el cheapo (RF) inductance meter - Page 1
Forum section: electronics / projects

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2015, 09:00:29 pm
content: Sometimes you have to measure inductance, especially small inductance for your RF projects. It can be hard to do - one way is to use the ESR meter I mentioned in a separate thread but that is good for high value inductors.How to measure small inductors?Well, one way is to use an oscillator of known frequency to excite an L/C tank where you know the capacitance. If you get resonance to the known frequency by varying the capacitance, you know the inductance.

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2015, 09:07:14 pm
content: In my actual implementation, V1 is a crystal oscillator (Plextronics, 12Mhz), and C2 is a 7-270p tuning capacitor. L1/DUT is some random inductor.Other parts are not critical.

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2015, 09:18:57 pm
content: Here is the inductance meter on the breadboard.Shown is the 12Mhz oscillator + resonance detector. The tank is not shown.

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2015, 09:29:01 pm
content: Here is the meter at work, measuring a small inductor (less than 1uh).The DUT is the inductor in the middle, with a blue core.The variable capacitor is on the bottom, under that black tuning wheel.The meter reading is on R2, from the detector.The left most picture is when the capacitor is at its smallest capacitance. The l/c tank is not resonating. The reading from the detector is high;As you turn up the capacitance, the l/c tank starts to resonate -> the middle picture is when the l/c tank is at resonance -> the detector output is the lowest.As you continue to turn up the capacitance, the l/c tank is no long resonating so the detector's output goes up again.You can observe the capacitor's position by following the green arrow on the black tuning wheel.I estimated that at resonance, the capacitance is about 250p so the inductor is 0.7uh -> just about right.

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 01, 2015, 09:35:10 pm
content: An alternate approach which I find easier/more reliable is to wire the unknown inductor in series with a 50 Ohm low-inductance resistor then apply at least 1V - the higher the better - across the series pair with a function generator that has a 50 Ohm output. Measure the voltage across each with a scope to determine the inductance reactance at that frequency then plug the relevant values into L = Xl / (2*pi*f).

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2015, 09:52:25 pm
content: QuoteNo plan whatsoever.

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2015, 09:54:22 pm
content: QuoteThat will work.Also, since you have a function generator, you can simply put the inductor in serial with a known capacitor and drive them via the function generaetor -> variable the frequency until resonance -> Done.

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 01, 2015, 10:21:33 pm
content: Quote from: dannyf on March 01, 2015, 09:54:22 pmYep, plenty of ways to skin this particular cat but a couple of caveats with this approach are that the function generator will almost certainly not like driving an LC circuit at or near resonance, especially if high Q, so you will need to insert a series resistance for the series LC to limit current, or a parallel resistance for the parallel LC to limit voltage, and even still you might get some weird/false readings if the frequency is well off resonance (particularly if the impedance seen by the F.G. is predominantly capacitive).Forgot to mention before that the resistors I like to use are in TO-220 or TO-247 packages (e.g. - Caddock MP9xxx series) with stray inductance in the 10-20nH range.

author: dannyf
author_rank: Super Contributor
timestamp: March 01, 2015, 11:55:45 pm
content: Reduced R1 to 470 to provide me with more sensitivity.