FM Transmitter help!

cs200939

Nov 20, 2010
7
hey guys I've been working on this FM transmitter for like a month and eventually my efforts payed off and i was able to make it work and it works awesome.Now as a matter of fact (which may be a little matter of shame too),I dont know a single bit how it works!.I Know theres and amplifier and an oscilattor used, but i dont know for sure and i also dont know what all the other capacitors and resistors,inductors doing there.I mean I am totaly nill about this project and it was just a luck!.So kindly tell me what each part of this circuit does and since i really want to study this FM phenomanen ,tell me any relative theories or articles that i should study in order to get in depth knowledge about this circuit's working and the theory behind it.Your help is highly demanded and appreciated.thanks

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barathbushan

Sep 26, 2009
223
i have divided your transmitter into three stages

1) THE capacitor at the supply end is used to smooth out the ripples present in your voltage source

2) STAGE 1 - the voice signal which is an ac signal is fed to the base of transistor q1 ,
notice c2 is used to block dc noise which may corrupt your signal . R1 and R2 are used
to bias the transistor q1 to operate in amplification mode . At the collector of q1 ,
the amplified message signal is obtained

3) c3 capacitor is once again used to block dc , which may enter through R2

4)stage 2 - q2 transistor is biased as an Class c Tuned amplifier , which performs the
oscillation at its collector which is tuned to the frequency of

f=1/2pi*sqrt(LC)

the signal at the base is obtained at the emitter of the transistor Q2 [emitter follower]
which is then passed via capacitor c6 to the oscillator , where the MODULATION occurs

5)the signal is then transmitted via an antenna

barathbushan

Sep 26, 2009
223
this is the simplest FM transmitter i have ever come across with only two transistors .
where did you find this design ??

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Does anyone else find it unusual that C1 is only 470 pF?

I imagine it makes the 9V battery look like a fairly low impedance at the oscillator frequency, bit I'd normally expect to see a higher value capacitor here.

barathbushan

Sep 26, 2009
223
i think the capacitor value is low because you are just filtering an already smooth DC wave

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Normally you have a capacitor in parallel with a battery (especially small ones), not to smooth the DC per se, but to provide an alternate low impedance source for short term transient loads.

barathbushan

Sep 26, 2009
223
in many circuits that i have built , many components required that sort of an input capacitor , for example in the datasheet of a 7805 a pf capacitors are used at ip and op ,
but i ignore all those caps , and the circuit works just fine .

by the way i didn't know the concept of transient loads , so thanks for the info

cs200939

Nov 20, 2010
7
This circut and the similar ones are all over the web but none of them are so much clearly explained as you just did.Thanks alot.Even though i dont have much expierence with electronics ,i am still able to make up a working picture of this transmitter's working.Anyway could suggest any suitable articles or topics that i should study to get more in depth and clear knowledge of all the components used and their role in this circuit,like filtering with a capacitor,transistor as an oscilator,bank circuit,amplification etc,that would be really nice of you thanks

barathbushan

Sep 26, 2009
223
This circut and the similar ones are all over the web but none of them are so much clearly explained as you just did.Thanks alot.Even though i dont have much expierence with electronics ,i am still able to make up a working picture of this transmitter's working.Anyway could suggest any suitable articles or topics that i should study to get more in depth and clear knowledge of all the components used and their role in this circuit,like filtering with a capacitor,transistor as an oscilator,bank circuit,amplification etc,that would be really nice of you thanks

Im glad you understood what i told , for starters you can read topics such as
1)transistor biasing techniques
2)types of transistor amplifiers like class a , class b class ab etc
3)oscillator circuits like wein bridge , RC coupled , hartley , colpitts etc
4)a bit about operation of devices such as capacitors ,inductors, diodes , resistors etc
5)circuit theorems such as KVL, nortons ,thevenins
6)concepts like voltage divider rule , current divider rule ,series/parallel reduction of resistors and capacitors

i think for beginners the book called "THE ART OF ELECTRONICS" , contains all the necessary topics that cover electronics .

Last edited:

Nov 20, 2010
7
thanks alot

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
"The Art of Electronics" is a great book, but to describe it as something for a beginner is a bit of a stretch.

It's great for someone who has a real interest in electronics and who wants a practical understanding without all the math that goes along with it.

It's also not cheap.

But if you're interested, and you can browse through it before you buy it, I would encourage you to have a look at it.

cs200939

Nov 20, 2010
7
so which book will you suggest for a beginner?

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
It all depends on how you define beginner.

If you understand the description of the FM transmitter, then you may have enough background for The Art Of Electronics. Like I said, find a copy and leaf through it.

