FM transmitter (mod4)

[Hero999]

I'm not able to buy online since I'm only 20, can't apply for credit card yet, and I can get a 56hm: easily from the lab .

I thought the minimum age was 18 or is it 21 in your area?

Anyway I didn't say online, I said in line.

I ment a dummy load resistor that you connect between the coaxial cable and the scope to match its impedance to the cable.

Anyway a simple feild meter is probably a simpler solution if all you want to do is tune the output stage to the correct frequency.

 

[bananasiong]

Ops, I read it wrongly
Minimum age is 21, but even if I'm 21, I can't get it with only part time job until I graduate.

I ment a dummy load resistor that you connect between the coaxial cable and the scope to match its impedance to the cable.

Now you can see how poor my English is, I can't figure out how does it look like. Do you mean the resistor paralleled with the coaxial cable? Or touch one end of the resistor to the place to be measured and the other end grounded? Because there're 2 terminal at the coaxial and the scope as well.

Thanks

 

[Hero999]

That's alright, this isn't an easy thing to explain. I woudln't be very supprised if some native English speaking people didn't understand.

I meant a resistor connected across the terminals of the scope; I think you might be able to by coaxial leads or adaptors with this bult in.

Try Googling a bit about transmission lines and characteristic impedance and it might become a bit more clear.

 

[bananasiong]

Thanks, I think I got it. I'll try it soon.

 

[bananasiong]

Can this be used as a field strength meter? I've seen one, the coil is connected where the 1khm: is. The multimeter should be set in AC mode or DC mode?

Thanks

 

[audioguru]

I took apart my Field Strength Meter. I forgot it needs a small input capacitor to block mains hum and AM redio stations. Also the 1st resistor is 10k.
Its output is DC or AM modulation.

 

[bananasiong]

What is the frequency range of this meter? Only in FM range? Why it doesn't need to be tuned to the right frequency?
What I need to do is just put the meter next to the transmitter and measure right? Otherwise the value I see will be the strong signal from FM station.
Can this meter measure the reception of the receiver? I think no.

Thanks

 

[audioguru]

What is the frequency range of this meter?

It works up to about 250MHz.

Why it doesn't need to be tuned to the right frequency?

It shows the strongest level of any high frequency signal near it. You don't need to tune it.

What I need to do is just put the meter next to the transmitter and measure right?

Mine was 1m from my transmitter.

Can this meter measure the reception of the receiver?

The circuit is just a rectifier without any gain so it shouldn't pickup radio stations unless they are very closeby.

 

[bananasiong]

Even it is simple, the capacitance in the breadboard can affect the measurement right? I think I'll make a PCB.

Thanks a lot

 

[audioguru]

A breadboard shouldn't be used for frequencies above zero Hz.
It is OK for small DC motors and small light bulbs.

 

[bananasiong]

My transmitter cannot goes higher or lower frequencies, just around 96MHz. I've tried 99MHz, the quality is a little bit poor compared to 96MHz. Is there anyway lowering or increasing the frequency? When I used the 'scope, it shows a flat line at certain time, I think around 120 degree show waveform.

Thanks

 

[audioguru]

C6 in my FM transmitter tunes it from below the FM broadcast band maybe down to 85MHz up to about 106MHz. I leave C13 tuned to 98MHz because its tuning is very wide and it covers the whole band.

 

[bananasiong]

It is illegal to transmit voice below or above FM range also, right?

 

[Hero999]

That depends on where you live and it is illegal where I live.

 

[audioguru]

Here there is a TV channel directly below the FM band. Many FM radios can pickup its audio. You will cause interference to its picture or sound and the interference is illegal.

Above the FM band is the aircraft and air traffic controllers band which you don't want to interfere with. Very illegal.

 

[bananasiong]

Oh, very illegal..
It's better to stick on FM radio range. Can I hear anything from TV if I lower the transmitter frequency? Or just interference?

Thanks

 

[audioguru]

The FM sound on our TV channel 6 is at 87.75MHz. Most FM radios can get it and my FM transmitter tuned to it can be picked up by a TV.

I don't know if you have a TV frequency near 88MHz.

 

[Nigel Goodwin]

Oh, very illegal..
It's better to stick on FM radio range. Can I hear anything from TV if I lower the transmitter frequency? Or just interference?

So is transmitting on FM radio!, depending what country you're in - as you don't have your location filled in we can't comment on the possible legality of it!.

 

[fever]

its really a nice thread for FM stuff.i cleared lots of doubts.
thanks guys.
coming in to the topic.i built FM transmitter using transistors.but the quality is not good.so in one thread audioguru said transmitter shld be stereo.i did a search and found two chips.
1.max2606 (later i came to know its not stereo)
2.BA1404 (some one said it produces distortion while working)
but in the below site the creator said that they did lot of experimentation and come up with a good design.
**broken link removed**

so i would like to hear comments from audio guru.
can u help me here
thanks

 

[audioguru]

Hi Fever,
Rohm company made the lousy BA1404 FM stereo tramsmitter IC long ago. They discontinued and replaced it about 5 years ago because its performance was lousy. A few kits still have it.

Their new BH1417 IC and a few others are much better and are used in the Micromitter project:
https://electronics-diy.com/schematics/BH1417_fm_transmitter.htm
https://electronics-diy.com/BH1417_PLL_Stereo_FM_Transmitter.php