help with transmitter

lokeycmos

Joined Apr 3, 2009
431
ive been working on this fm transmitter from this author:

and schem:

for the life of me i just cant get it working. for the resonance, im shooting for about the middle of the dial. 100pF and about .01uH. ive done hours of experimenting with different caps, coils, variable caps, and adjustable coils. ive tried using fixed values and tuning the radio. ive fixed the radio and tried tuning the circuit. i cant even get anything if i clip the output directly to the antenna of my fm radio. really hoping someone can aim me in the right direction!!

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upand_at_them

Joined May 15, 2010
940
Your voltage regulator really should have before-and-after filter caps (electrolytic). And it looks like you're using an electrolytic cap for the input audio coupling to the transistor base, which should probably be a ceramic and probably 0.1uF instead of 1uF.

The bias resistors keep the transistor always on...Do you see a varying voltage at the transistor output?

k7elp60

Joined Nov 4, 2008
562
I agree with up and at them for the capacitors for the input and output of the 5V regulator. Your basic values calculate to a resonate frequency of 159MHZ. There are some capacitance values on the breadboard between the various terminals, I have no idea how much it is. If your circuit is working, even without an audio input, when you have a FM receiver near your circuit the FM receiver should be quiet. With the value of your inductor I calculate your capacitor should be near 250pf. This formula is one that is taken from the standard resonate frequency formula..but it only relates to frequencies in the MHZ region. LC=25330/(f x f). Where L=inductance in uh and C=capacitance in pf. So if we use your desired frequency of 100MHZ we have LC=25330/10,000. So LC=2.53 With a 100pf capacitor 2.53/100= 0.0253uh.

Keep in mind that when the circuit is working it won't be very stable, that is to say the frequency will vary if any conductor is placed close to the components, and the modulation will be poor.

lokeycmos

Joined Apr 3, 2009
431
thanks guys! got it going. added 2 caps, one to each side of the regulator and changed the coupling cap to a .1uf

will putting the circuit in a grounded metal box stop the capacitance thing when i move my arm around it?

Audioguru

Joined Dec 20, 2007
11,248
A frequency as high as 100MHz does not work properly on a breadboard.
Usually the inductor has an air core.
Usually the inductor has a tuning capacitor in parallel with it.
Usually the base of the transistor has a small capacitor to ground so it acts like a common-base amplifier.

My FM transmitter uses an RF amplifier to isolate the capacitance of something moving towards or away from its antenna from affecting the oscillator frequency.
Here is the oscillator of my FM transmitter:

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lokeycmos

Joined Apr 3, 2009
431
A frequency as high as 100MHz does not work properly on a breadboard.
Usually the inductor has an air core.
Usually the inductor has a tuning capacitor in parallel with it.
Usually the base of the transistor has a small capacitor to ground so it acts like a common-base amplifier.

My FM transmitter uses an RF amplifier to isolate the capacitance of something moving towards or away from its antenna from affecting the oscillator frequency.
Here is the oscillator of my FM transmitter:
the variable cap i have is a 15-145pf. how do i modify the coil to match it?

Audioguru

Joined Dec 20, 2007
11,248
Your variable cap has too much capacitance for 100MHz. It might work if the coil has only 2 or 3 turns of wire.