHz is fine), because the edge speed should be fast enough to excite the resonance of the tank.

author: Deckert
author_rank: Regular Contributor
timestamp: May 14, 2013, 12:39:07 am
content: Quote from: w2aew on May 04, 2013, 03:05:15 amThanks w2aew!Just wow .. and it's so accurate! Had no issues measuring this using my cheap Atten/Siglent 100MHz DSO. Even at very short time-bases it works perfectly. I used a 32kHz signal, fed it to a 74HC4040 and tapped off a 2kHz signal from the divide-by-16 output. The old 150MHz HP scope measures rise time at 2.8ns, so it should okay in terms of fast enough rise time, yes?In any event, measuring the caps (a 4.7nF and 40pf cap) was perfectly accurate. Gotta have vernier adjustment on your scope to use that 5/8-division trick though!--deckert

author: vk6zgo
author_rank: Super Contributor
timestamp: May 14, 2013, 01:01:43 am
content: I've done the LC trick,just using the "cal" output on my 7613,but the RC would need the faster rise time.I'm 1000km away from my hamshack/lab,or I would be out there throwing the fast risetime "thingie" together!As always,Alan,great video!

author: notsob
author_rank: Frequent Contributor
timestamp: May 14, 2013, 01:26:13 am
content: There's a version of the 74AC14 circuit which is a pulse generator with variable frequency for differing cable lengths.http://www.epanorama.net/circuits/tdr.htmlkeep up the good work alan.

author: JuiceKing
author_rank: Regular Contributor
timestamp: May 14, 2013, 02:17:41 am
content: Quote from: w2aew on May 13, 2013, 04:24:10 pmIt was pilot error. I fixed my mistake and got 5.66 Mhz, which was still high, but in the right ballpark. I rechecked my measurements of the capacitor and choke. The second time I got L = 1.18 uH and C = 967, which would put f = 4.7 Mhz. I measured these on bridges at 1 KHz.Interestingly, the pulse width didn't make a difference; the ringing was 5.62 MHz with a 1 KHz square wave or a 20 ns pulse.- Ken

author: w2aew
author_rank: Super Contributor
timestamp: May 14, 2013, 02:37:38 pm
content: Quote from: JuiceKing on May 14, 2013, 02:17:41 amYes, the pulse width won't make a difference. I chose a slow pulse so that the same source could be used for TDR, and a wide variety of L and C values. For low values, you have to take into account for probe capacitance, etc.

author: Rick Law
author_rank: Super Contributor
timestamp: May 15, 2013, 04:31:13 am
content: Quote from: w2aew on May 04, 2013, 03:05:15 amThat is exactly where I learned the inductance measurement technique! Very useful and many thanks!!