LC_Meter

[Adam2014]

Hi
i create the thread again in a Electronic form , because a have no answer in a micro controller form , maybe because the users think the main idea of my questions is about a micro , but it a bout a circuit design ..

Hi
i upload the photo of circuit that i used in the LC design
but i have some question ::
Q1 ) what the function of : C9 , R9 , why we put this value ?
Q2) the second photo is for the signals : 1.comparator output (scale 5v) 2.non-inverting input (scale 1v) ,,, is the non-inverting input should be a sin wave (as shown) or a square wave (as i think)?

 

[Adam2014]

figure 2 (for Q2)


figure1 (for Q1) in this link https://www.rfcandy.biz/communication/pic/lcimpsch.gif

 

[alec_t]

R9 provides positive feedback to make IC2 an oscillator and its value equals that of bias resistors R7,R11 so that the hysteretic switching levels of the oscillator are at 1/3 and 2/3 of the supply voltage (5V). C9 provides AC coupling from pin 2 of IC2 to a parallel-resonant circuit made up of C6, L1 and whatever other L and C values are switched in via the relays or connected at Y1,Y2. The value of C9 is chosen to have a low AC impedance relative to that of the parallel-resonant circuit at resonance. It is this resonance which results in the sinewave input to pin 2 of IC2.

 

[Adam2014]

thanks alec_t !
Could you give me more details just a bout R9,C9 values ?
and what did you mean "C9 provides AC coupling" , i think it to block DC signal in IC2 but why ?
is the signal in inverting input increase/decrease as an exponential form ?

 

[Nigel Goodwin]

and what did you mean "C9 provides AC coupling" , i think it to block DC signal in IC2 but why ?

That's the same thing - a coupling capacitor is there to both pass AC and block DC.

 

[Adam2014]

thanks
that what i think , should it has a large value for a lower impedance to AC signal ?

 

[Nigel Goodwin]

thanks
that what i think , should it has a large value for a lower impedance to AC signal ?

Obviously the larger the value, the lower the impedance at lower frequencies.

 

[alec_t]

Could you give me more details just a bout R9,C9 values ?

Nigel's covered the C9 value. R9 value isn't critical, but some positive feedback is needed to make IC2 oscillate.

 

[Adam2014]

thanks
depending on the Circuit How i can determine the range of inductance/capacitance can i measure ?
is i should to change the capacitor value and monitoring the Oscillation , and when there is not oscillation i have one side of the capacitance range ?

using that method ........
when i simulate the circuit using proteus for L=100uH , C=1nF
i get this result ::
there is an Oscillation for a capacitor value between (10pf , 1uf)
for inductor value between (600nh , 100mh)

 

[alec_t]

How i can determine the range of inductance/capacitance can i measure ?

using that method ........when i simulate the circuit using proteus for L=100uH , C=1nF i get this result ::
there is an Oscillation for a capacitor value between (10pf , 1uf)
for inductor value between (600nh , 100mh)

You've just answered your own question .

 

[Adam2014]

I just want to make sure a bout the method
Q1) why the signal in inverting input is DC (2.5 v) ?
may be C10 doesn't discharge through R10 but why ?
Q2)

the hysteretic switching levels of the oscillator are at 1/3 and 2/3 of the supply voltage (5V).

more details please !

 

[alec_t]

Q1) R7,R11 (the bias resistors) are of equal value so form a voltage divider which halves the supply voltage.
R10,C10 form an integrator (low-pass filter) such that the voltage across C10 becomes the average of the changing voltage at pin 7 of IC2, i.e. becomes approximately 2.5V (half the supply voltage).
Q2) I should have referred to hypothetical hysteretic levels, since in practice C9 and the parallel-resonant circuit components load the pin 2 signal and prevent them being reached. If C9 were absent, but pin 7 could switch either high (5V) or low (0V), then R10 would be switched in parallel with either R7 or R11. R10 in parallel with R11 have a combined resistance Rpar of 50k, so the voltage at pin 2 would be 5V x Rpar/(R7 + Rpar) = 5V x 50k/150k = 5V x 1/3. Similarly, R10 in parallel with R7 have a combined resistance Rpar of 50k and the pin 2 voltage would then be 5V x R11/(R11+Rpar) = 5V x 100k/50k = 5V x 2/3.

 

[Adam2014]

R10,C10 form an integrator (low-pass filter) such that the voltage across C10 becomes the average of the changing voltage at pin 7 of IC2.

but the integrator equation is 1/RC x ∫Vin dt ?

 

[alec_t]

That looks about right .
Here's a simulation of the integrator action:

 

[Adam2014]

So how the value it 2.5v ?
depending on the equation on post # 13 it should be 27 v !!!

 

[alec_t]

How do you get 27V? What are you using as Vin (remember the voltage at pin 7 is oscillating, not constant)?
See my edit to post #14.

 

[Adam2014]

So how i get 2.5v from the equation ?
and how match time it take to gave a fixed 2.5 volt , how i can calculate it?

 

[alec_t]

how match time it take to gave a fixed 2.5 volt , how i can calculate it?

The time taken will depend on at least the initial state of charge of C10, the output impedance of IC2, leakage current through C10, and resistor tolerances. In other words the calculation would be very difficult. If you assume the pin 7 oscillation has a 1:1 mark/space ratio and that pin 7 can be pulled rail-to-rail then the long-term average voltage is close to 50% of the supply voltage.

 

[Adam2014]

Thanks
i have a question a bout LC tuned circuit ,,,Q1) why the value of their component are (L=100u , C=1n) ?
Q2) How it start oscillation with out any source ? i know it can continue oscillation depending on feedback from the comparator
Q3) is the Ac_ coupling capacitor C9 block Dc signal from Vcc (through pin2) to LC Oscillator & pass Ac signal from LC Oscillator to comparator ?
Q4) how i calculate the out voltage of LC circuit ? it is effected by C9 ?

 

[alec_t]

Q1) Designer's choice of minimum values (presumably found to work reliably).
Q2) Thermal noise, interference signals picked up by circuit wiring, etc.
Q3) If by 'oscillator' you mean the LC parallel-resonant circuit then yes. (The comparator forms part of the oscillator proper).
Q4) If by 'out voltage of LC circuit' you mean the voltage at pin 2 then I don't know. The PIC (IC1) doesn't care about the voltage: it measures the frequency of the output from pin 7.