# S11 inductance measurement

1. ## Re: S11 inductance measurement

If a default RLC resonant circuit appears, then click the Circuits menu, and go to Op Amps >> Gyrator.

https://tinyurl.com/yah5tp4t

I wonder why your tinyurl fails?

http://tinyurl.com/y983s4bo

•

2. ## Re: S11 inductance measurement

Originally Posted by SunnySkyguy
Thanks, Tony. Your tinyurl link works fine. It displays the gyrator circuit.

My link was produced by Falstad's version 1.5n which is no longer up-to-date with the website version. I suspected it would misbehave. I use the older version because it allows me to rearrange scope traces by editing the file directly in the Export window.

3. ## Re: S11 inductance measurement

Originally Posted by promach
Have anyone used S11 result to calculate inductance value of active inductor as decribed in https://www.ieee.li/pdf/essay/s_para...nts.pdf#page=5 ?

Attachment 144460

Attachment 144459
I must confess I've never computed L from s11.

From your datasheet. 50 Ohms @ 100MHz has s11 =0.333, 30 degrees which is a VSWR of 2.0 and return loss of 9.6 dB

Z = Zo * ((1 + Γ) / (1 - Γ))
Γ = (Z - Zo) / (Z + Zo)
S11 = 20 * Log10(mag)

Reflected power is the square of reflection coefficient or 1/9th or 11.1% in this example.

same example
Z= 82.9Ω +j30.7Ω

L=Z_Im/2πf
L= 30.7/(2π*1e8Hz) = 49 nH

have fun.

1 members found this post helpful.

•

5. ## Re: S11 inductance measurement

Z_denominator_im =S.11_im*(-1)
Do you guys have any idea how the author arrived at this expression exactly ?

6. ## Re: S11 inductance measurement

Originally Posted by promach
Do you guys have any idea how the author arrived at this expression exactly ?
Surely learn and study very basic things.

Z=(1+S.11)/(1-S.11)

Z_numerator=1+S.11=(1+S.11_re) + j*S.11_im
Z_denominator=1-S.11=(1-S.11_re) + j*(-S.11_im)

Z_numerator_re=1+S.11_re
Z_numerator_im=S.11_im

Z_denominator_re=1-S.11_re
Z_denominator_im=-S.11_im

1 members found this post helpful.

7. ## Re: S11 inductance measurement

Could anyone tell me why I am not getting Leq to be 1nH ?

- - - Updated - - -

The following is the netlist for the test circuit :

Code:
```* gnetlist -g spice-noqsi -o test_active_inductor.net test_active_inductor.sch
* SPICE file generated by spice-noqsi version 20130710
* Send requests or bug reports to jpd@noqsi.com
RZ1 Vtest Ltest 50
Vtest Vtest GND dc 1
+ac 1
.GLOBAL Vdd Vss
.PARAM SUPPLY=3.3v
.options TEMP=25
.MODEL n1 NMOS
.MODEL p1 PMOS
.INCLUDE CMOS_Inverter.net
.INCLUDE Gm2.net
Vd Vdd GND 'SUPPLY'
Vs Vss GND 0V
Ltest Ltest GND 1n
.control
AC LIN 1000 100Meg 200Meg
run
let S11=(2*v(Ltest)/v(Vtest)-1)
settype s-param S11

let S11db=db(S11)
settype decibel S11db

let P11=180*ph(s11)/pi
settype phase P11

plot s11db ylimit -0.5 0
plot P11

print S11 > S11.log
*plot smithgrid S11
*wrs2p s3046.s2p \$ write touchstone vers. 1 file s3046.s2p
.endc```

8. ## Re: S11 inductance measurement

Found the root cause for the miscalculation.

I forgot to multiply by Zo which is 50ohm

•