FM transmitter.

[mstechca]

Lets take a simple FM transmitter.

What components should I deal with and how should I change them? I want to optimize it for distance, but I also want to hear the transmitted tone on the receiver as well. I don't care if the tone changes frequency, but I still want to hear it when the transmitter and receiver are far away.

 

[audioguru]

Lets take a simple FM transmitter.
What components should I deal with and how should I change them? I want to optimize it for distance

Then use a high supply voltage and a low-value emitter resistor to get more power output. Adding an RF power amplifier will also extend its range. Adding a multiple-stage RF power amplifier will extend its range a lot (and put you in jail).

but I also want to hear the transmitted tone on the receiver as well. I don't care if the tone changes frequency, but I still want to hear it when the transmitter and receiver are far away.

What tone? Are you modulating it with an audio tone?

Maybe you mean its RF carrier frequency. A simple FM transmitter keeps changing its RF frequency if you or something gets near it, if the temperature changes or if the battery voltage changes. When its RF frequency changes then you must re-tune the radio to find it again on the dial. It might even change its frequency to the same as a broadcast station and be masked and cause interference.

You can fix the frequency change caused by proximity by adding a buffer/RF amplifier stage between its RF oscillator and its antenna.
You can fix the frequency change caused by temperature by using appropriate negative tempco capacitors.
You can fix the frequency change caused by voltage by using a regulator.

While you are fixing it, don't you want to add pre-emphasis to its audio preamp so it sounds correct? Without having pre-emphasis, irs received audio will have no treble, especially in North America.

What about stereo? Simple FM transmitters are mono.

What about distortion? Most simple and even many stereo FM transmitters sound awful.

Try this one that I designed and built, it works and sounds pretty good:

 

[TV-Engineer]

Funnily enough, as of yesterday I decided I wanted to build an FM transmitter for my father - whose hearing isn't so good anymore but likes his radio - to replace the one he has now which isn't working. He likes listening to sat receiver audio outputted by the transmitter.

Because of his bad hearing, it needs to be very good quality, and for ease of use, no frequency drift.

The one he has now was a kit, and it uses a crystal to keep its frequency steady.

I've searched high and low on the internet for a schematic for a good one, but I can only find the type which drift, or PLL controlled ones which use PIC chips programmed at home, which I don't want to bother with.

I don't suppose there's a nifty circuit about which would combine the high quality stereo audio with crystal control to stop frequency drift, take around 12V and transmit at about 300 to 500 mW (enough to get radio down to the bottom of the garden, at least, preferrably further since the walls are breeze blocks and not so kind to radio waves)? Doesn't matter if it needs a fair few components, that's no problem.

I'd be grateful to anyone who can help, it'd be a nice gift for my father, and allow me to use my new photo-resist board with a laser printer that's come my way today!

Thanks.

Over and out.

 

[audioguru]

Hi TV,
There is a crystal/ppl controlled stereo transmitter project by Silicon Chip mag. A few Australian and New Zealand companies have it as a kit. It is used in many of those portable MP3 player to car radio gadgets. You select its 14 FM channels with dip switches. It is fairly good, except rolls off the highs at 10KHz and above, and has a fairly high 3% distortion.
The project has an output attenuator to keep the level below Australia's 10mW limit. The IC's manufacturer doesn't say how much its power is. You could make it really illegal and cover your entire property by adding a simple RF power amp like in my circuit.


Go to Google on the web and search for Stereo Micromitter. A link to the project will present the entire article for free. I've posted its schematic on this one and other sites a few times.

 

[mstechca]

It seems that 99% of the transmitters out there have a microphone input. I dont want a microphone input because I dont want my voice on the air. Instead I want a continuous tone.

If you still dont understand, then tell me how I can make a wireless morse-code transmitter that can transmit audio tones?

To start, I have been figuring out exactly what makes FM transmitters work.

I rip apart the smallest FM transmitter I could find on the net, and I have discovered the following circuit will oscillate provided that:

a) the feedback capacitor is the correct value (about 10pf)
b) R and C are high enough values

What I notice is that if I increase the feedback capacitor, the oscillator frequency is higher.

So, with this "SIMPLE" circuit, what equation can I use to determine the exact frequency?

AND, if someone could tell me the frequency of 100% silence, (its gotta be somewhere between 20Khz and 30Mhz) I can probably make a small transmitter that can block out a frequency and literally mute the audio.

I need help on this.

 

[audioguru]

Hi MS,
Your simple circuit might oscillate around 100MHz if you use the speaker's voice coil as its inductor and tank capacitance and connect the other end of its coil to the positive supply. Then connect an antenna to its collector.
But it isn't FM, nor AM. It isn't modulated. It transmits silence which is a non-modulated carrier. You need to add an audio oscillator to modulate it with an audio tone.

