Experiments with LC circuits  part 1

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Via some experiments en measurements I want to find out, how we can increase the Q of a LC circuit.

Description coils L1, L2,L3

First I made 3 coils (L1,L2 and L3) each with 12.5 meter litzwire (40x0.07mm) on the coils, and 2 leads of 0.25 m.
So the wire length is each time 13.0 meter.
The coils are spiderweb coils wound on corrugated cardboard formers.
The difference between the coils is the internal diameter, and so also the number of turns.

Coil L1: inside diameter: 50mm, coilformer diameter: 120mm, number of turns: 50
Coil L2: inside diameter: 40mm, coilformer diameter: 120mm, number of turns: 55
Coil L3: inside diameter: 100mm, coilformer diameter: 170mm, number of turns: 33.5

Description capacitor C1.

The tuner capacitor I used is shown in the picture above.
The maximum capacity is 495 pF.
The original version is named C1.
Later I made some improvements on this capacitor, like changing the screws and insulation rings, on the picture you see the old screws and insulation rings.
After each improvement, the name is changed in C1a, C1b, etc.
On the picture you see version C1b.

Measurement

In this measurement I measured the minimum and maximum tuning frequency, and the circuit Q at 600, 900, 1200 and 1500 kHz.

measurement
number
LC combination F min.
kHz
F max.
kHz
Q
600 kHz
Q
900 kHz
Q
1200 kHz
Q
1500 kHz
1 L1 C1 482 1710 * * * *
2 L2 C1 480 1730 200 129 75 58
3 L3 C1 495 1710 200 129 71 58

* = not measured.

Conclusion:

The minimum frequency of L3 is higher than L1 and L2, this indicates a lower inductance of L3 compared to L1 and L2.
The Q is rather low especially at high frequencies.

 

Improvements on C1

Now I replaced C1 temporary with a air trimmer capacitor with a maximum capacity of about 33 pF, this trimmer I connected to coil L2.
The minimum frequency was now 1650 kHz, and the Q was 165 at 1650 kHz.

Conclusion: The low Q of measurement 2 and 3 is mainly caused by the (bad) quality of C1.

In a tuner capacitor the quality is mainly determined by the kind of insulation between rotor (turnable part) and the stator (non turnable part).
In C1 the insulation between the plates is air, and this is a high quality insulation.
But there are also insulation parts needed to connect the rotor to the stator to each other, and these also have to be high quality, and also the capacity caused by these insulation parts must be as low as possible.

For reducing the capacity between rotor and stator, I replaced the metal screws with nylon screws.
The minimum capacity of C1 now drops from 33 pF to 20 pF.
This version of the tuner capacitor I call C1a.
Next I made this measurement:

Measurement
number

LC combination

F min
kHz

F max
kHz

Q
600 kHz

Q
900 kHz

Q
1200 kHz

Q
1500 kHz

4

L2 C1a

*

1950

240

225

171

107

Compared with measurement 2 the Q is much higher, especially at higher freqencies.
Because the lower minimum capacity of C1a, the maximum frequency of the circuit is also higher.

Next I replaced the original insulation rings by selfmade insulation blocks made of polyethylene (PE-UHMW), This version is named: C1b and has a minimum capacity of 14 pF.
This also gives a improvement of the Q.

Measurement
number

LC combination

F min
kHz

F max
kHz

Q
600 kHz

Q
900 kHz

Q
1200 kHz

Q
1500 kHz

5

L2 C1b

472

2060

300

300

200

166

 

Use of other tuner capacitors

For the next experiment I replaced C1b by the following tuner capacitors:

C2

390 pF tuner capacitor
with silvered plates.

C3

Small 500 pF tuner capacitor.
With plastic foil insulation between the plates.

 

This gave the following results:

measurement
number

LC combination

F min
kHz

F max
kHz

Q
600 kHz

Q
900 kHz

Q
1200 kHz

Q
1500 kHz

6

L2 C2

537

2020

300

300

214

195

7

L2 C3

*

*

133

128

109

94


Measuring coil L1 and L3

The next measurements are again made with capacitor C1b.
Now the capacitor is improved, I want to measure again the differences in Q of coil L1, L2 and L3.
L2 was already tested in measurement 5, here are the results for L1 and L3.

Measurement
number

LC combination

F min
kHz

F max
kHz

Q
600 kHz

Q
900 kHz

Q
1200 kHz

Q
1500 kHz

8

L1 C1b

473

2050

300

300

200

166

9

L3 C1b

487

2050

300

300

200

166

The Q of the coils L1, L2 and L3 are the same, the Q seems not to be dependent on inside diameter of the coils.

 

Coilformer made of foam PVC

Coil L4

Coil L1, L2 and L3 have corrugated cardboard coilformers.
This coil (L4) has a coilformer made of white foam PVC, this is PVC with small aircells in it.
The foam PVC is 3mm thick.
Wirelenght is again 13.0 meter (12.5 m on the coilformer, and two leads of 0.25 m).
The wire is also 40x0.07 litzwire.
The internal diameter of L4 is 50mm.

 

Measurement
number

LC combination

F min
kHz

F max
kHz

Q
600 kHz

Q
900 kHz

Q
1200 kHz

Q
1500 kHz

10

L4 C1b

480

2085

326

310

261

200

Conclusion: the foam PVC coilformer gives a bether Q than the corrugated cardboard former.

Coil L5

Coil L5 has a former made of 3mm foam PVC.
There is as much as possible material removed from the former.
For the rest it is the same as L4.

 

Measurement
number

LC combination

F min
kHz

F max
kHz

Q
600 kHz

Q
900 kHz

Q
1200 kHz

Q
1500 kHz

11

L5 C1b

484

2050

360

346

279

242

So, removing material from the former also has a positive effect on the Q.

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