# FM frequency

Discussion in 'Wireless & RF Design' started by nerdegutta, Oct 30, 2010.

1. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
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Hi,

I have built a FM transmitter. When I connect it to the computer speaker, and power it up, I hear it in my radio. And by turning the variable capacitor, the frequency changes.

On my frequency counter, DVM13MFC2 from Velleman, the frequency shown is from 30-55MHz, according to the turning on the capacitor.

The question is: Why do I hear it on the radio? Does the radio pick up the 2nd harmonic or something? I'm a little bit in the wild here...

I'm aware of the law and regulations in the 88 - 108 MHz FM range.

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2. ### bertus Administrator

Apr 5, 2008
15,799
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Hello,

It is most likely the 3th harmonic.
What are the dimension of the coil and the value of the Ctrim?

Bertus

3. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
2,565
805
Hi,

The coil is an air core coil.

Diameter : 10 mm
Length : 7 mm
Turns : 4
Wire: copper 0,5 mm
Antenna tap in center.

The ctrim is 6-70pF, I guess...

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4. ### bertus Administrator

Apr 5, 2008
15,799
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Hello,

Did you see this thread?

To calculate the value of the coil, take a look at this page about aircoils:
http://info.ee.surrey.ac.uk/Workshop...air_coils.html

To calculate the resonance frequency take a look at this page of the AAC eBook.

I used the calculator from this page for the value of the aircoil:
http://www.mantaro.com/resources/imp...calculator.htm

I have done some little calculations:
L = 100.440 nH (when dia = 6 mm and lenght = 10 mm and 6 turns)

Lower frequency ( C = 30 pF) = 91.6 Mhz
Higher frequency ( C = 5 pF) = 224.6 Mhz
Middle frequency ( C = 15 pF) = 129.7 Mhz

Greetings,
Bertus

Last edited: Oct 31, 2010
5. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
2,565
805
That's weird. I do not get the same numbers you do.

Here's what I got:

L=0.001N²r² / (228r + 254l)

L=0.001 x 6² x 0.003² / (228 x 0.003 + 254 x 0.01)

L=0.001 x 36 x 0.000009 / (0.684 + 2.54)

L=0.000000324 / 3.224

L=0.000000100

Where do I take the wrong turn?

I do not get the same numbers with this either...

6. ### bertus Administrator

Apr 5, 2008
15,799
2,386
Hello,

As you can see I can even go wrong.
Your calculations are correct.
I found an other calculator page that also gives the value you calculated:
http://www.mantaro.com/resources/impedance_calculator.htm
The answer for the transmitter tread is 100.440 nH

Your coil is : 136.939 nH

I will correct the page also in that thread.

With your coil and capacitor the frequency will be between:

Ctrim = 6 pF : freq = 175.6 Mhz
Ctrim = 70 pF : freq = 51.4 Mhz

Bertus

Last edited: Oct 31, 2010
7. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
2,565
805
Thank you. I can now stop pulling my hair...

I got the same result with this formula:

Now I'm back on track.

So, by manipulating the coil, I can narrow down the frequency. Nice!

8. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
2,565
805
Hm... How did you get this? I've tried

with no luck. *start-pulling-hair*

9. ### bertus Administrator

Apr 5, 2008
15,799
2,386
Hello,

The highest frequency is :
1 / (2 * pi * sqr(136.969 * 10^-9 * 6 * 10^-12)) =
1 / (2 * 3.141592 * sqr(821.634 * 10^-21))=
175.582 Mhz.

The lowest frequency is :
1 / (2 *pi * sqr(136.969 *10^-9 * 70 * 10^-12)) =
1 / (2 * 3.141592 * sqr(9.58273 * 10^-18))=
51.413 Mhz.

As you can see the spread in frequency is large due to the large change in capacity.

Bertus

10. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
2,565
805
Thank you, Bertus. Now I got the same numbers.

11. ### Audioguru New Member

Dec 20, 2007
9,411
896
If you make the FM transmitter on a pcb then add about 10pf of stray capacitance to everything.
If it is made on a breadboard then add about 30pf to everything plus add stray series inductance to each wire. No wonder its frequency is too low.

Most cheap FM radios are overloaded by a little FM transmitter close-by. Then the signal is received all over the dial and beyond.

12. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
2,565
805
Hi,

all my RF projects are built on PCB I etch myself. I've read about stray capacitance, skin-effect and "micro-phony" in a few books.

So far I've only noticed "micro-phony", but as long as the PCB is laying still, I'm not to bothered with it. I guess it'll be better if I mount the pcb in a metal casing, or if I put aluminum foil inside a plastic box I have. (as long as the foil is not connecting/touching the pcb.)

I didn't know I should add 10pF.

Thanks.

13. ### bertus Administrator

Apr 5, 2008
15,799
2,386
Hello,

As Audioguru said the frequency will go lower when the stray capacity is larger.
The calculated frequencies are in ideal cases where there are no stray capacities.

I have attached a PDF with a lot of information on RF prototyping.

Bertus

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14. ### nerdegutta Thread Starter Moderator

Dec 15, 2009
2,565
805
Thanks.

This is a good document.