Improving 100mW FM Transmitter Design

[neeraj kaira]

What is the use of inductor in a receiver? It amplifies the signal or it only blocks the high frequency signals?

 

[PhysicoRaj]

The inductor in a reciever is the part of a tank circuit, you may call it a tuning circuit which selects the frequency ( your station).

 

[neeraj kaira]

The inductor in a reciever is the part of a tank circuit, you may call it a tuning circuit which selects the frequency ( your station).

To tune the frequency i think there is a variable capacitor which selects the transmitted frequency.

 

[PhysicoRaj]

The variable capacitor along with the inductor acts as the tank ckt. See This.

 

[meBigGuy]

Radio receivers essentially use an RC tank circuit that resonates at the desired frequency. The variable capacitor certainly adjusts frequency, but it cannot do that without an inductor. Generally, the frequency at which the inductive reactance equals the capacitive reactance is called the resonant frequency.

http://en.wikipedia.org/wiki/LC_circuit

 

[neeraj kaira]

Yeah i got this thanks.
I have a transmitter which sends the signals 90-120 mhz due to a variable capacitor 0-100 pf but the range of this arrangement is only 10-15 metres :( i want to increase the range what should i do?

 

[PhysicoRaj]

Are you capable of altering it's design or it's a ready-made?

 

[neeraj kaira]

This is the circuit.

 

[PhysicoRaj]

Not concerning the details of the circuit, I hope this will do: Increase the input voltage. Add an amplifier (you will find details on these at the flick of a search in google). Increase your antenna length, or try to direct the signals in a particular direction you want. And this one :http://www.zen22142.zen.co.uk/Analysis/efftxd.htm

 

[neeraj kaira]

Not concerning the details of the circuit, I hope this will do: Increase the input voltage. Add an amplifier (you will find details on these at the flick of a search in google). Increase your antenna length, or try to direct the signals in a particular direction you want. And this one :http://www.zen22142.zen.co.uk/Analysis/efftxd.htm

Thanks :)

 

[jtbell]

I have a transmitter which sends the signals 90-120 mhz due to a variable capacitor 0-100 pf but the range of this arrangement is only 10-15 metres :( i want to increase the range what should i do?

Keep in mind your country's regulations about "private" radio transmissions. There is likely a limit on the effective power that you can radiate before potentially getting into trouble with the authorities. Or with your neighbo(u)rs.

 

[anhnha]

...
Generally, the frequency at which the inductive reactance equals the capacitive reactance is called the resonant frequency.

I always wonder why the LC tank resonates at the resonant frequency not other frequencies.
It would be great if you could explain it.

 

[Jony130]

I always wonder why the LC tank resonates at the resonant frequency not other frequencies.
It would be great if you could explain it.

Because only for this one frequency Xc = XL and for the parallel tank circuit equivalent impudence reach his maximum value. So only for this frequency no current is drawn from the source. And this means that the current is circulating between the inductor and the capacitor, so we have a resonance circuit.

 

[sophiecentaur]

I always wonder why the LC tank resonates at the resonant frequency not other frequencies.
It would be great if you could explain it.

An arm waving explanation about how a tank circuit works: If you charge a capacitor, it holds energy. When you connect an L across it, current will start to flow through the L, discharging the C. The current in the L produces a magnetic field, which stores energy. When the C has discharged, all the energy is now in the L. Current continues flowing in the L, charging the C in the other polarity, until the current is zero and the C is fully charged again - in the opposite sense. The process continues, with current sloshing around and the energy going back and forth at a frequency determined by the L and C values. This is the natural frequency of the circuit. If you apply a small signal at this frequency to an LC circuit, the supplied energy will build up in the circuit because each cycle of the applied signal is in step with the cycles of the energy sloshing back and forth. The amplitude will build up - in the same way that a pendulum / swing will build up its oscillations if you push it at the right rate. This only happens for applied signals with around the right frequency. Signals off frequency get out of step and there is no build up. This means the resonant circuit will 'select' only signals around the resonant frequency and reject others.

Making a resonant circuit is relatively easy at lowish Radio Frequencies as you can calculate the values of L and C. At frequencies around 100MHz, the components may not do exactly what's been calculated and there is a certain amount of 'green fingers' and experience needed to make a good resonator and an amplifier that actually works. You should look at Google Images of 100MHz amplifier circuits and see if there are any that you think you could build. (they can be very complicated)
This is an example of a circuit design for a 1W transmitter.

Unfortunately, an RF amplifier is not the easiest thing to start a career of circuit building with. Perhaps an off the shelf transmitter would be better - but not cheap, of course.

 

[meBigGuy]

The capacitive reactance is opposite sign of the inductive reactance, and they are equal at the resonant frequency, so they sort of cancel (in a series resonant circuit). But only at the resonant frequency. The remaining resistance (which cannot cancel) determines the quality of the circuit, which is called the Q. The higher the Q the narrower the tuning bandwidth.

http://www.electronics-tutorials.ws/accircuits/series-resonance.html
http://www.electronics-tutorials.ws/accircuits/parallel-resonance.html

Regarding your transmitter... How do you know the fault is not with the receiver? Or the antennas? Are you using properly designed antennas and connecting them with properly matched transmission lines?

