Author Topic: LC in Tank Configuration Confusion  (Read 886 times)

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Offline dkggpetersTopic starter

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LC in Tank Configuration Confusion
« on: March 24, 2019, 02:45:52 pm »
I setup an LC in a tank configuration using a 51uH Inductor and if I use I higher value capacitor the frequency cutoff range comes out to what I calc but when I use a lower value capacitor it is about 1/2.

Inductor     51 uH
Capacitor    330 pf
Calc Freq    1.226 Mhz
Actual Test 1.19 Mhz

Inductor      51 uH
Capacitor     20 pf
Calc Freq    4.983 Mhz
Actual Test  2.64 Mhz

I have used different types of capacitors and even different values of Inductors and see the same thing.  I calced using a Bode plot and also observing sine wave amplification by manually adjusting frequency and finding the highest amplitude and get the same results.

What in the world am I doing wrong?
 

Offline Benta

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Re: LC in Tank Configuration Confusion
« Reply #1 on: March 24, 2019, 03:44:22 pm »
With inductor and capacitor values that low, there are lots of parasitic effects: inter-winding capacitance, lead inductance etc.
 

Online Stray Electron

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Re: LC in Tank Configuration Confusion
« Reply #2 on: March 24, 2019, 03:53:57 pm »
   Where did you get your parts?  If they're Chinese these they usually aren't anywhere near the specified value.  If old parts, they could have drifted out of spec.  Running too much power through a lot of devices will also cause them change value.

  This is why things like a GOOD LCR meter is so valuable, they let you check your parts for the proper value before you build the circuit.  Then if you built the circuit and it is out of spec then you know the problem is in the construction or operation and not in the components.
 

Offline dkggpetersTopic starter

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Re: LC in Tank Configuration Confusion
« Reply #3 on: March 24, 2019, 05:32:23 pm »
   Where did you get your parts?  If they're Chinese these they usually aren't anywhere near the specified value.  If old parts, they could have drifted out of spec.  Running too much power through a lot of devices will also cause them change value.

  This is why things like a GOOD LCR meter is so valuable, they let you check your parts for the proper value before you build the circuit.  Then if you built the circuit and it is out of spec then you know the problem is in the construction or operation and not in the components.

I have a DE-5000 so and used measured values which were close to reported.  They are not old parts as well.  I believe that the previous post answered my question well.  I will test high higher values and wider ranges to see how they respond.
 

Offline dkggpetersTopic starter

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Re: LC in Tank Configuration Confusion
« Reply #4 on: March 24, 2019, 07:38:54 pm »
Since the capacitance levels are so low, can it be the capacitance of the scope probes and breadboard?  I believe that this is driving the majority of the change.  Since the capacitance levels were so low, it doesn't take much to double their values.  The scope states 15 pf on inputs and when I run the circuit not on a breadboard I get higher frequency values as well.  The combination of the two has a huge impact on low capacitance values.
 

Offline Benta

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Re: LC in Tank Configuration Confusion
« Reply #5 on: March 24, 2019, 08:09:25 pm »
Then solder a 100k resistor in series with you probe. For determining the resonant frequency it doesn't matter if the amplitude is not quite right.
 


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