VK3BHR LC meter inductance measurement problem

Thread Starter

ashok.das81

Joined Jan 4, 2017
16
Hi all,

I am making the LC meter V2 designed by VK3BHR Phil RIce.

https://sites.google.com/site/vk3bhr/home/index2-html

I have seen many have done this and it works fine, but in my case it is not measuring inductance properly. For example a 90uH coil shows 52uH, 3.3uH shows 0.3uH. The measurement is completely wrong. Capacitance measurement is looking OK. Calibration was done using a 1% 100pF NP0 capacitor. I checked F1 and F2.

F1=00042879 (little lower than recommended value of 00050000 +- 5%)
F2=00032223 (within 71% of F1)

I have used all new components and L is a 100uH +/-20% toroid. C and Ccal are 1000pF NP0 +/-5%. I am using the PCB designed by VK3BHR. The relay is driven by a BC548B transistor as I cannot find a low current relay and I have checked the RELAY its clicking and contact resistance is less than 1 ohm. I am thinking that the +/-20% tolerance for the 100uH toroid is too high. What else could be wrong ? I will provide any other info that is useful for troubleshooting.

Can anyone help me to find out the problem.

Thanks
Ashok
 

DickCappels

Joined Aug 21, 2008
7,442
Often, when people have trouble with the accuracy of this oscillator (the basis of the meter), the problem is the Q of the inductor. Reading inductors with large resistances leads to large errors in inductance measurements, similarly when measuring capacitance if the reference inductor has significant resistance it throws off the capacitance measurement. This is because resistance in the LC circuit makes the oscillator oscillate at other than the expected f = 1 / (2π √L C).

Let me know if you want me to dig up the definitive analysis by Chris Krah. I know it is somewhere on my hard drive...
 

Thread Starter

ashok.das81

Joined Jan 4, 2017
16
Hi DickCappels
Thanks for your reply. I will certainly like to have more analysis on this, so kindly share whatever information you have with you. This project is very important for me as its only way for me to measure inductors for RF filters. I am appearing for my amateur radio license exam and building radios cannot be done without measuring inductance accurately.
 

OldTech

Joined Jul 24, 2009
3
Thanks for the documents. Now I need to disconnect up the L and measure its internal resistance.
You didn't say what kind of toroid core you used to wind the coil. The "Q" of the inductor you made with that core could easily be your problem.
Get rid of the toroid-core coil and get a leaded inductor such as this one: https://www.mouser.com/ProductDetail/Fastron/SMCC-101J-02/?qs=R2jSSvul3fDqs1EnZiUM9g==.
Its Q and DC resistance should get your LC meter up and running.

Cheers,
Dave M
 

Thread Starter

ashok.das81

Joined Jan 4, 2017
16
Hi Dave,

I used a ready made coil from Multicomp. As per VK3BHR, the inductor should have low internal resistance, at least less than 1 ohm. Attached is the datasheet of that toroid inductor. Its internal resistance is 0.3 ohm, but in datasheet the test frequency is 10 khz. In my circuit the operating freq is approx 500 khz. Could that be a problem? The inductor that you have suggested have internal resistance more than one ohm. Can I have something which have internal resistance less than 1 ohm.
 

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Thread Starter

ashok.das81

Joined Jan 4, 2017
16
Hi Dave,
I have found one TDK part. Specs are looking good having 0.7 ohm series resistance and recomended operating freq at 796 KHz, 100uH +/- 5%, self resonant at 3.5MHz. Waiting for this to arrive.
 

Thread Starter

ashok.das81

Joined Jan 4, 2017
16
Update:
I have replaced the inductor with the TDK part, I mentioned earlier 100uH, 0.7ohm, 2MHz SRF. Also replaced both C and Ccal with 1% polystyrene capacitors. Removed the relay to a separate PCB (thinking that Relay coil might be interfering with 100uH inductor). Now the circuit is working as it should. Thanks to all for their valuable feedback to make this a success.
 

upand_at_them

Joined May 15, 2010
766
I was working with this circuit recently. Polystyrene caps make a big difference in stability.

@ashok.das81 Also, be aware that those polystyrene caps are "kind of" polarized. Not in the way that electrolytics are, but one of the leads is, of course, connected to the outer plate of the cap and is near the outer surface of the cap. It will be affected by such things as your finger being too close if the cap is oriented "backwards". The end of the cap that is colored (probably red or orange) should be connected to the ground side.
 
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