# FM transmitter

#### HaneenSu

Joined Apr 21, 2013
2
Hello Electronics geeks

I'm trying to build and understand an FM trasmitter circuit. I found one on the internet :
http://s.eeweb.com/members/circuit_...short_distance_88Mhz_to_108Mhz-1326815617.gif

Implementing is not hard, anyone can do it. But I want to understand the circuit perfectly.

What I know about this circuit that it makes use of Colpitts oscillator. (In the middle of the circuit).

But what about the other parts of the circuit?
Please if you have any comment, even if a minor one, about the circuit parts, let me know !

#### HaneenSu

Joined Apr 21, 2013
2
Well, the first stage I guess is an amplifier to the speech signal. and It is followed by the oscillator. But I can't spot the mixer .. can any one help me in that?

#### vk6zgo

Joined Jul 21, 2012
677
That's easy--it doesn't have a mixer!
Have a look at the components marked CD1 & CD2------What is the significance of the "C" part of the designation?

#### TecknoTone

Joined May 20, 2012
21
Capacitance; they are varicap diodes and function to frequency modulate the signal by varying the amount of C in the free running oscillator in sympathy with the audio signal.

#### vk6zgo

Joined Jul 21, 2012
677
I know what they do.
I was trying to lead the OP to make the same conclusion that you did.

#### haneen

Joined Nov 29, 2010
1
Thanks a lot for you help and comments vk6zgo and TecknoTone!
I was trying to simulate the circuit and then I found it has a double gate transistor which I couldn't find, so I thought of finding another circuit, which is this:

Now for this one, the first stage is also an amplifier. Q2 forms the oscillator with the variable capacitor and L2. C8 C7 and L1 are elements used for matching.

Is what I mentioned right? Correct me please if not.

One more thing, why the diode is useful here? And what about C2 (The tantalum capacitor) ?

#### fjord

Joined May 10, 2012
14
I have a question related to FM transmitters too and I don't want to start a new thread just to ask.

My question is this:

This seems to be about the simplest possible FM transmitter.

I understand everything, except, how does the transistor modulate the frequency that the LC tank circuit oscillates at?

The resonant frequency for an LC tank circuit is f = 1/(2pi * sqrt(LC) ) and none of those parameters are being modulated by modulating the current through the transistor????? Do I need to think about it in small signal model terms, which would give me an L // C // R // Controlled current source

Thanks

#### THE_RB

Joined Feb 11, 2008
5,438
As the transistor turns more or less on, it has an affect on the cap that is between its collector and emitter.

The general effect would be an effective change in capacitance affecting the tuned circuit so freq will change.

#### fjord

Joined May 10, 2012
14
How does it affect the cap?

I've never seen a capacitor connected like this before

#### fjord

Joined May 10, 2012
14
I'm really hoping to take analog filter design next semester, because I feel like as soon as there are any passive components that aren't/cant simply be treated as simply coupling capacitors or simple filter capacitors or meant to be open circuit to AC (inductor), I'm lost

#### TecknoTone

Joined May 20, 2012
21
I like the design of the first circuit best. The second one looks dirty.
There must be an equivalent FET for that application if you can't source it locally.

#### vk6zgo

Joined Jul 21, 2012
677
I like the design of the first circuit best. The second one looks dirty.
There must be an equivalent FET for that application if you can't source it locally.
I agree,but it looks like he wants to simulate it & his simulation software doesn't include a twin gate MOSFET.

He could just swap a JFET into the circuit,ditch the bias circuit for the second gate,& it would probably simulate OK.

I think simulation is a waste of time,anyway,as it isn't going to look anything like the physical device they end up with if they make a real one.

The whole idea of Newbies building self excited FM "bugs" has always struck me as dumb,as they are thrown into realms which they don't really understand.
You get dozens of beginners trying to build VHF circuitry on breadboards & wondering why they are unsuccessful!