How do I calculate Rs and Cs from these waveforms?

Discussion in 'General Electronics Chat' started by Bakez, Jul 23, 2013.

1. Bakez Thread Starter Active Member

Aug 21, 2012
31
0
I am trying to work out how an LCR meter works as I want to build one of my own to measure small capacitors, nF range

As far as I have read, an LCR meter applies a sinusoidal signal to a capacitor and calculates the parameters from the resulting waveform (using phase and magnitude)?

So I put a capacitor of 2nF and ESR 2Ω in LTspice and a voltage source of 500mV, 10Mhz

These are my resultant waveforms. How do I now calculate the 2nF and ESR from these waveforms?

Am I missing something?

File size:
104.7 KB
Views:
22
File size:
24.9 KB
Views:
20
2. MrChips Moderator

Oct 2, 2009
14,509
4,275
Put a known resistance in series between V1 and C1.

3. Bakez Thread Starter Active Member

Aug 21, 2012
31
0
OK I put 2 ohm resistance between cap and voltage source

File size:
63 KB
Views:
19
File size:
86 KB
Views:
19
4. MrChips Moderator

Oct 2, 2009
14,509
4,275
Try R1 = 20Ω.

5. Bakez Thread Starter Active Member

Aug 21, 2012
31
0
ok

What am I supposed to be measuring?

File size:
95.8 KB
Views:
13
6. MrChips Moderator

Oct 2, 2009
14,509
4,275
Measure the voltage across R1 or C1.
Do the calculation.

btw, this is not how an LCR meter works unless you build a computerized LCR meter.

Aug 21, 2012
31
0
8. MrChips Moderator

Oct 2, 2009
14,509
4,275
Why did you choose 10MHz test frequency?
What is the relationship between the 10MHz input signal and the measured quantity?

9. Bakez Thread Starter Active Member

Aug 21, 2012
31
0
Xc = 1 / (2 * pi * f * C)

Dont i want to make Capacitive reactance small so I can measure the resistance properly, so I have to make a large frequency?

10. MrChips Moderator

Oct 2, 2009
14,509
4,275
Depends on what you want to measure.
Do you want to measure capacitance or ESR?

11. Bakez Thread Starter Active Member

Aug 21, 2012
31
0
Rs, ESR predominantly