bananasiong

Hi,
I have some question regarding this mod4 FM transmitter.
1. From the circuit, I can see that the coil is 10 turns, but I see the coil with measurement is only 9 turns.
2. If I use thinner magnet wire, the coil should be stretched to achieve the length right?
3. C13 is used to peak the output power so that it doesn't interfere with TV, must the tank of this be the same as the tuner tank?

Thanks.

Attached Images

	fm_transmitter_mod4_pic_schem_181.gif (115.8 KB, 279 views)
	FM coil.gif (6.5 KB, 83 views)

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Hayato

1 - The coils is 9 turns. The first and the last turn are only half-turns.

2 - For the same inductance, thinner the wire, less the spires.

3 - Did not understand this one.

bananasiong

I mean, from the mod4 circuit, I see that the tank circuit for the tuner is same as the tank used to peak its output power. Must they be the same? This is because I have made a few 6 turns coils (around 76nH by calculation).

What is less the spires??

Thanks!

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Hayato

Yes they need to be the same.

For example, you have an 10-spire (turns) coil, made of #18 AWG, with diameter of X and inductance of Y.

If you want to obtain the same Y inductance with the same X diameter, but of a #28 AWG, you would need only 9 ~ 8 spires (turns).

audioguru

The circuit has trimmer capacitors to adjust the frequency. A coil with 9 or 10 turns or a different size of wire will make very little difference, since the circuit layout's stray capacitance will probably change the tuned frequency much more anyway.

Why not make both coils the same and use the same trimmer capacitors like I did?

C13 is used to peak the output power. Its tuning is very broad and when I peaked it at 98MHz then the power at 88MHz and 108MHz was reduced only a little. Because it is a tuned circuit then the 2nd and higher harmonics are reduced a lot to avoid TV interference.

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Hero999

That's true and large thick bus bar has an inductance of about 50nH/m. Also using litz wire lowers the inductance slightly and multiplate bus bar can be as low as 10nH/m. Using thik or multistrand wire is good though because it a lower resistance and the latter a lower skin effect so the Q will be higher.

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audioguru

I used 1mm heavy wire for the coils so they are strong enough to remain in the same shape after a bump. I stood the coils up away from the circuit board so they had to be stong. Thin wire gets bent just by looking at it.

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Uncle $crooge

Hayato

That's true.
When I use thin wire to coil, I make it old way.
After I finish coiling, I cover the inductor with parafine to give it some strengh.

bananasiong

Okay, then I will follow audioguru's coil, so I have to change my rx coil, to make them to be the same.
Thanks.

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bananasiong

Hi,
I couldn't find an electret mic here, I can get only a 2 leads mic (i think it is called capacitor mic)
look quite similar to this:

the impedance of this kind of mic is much lower than electret mic?

Thanks

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audioguru

It is an electret mic and will work fine. Its ground wire terminal is connected to its metal case. I got mine from a broken cell phone.

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bananasiong

How come the shop owner told me that they don't sell an electret mic and he gave me this kind of mic. But I don't see the polarity of it, I've tried using ohmeter to measure which terminal is connected to the metal case, but I couldn't. Because the ohmeter shows that both end to the metal case have resistance; either one of the resistance is the impedance of the microphone.
Just want to be sure, is this kind of mic is electret mic? No any other mic that look similar to this right?

Thanks.

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Hero999

Probably because he doesn't know what he's talking about.


Like audioguru said, the 0V line is connected to the metal case. Look at the back, the pad with a trace going to the metal case is the 0V line.

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Hayato

Did you ajusted the ohmmeter's offset null before the test?

audioguru

I got this pic of an electret mic from Google.
You can see that the rear solder pad is connected to the metal case.

Attached Images

electret mic.JPG (10.2 KB, 49 views)

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