 

[neeraj kaira]

The capacitive reactance is opposite sign of the inductive reactance, and they are equal at the resonant frequency, so they sort of cancel (in a series resonant circuit). But only at the resonant frequency. The remaining resistance (which cannot cancel) determines the quality of the circuit, which is called the Q. The higher the Q the narrower the tuning bandwidth.

http://www.electronics-tutorials.ws/accircuits/series-resonance.html
http://www.electronics-tutorials.ws/accircuits/parallel-resonance.html

Regarding your transmitter... How do you know the fault is not with the receiver? Or the antennas? Are you using properly designed antennas and connecting them with properly matched transmission lines?

Receiver is my cell phone so i think it can't be the problem ..

 

[sophiecentaur]

Receiver is my cell phone so i think it can't be the problem ..

What frequency do you think your phone is using? How do you think it can receive the signals you are producing?

 

[neeraj kaira]

What frequency do you think your phone is using? How do you think it can receive the signals you are producing?

About 100 mhz

 

[neeraj kaira]

I transmit the signal using a mic and receive via my cell phone ...but range only 10 metres :(

 

[sophiecentaur]

Are there any Analogue cell phones around, these days?

And, if you look at this link you will see the operating frequencies.

 

[sophiecentaur]

Something is not making sense here.

 

[neeraj kaira]

Are there any Analogue cell phones around, these days?

And, if you look at this link you will see the operating frequencies.

Analoge cell phones .!
I used fm radio in my cell to receive the transmitted signal

 

[sophiecentaur]

OMG Where do you live?

 

[sophiecentaur]

I transmit the signal using a mic and receive via my cell phone ...but range only 10 metres :(

This is what you wrote.

 

[neeraj kaira]

Listen i made a circuit which sends the signal at 90 - 120 mhz and i receive them using my cell phone by setting the fm radio at the desired frequency...

 

[sophiecentaur]

Listen i made a circuit which sends the signal at 90 - 120 mhz and i receive them using my cell phone by setting the fm radio at the desired frequency...

So you are telling us that your Cell phone receives Analogue FM ??
No wonder you can only get your set up to work partially. It seems to me that you are just receiving what would be referred to as Interference to your Cell Phone's normal mode of operation.

This is a great example of where a simple block diagram of a set up would have resolved most of these problems in just a couple of posts.

 

[sophiecentaur]

On second thoughts, I see that you mean you have a Cell Phone that also has an FM receiver in it. The CellPhoneness is irrelevant - except in as far as the receiving antenna is basically the earphone lead? Have you thought of modifying the receive antenna to be more suitable than this?

 

[neeraj kaira]

Yes the earphones works as antenna and i didn't try any modification with the antenna

 

[neeraj kaira]

Sir any method which could enhance the strength of the signals so as to transmit the signal to long distances

 

[sophiecentaur]

I should think you would be better to search for a design that produces more power and ditch what you have. But first. you need to make sure that your set up is, in fact, working properly. Borrow another FM radio receiver and see how that works. I seem to remember having a phone with an FM receiver in it and it wasn't very good - nothing like as good as my personal FM (Sony, perhaps) for reception when walking around the town.

 

[sophiecentaur]

One more comment. Was your transmitter bought or did you make it? It may just not be working optimally. You can make a vast difference to equipment, working at those frequencies by bending and tweaking coils and leads. You have the problem of not much available test gear, of course, which makes things doubly difficult.

 

[neeraj kaira]

One more comment. Was your transmitter bought or did you make it? It may just not be working optimally. You can make a vast difference to equipment, working at those frequencies by bending and tweaking coils and leads. You have the problem of not much available test gear, of course, which makes things doubly difficult.

I made it
This is the circuit diagram
What should i do please suggest some tips

 

[neeraj kaira]

Transmitter

 

[meBigGuy]

Is unlicensed fm transmission still limited to 100mW? I think it is.

The schmatic has been posted already in post 8.

I'd try making C2 a variable cap and see if it helps the antenna match.
What are you using for a transmit antenna?

Look at http://www.circuitstoday.com/2-km-fm-transmitter for a better design.

 

[neeraj kaira]

Is unlicensed fm transmission still limited to 100mW? I think it is.

The schmatic has been posted already in post 8.

I'd try making C2 a variable cap and see if it helps the antenna match.
What are you using for a transmit antenna?

Look at http://www.circuitstoday.com/2-km-fm-transmitter for a better design.

I'm using a wire as antenna

 

[sophiecentaur]

Is unlicensed fm transmission still limited to 100mW? I think it is.

The schmatic has been posted already in post 8.

I'd try making C2 a variable cap and see if it helps the antenna match.
What are you using for a transmit antenna?

Look at http://www.circuitstoday.com/2-km-fm-transmitter for a better design.

The problem with building circuits at 100MHz is that the way it's laid out is so important. Use of a ground plane and proper decoupling where it counts can make all the difference.
If neeraj kaira wants something that will just 'work for him' then he may not want to develop too much expertise as a result of 2nd and 3rd versions for laying out that circuit. That other schematic you provided is potentially better and more stable but it still needs to be soldered together with short lead lengths, disc ceramics in the right places, physically, etc.. The least he would need, I should say, would be a construction leaflet with a picture of a working model to work to - in addition to the schematic. You can often buy kits with a PCB which you just have to populate. That's the ideal way to get results, for starters.