I think Google will show you some pages

davenn

Moderator
Sep 5, 2009
14,011
in many circuits that i have built , many components required that sort of an input capacitor , for example in the datasheet of a 7805 a pf capacitors are used at ip and op ,
but i ignore all those caps , and the circuit works just fine .

you shouldnt ignore the caps in the cct either side of the regulator. They are there for a reason ..... apart from steve's valid comments, the other reason there are different values of decoupling caps on PSU rails are for covering a range of freqs.

get into the habit of using them !!

This is really important in say where that regulator is feeding a RF circuit where you need not only to decouple low freq ripple on the DC rail but also any much higher freq that appears on the DC rail at what the circuit is operating at.

In the RF work that i do its not uncommon to have 3 or 4 wide spread values of caps decoupling a DC rail. For example you mite have a 10uF a 0.1uF and a .001uF cap side by side from the DC rail to ground.

without it you can get crazy things happening with say VCO's operating at UHF and SHF freqs where instability on the DC rail translates to instability on the output freq of the VCO.

Dave

barathbushan

Sep 26, 2009
223
you shouldnt ignore the caps in the cct either side of the regulator. They are there for a reason ..... apart from steve's valid comments, the other reason there are different values of decoupling caps on PSU rails are for covering a range of freqs.

get into the habit of using them !!

This is really important in say where that regulator is feeding a RF circuit where you need not only to decouple low freq ripple on the DC rail but also any much higher freq that appears on the DC rail at what the circuit is operating at.

In the RF work that i do its not uncommon to have 3 or 4 wide spread values of caps decoupling a DC rail. For example you mite have a 10uF a 0.1uF and a .001uF cap side by side from the DC rail to ground.

without it you can get crazy things happening with say VCO's operating at UHF and SHF freqs where instability on the DC rail translates to instability on the output freq of the VCO.

Dave

Thanks for the info dave , ill definitely use the caps .

barathbushan

Sep 26, 2009
223
"The Art of Electronics" is a great book, but to describe it as something for a beginner is a bit of a stretch.

It's great for someone who has a real interest in electronics and who wants a practical understanding without all the math that goes along with it.

It's also not cheap.

But if you're interested, and you can browse through it before you buy it, I would encourage you to have a look at it.

Hi steve , the art of electronics is the cheapest book about electronics that i have bought .
In INDIA it is retailed at 295 rupees [around 5 to 6 $] , but i bought an used book for way more cheaper. published by FOUNDATION PUBLISHERS , not cambridge , but the book quality is pretty good . and its very much the same containing 1125 pages . But yes of course the one from cambridge is very expensive as you said around 47$

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
In the RF work that i do its not uncommon to have 3 or 4 wide spread values of caps decoupling a DC rail. For example you mite have a 10uF a 0.1uF and a .001uF cap side by side from the DC rail to ground.

I'm glad you brought that up. I was thinking along those lines when I was crafting one of my replies but thought I'd gloss over it.

In addition to having multiple capacitors of different values, they (especially the smaller valued ones) can also often be duplicated and spread throughout the circuit (especially where there are relatively long power rails).

The same techniques are also often seen in logic and power supply (especially smps) circuits.

At low frequencies the internal resistance of power sources tends to be an issue. As the frequency rises, the issue shifts to being one that is more dominated by (stray) inductance.

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Hi steve , the art of electronics is the cheapest book about electronics that i have bought .

Mine cost the equivalent of Rs 6750 (\$AUD150) so I guess one of us got a better deal.

Having said that, for the amount of value I've got from it, I would probably have paid double that.

But I bought a few books on my recent trip to India too.

cs200939

Nov 20, 2010
7
hi once again.I ve some problem understadning the stage 3 of the circuit.I have read about the class C amplifier of the transsitor but i am having difficulty in understanding it.Can anybody explain it in easy terms?.I mean why we connect a tank circuit in parallel to the amplifier?.what property of the tank circuit makes it obvious to be used here?and which part of the stage is modulating the input signal and sending out the signal to the antenna thanks

davenn

Moderator
Sep 5, 2009
14,011
hi once again.I ve some problem understadning the stage 3 of the circuit.I have read about the class C amplifier of the transsitor but i am having difficulty in understanding it.Can anybody explain it in easy terms?.I mean why we connect a tank circuit in parallel to the amplifier?.what property of the tank circuit makes it obvious to be used here?and which part of the stage is modulating the input signal and sending out the signal to the antenna thanks

Simply put .... The tank cct is part of the oscillator and its freq of operation.

modulation from the simple 1 transistor microphone amplifier, Q1, is being applied to the base of Q2 via C3. With no modulation present the bias voltage on the base of Q2 is stable, producing a steady transmitted carrier at the osc. freq. But when modulation is applied to the base of Q2 the bias is varied and this varies the freq of the osc. producing an Frequency Modulated signal.

cheers
Dave

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