Silence isn't a frequency, it is a lack of sound. Ultrasonic frequencies are real, but your hearing can't detect them. You can add ultrasonic frequencies to normal audio and the audio won't be muted nor affected in any way.

AM transmissions on the same RF frequency add together. FM transmissions don't, because good FM radios have so much gain and limiting in their IF strips that they have an excellent "capture ratio". The capture ratio allows a good FM radio to lock-in on a transmission on the same RF frequency that is slightly stronger than another one, completely blocking it.

 

[audioguru]

I forgot, your simple transmitter also needs an emitter resistor.

 

[mstechca]

how did you figure 100Mhz?

Lets say the oscillator comes with the emitter resistor. What equation can I use to determine the frequency?

 

[Nigel Goodwin]

how did you figure 100Mhz?

Lets say the oscillator comes with the emitter resistor. What equation can I use to determine the frequency?

He 'guessed' it :lol:

The formula is the standard one for working out resonance of an LC circuit (which you should already know!). The problem is working out the L and C values, there are far too many things affecting those values - so there's not really any way to calculate it (at least not with any accuracy).

The only way is to build it and measure it, then adjust the values accordingly.

 

[mstechca]

if you say LC determines the frequency of the output tone, then why is it that when I change any component shown in my oscillator diagram, the frequency changes?

Are you telling me that I can just change those values around, and the tone frequency doesnt change? I dont think so.

What I have learned from experience is that two frequencies exist in a transmitter, the Radio frequency, and the audio (tone or voice) frequency. And in a radio transmitter, the audio or voice gets passed through the radio frequency. The simple LC equation would reveal the radio frequency, and I understand that. But, what I need is the equation for the audio frequency with the feedback capacitor. For a simple oscillator, I can just do R * C, but here, it doesn't exactly work that way because of the feedback capacitor.

I need an equation for the audio frequency.

 

[Nigel Goodwin]

if you say LC determines the frequency of the output tone, then why is it that when I change any component shown in my oscillator diagram, the frequency changes?

Because you are altering L or C (most probably C), changing any component around the oscillator will affect these.

Are you telling me that I can just change those values around, and the tone frequency doesnt change? I dont think so.

Having finally realised what you are on about, there IS NO TONE.

What I have learned from experience is that two frequencies exist in a transmitter, the Radio frequency, and the audio (tone or voice) frequency.

That's not really true, you need to 'learn' some more 8)

A transmitter has a carrier (RF) and 'may' include modulation at a lower frequency, but this is seperate to the RF transmitter part.

And in a radio transmitter, the audio or voice gets passed through the radio frequency. The simple LC equation would reveal the radio frequency, and I understand that. But, what I need is the equation for the audio frequency with the feedback capacitor. For a simple oscillator, I can just do R * C, but here, it doesn't exactly work that way because of the feedback capacitor.

I need an equation for the audio frequency.

There is NO AUDIO FREQUENCY, just an RF carrier signal, modulated by any input fed to it - if you want an audio tone you need to build an audio oscillator and feed it to the input.

 

[audioguru]

I think the "tone" is caused by the self-quenching frequency of his super-regen tuner. He is a super-regen-o-maniac!

 

[mstechca]

Me: if you say LC determines the frequency of the output tone, then why is it that when I change any component shown in my oscillator diagram, the frequency changes?

Nigel: Because you are altering L or C (most probably C), changing any component around the oscillator will affect these.

in my original circuit, there is no L.

Me: Are you telling me that I can just change those values around, and the tone frequency doesnt change? I dont think so.

Nigel: Having finally realised what you are on about, there IS NO TONE.

Uhh, I built and tested the circuit, so I do know what I am talking about.

Me: What I have learned from experience is that two frequencies exist in a transmitter, the Radio frequency, and the audio (tone or voice) frequency.

Nigel: That's not really true, you need to 'learn' some more 8)
A transmitter has a carrier (RF) and 'may' include modulation at a lower frequency, but this is seperate to the RF transmitter part.

How about a tracking transmitter. How can it transmit to a receiver if a tone isn't passed?

Me: And in a radio transmitter, the audio or voice gets passed through the radio frequency. The simple LC equation would reveal the radio frequency, and I understand that. But, what I need is the equation for the audio frequency with the feedback capacitor. For a simple oscillator, I can just do R * C, but here, it doesn't exactly work that way because of the feedback capacitor.

I need an equation for the audio frequency.

Nigel: There is NO AUDIO FREQUENCY, just an RF carrier signal, modulated by any input fed to it - if you want an audio tone you need to build an audio oscillator and feed it to the input.

