• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

amatuer FM transmitter

Status
Not open for further replies.

chingyg

New Member
We decided to build a radio transmitter for our boarding school's hostel, its need about 100m radius indoor area to be covered.

I build this circuit on breadboard


but it is not working, by the way I changed the inductor to a 3.3uH and the capacitors to 1.8pF, which should oscillate at 92.3MHz.

I tried to test the circuit with my oscilloscope but it is caped at 20MHz so I can't test it.

Any one of you know what is wrong or know a good simple design of a PLL or even better coil less transmitter.
 

Hero999

Banned
A 3.3µH will have a higher parasitic capacitance than 0.9pF so will oscillate at a much lower frequency, probably around 50MHz.

The problem with that simple transmitter is that:

  • It doesn't have any pre-emphasis so it will sound like and AM radio.
  • It doesn't have an RF amplifier so the frequency will drift when an object is placed near antenna.
  • The power to the oscillator isn't regulated do the frequency will drift as the battery discharges.


This subject has been discussed here on many occasions so audioguru decided to design a better transmitter.
http://www.electro-tech-online.com/threads/fm-transmitter-mod4.24887/
 

chingyg

New Member
Thank you, I understand now, but would you think


Will work? because It has no inductors, I hate winding coils.

Can I add a RF at the out put of this to make it stronger?
 

MikeMl

Well-Known Member
Most Helpful Member
If N2 oscillates at all, it will oscillate at 10.7MHz. Driving that through the N4-6 will square it up and produce harmonics, but the ninth or tenth harmonic (which is what can be tuned by a 88-108 Mhz FM receiver) will have an effective radiated power which is tiny. You will be lucky if you can receive it over a distance of a meter.
 

chingyg

New Member
So you think I should just use the FM transmitter for the mod 4 page?

btw on that old circuit I have my own voltage regulator.

What about this (I got all the parts for it already)
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
You can't build VHF circuits on breadboard, so it's never going to work.

It needs building carefully and properly in order to work, and layout is pretty critical.
 

chingyg

New Member
What about veroboard for the VHF and breadboard for the audio.

BTW I forgot to mention, I'd love to have the transmitter running on a audio line in from a computer audio out port. How strong is the signal, can I bypass all the audio amplifier stages if I use the jack input, or what about the jack and a pot?
 

Hero999

Banned
Veroboard will work but make it as small as possible,

You need another power supply, you can't power a decent transmitter from an audio line.

If you go digital, you could use a USB port but it won't be simple, you'll need a microcontroller and to write a driver for you PC.
 

chingyg

New Member
that sounds complicated. but about the audio line, well it can power a speaker loudly than that of a electret mic with that amplifier on the transmitter design.

of course I'll use a power supply, but I am just wondering about using the signal from the audio lines.
 

chingyg

New Member
Surely the pc out can have per-emphasis and equalizer and many audio signal related functions. which will replace the need of the audio amp.
 

Hero999

Banned
Yes, you can use the line out, as a signal source but you'll need an attenuator.

I don't see why you can't do the pre-emphasis using software on the computer.
 

chingyg

New Member
Well, I am reading the mod4 thread as soon as I started this thread, pg 19 now.

I am going to try that transmitter, the only thing that bother me is the trimmer, the only component shop in my town only have 1 - 10pF trimmer. I have to get my friend to try another town 100km down the road in the weekend. I am just 17 so, no online purchasing or anything, and no funny equipments but I'll make it.

BTW what's an "attenuator"
 

chingyg

New Member
maybe I should ask, how to make the attenuator for the mod4 circuit, I know it's like a bunch of resistors and pot.
 
Last edited:

Hero999

Banned
An attenuator is the same for anything else; it just reduces the amplitude of the signal.

You really need the correct value trimmer because using a larger inductor will add more parasitic capacitance.
 

chingyg

New Member
how about a 5k pot in series with the signal input?

on the output jack there's GND left and right, If you want to combine left and right channel what can you do, connect them together? or just ignore one?
 

Hero999

Banned
Connect them together via a 47k resistor.

Rather than connect the resistor as you describe.

Use a 10k resistor and connect is R1 is in the schematic below.
 

chingyg

New Member
Do you mean something like
attenuator.GIF
I don't quite get the 47K part.
 

Hero999

Banned
RV 1 is right.

Sorry I should have said you need two 47k resistors, one for each input.
 

Hero999

Banned
Yes, that's right.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top