Now you are really throwing me off. If I could just get AN EQUATION, maybe I wouldn't be in need of help right now.

 

[Nigel Goodwin]

Me: if you say LC determines the frequency of the output tone, then why is it that when I change any component shown in my oscillator diagram, the frequency changes?

Nigel: Because you are altering L or C (most probably C), changing any component around the oscillator will affect these.

in my original circuit, there is no L.

Try posting the circuit then, I can't imagine a VHF transmitter without an inductive component?.

Me: Are you telling me that I can just change those values around, and the tone frequency doesnt change? I dont think so.

Nigel: Having finally realised what you are on about, there IS NO TONE.

Uhh, I built and tested the circuit, so I do know what I am talking about.

Well post it then!, you may have multiple stages in this transmitter, including a modulating audio oscillator - but how would we know?.

Me: What I have learned from experience is that two frequencies exist in a transmitter, the Radio frequency, and the audio (tone or voice) frequency.

Nigel: That's not really true, you need to 'learn' some more 8)
A transmitter has a carrier (RF) and 'may' include modulation at a lower frequency, but this is seperate to the RF transmitter part.

How about a tracking transmitter. How can it transmit to a receiver if a tone isn't passed?

It only transmits an RF carrier, why would you need modulation on a tracking transmitter?.

Me: And in a radio transmitter, the audio or voice gets passed through the radio frequency. The simple LC equation would reveal the radio frequency, and I understand that. But, what I need is the equation for the audio frequency with the feedback capacitor. For a simple oscillator, I can just do R * C, but here, it doesn't exactly work that way because of the feedback capacitor.

I need an equation for the audio frequency.

Nigel: There is NO AUDIO FREQUENCY, just an RF carrier signal, modulated by any input fed to it - if you want an audio tone you need to build an audio oscillator and feed it to the input.

Now you are really throwing me off. If I could just get AN EQUATION, maybe I wouldn't be in need of help right now.

You're not listening! - you can't get an equation because what you're asking doesn't exist! - try posting your circuit and we'll offer advice - but from what you've posted so far it makes very little sense?.

 

[audioguru]

There are a few tracking "FM" transmitter circuits on the web that pulse the RF on and off and you can hear them due to the resulting bursts of quieting in normal FM radios that don't have inter-station muting.

Isn't that a good joke? He, he. They actually transmit silence. Maybe that is what MS meant when he wanted to modulate with ultrasonics.
Their authors say they can be detected at a much further distance than with modulation.

 

[mstechca]

audioguru is now starting to understand me.

I hope everyone understands this:

1. I want to make an FM transmitter using only transistors as the semiconductor devices.
2. I want the transmitter to transmit a continuous audio tone.
3. I want equations relating to every component in the circuit.

It seems that when I search for these answers elsewhere, I get 100 hits for request #1, maybe 1 or 2 hits if I am lucky for request #2 and ZERO hits for request #3.

I come here, because I need help!

So I would be grateful if I can get some equations together for the circuits.
and please nevermind handing me the LC equation because a zillion web site have already shown me that one.

 

[Nigel Goodwin]

audioguru is now starting to understand me.

I hope everyone understands this:

1. I want to make an FM transmitter using only transistors as the semiconductor devices.

How about the circuit Audioguru posted earlier in the thread?

2. I want the transmitter to transmit a continuous audio tone.

Why? - and it's not a 'silly' question!.

3. I want equations relating to every component in the circuit.

There are not equations for every component in the circuit, as already explained numerous times, in almost any circuit you have to make assumptions for some of the values. But in particular these types of circuits can't be calculated because their construction alters the values os inductance and capacitance.

The only equations you need are those you already know, LC resonance, and ohms law!.

 

[audioguru]

MS,
Here is a very simple RF oscillator that is used in many FM transmitter circuits on the web.
It transmits a weak signal on the FM band.
It transmits AM and FM when modulated with a tone.
It drifts its RF frequency all over the place if you or something gets near it, if the temperature changes or if its battery voltage runs down.
The function of its few parts should be obvious.
You can modulate it with an audio tone from a single transistor phase-shift oscillator or any of the many audio oscillator circuits on the web.
You can even gate it on and off with a low frequency oscillator like the tracking transmitters on the web.
Unlike your posted circuit, its collector and emitter go somewhere.

 

[zachtheterrible]

It transmits a weak signal on the FM band.
It transmits AM and FM when modulated with a tone.

out of curiousity, if i had an AM receiver that is around 88-108MHZ, would i be able to receive the signal easier, in terms of sensitivity?

 

[audioguru]

Hi Zach,
I think a simple FM transmitter has the same amount of AM as FM. The limiting factor with an AM receiver sensitivity is that it detects its own noise, and atmospheric noise is also detected.