Hi
I'm thinking of making my own fm transmitter (tiny range) and as I have never worked with 100Mhz, I realized I need the help of more qualified people.
One fm modulator I really like is the armstrong one:
(Sorry but the french but I believe it is easily understandable as the words look the same).
Anyway, such a diagram will need AO to phase shift or add them and I'm not sure an AO can handle 100Mhz without significantly changing the signal. Am I right?
Secondly, for a circuit like this one (just an example):
No matter which circuit I am going to use, I will need a variable capacitor to adjust the oscillation.
I've seen a lot of 'trimmer capacitor' like that:
But they all need a screwdriver, do you know something that is manually adjustable like a potentiometer.
TLDR:
Can you confirm there is no easy way to phase shift/multiply/add signals which are >100Mhz?
How can I have a variable capacitor (<20pF /30pF) that can be manually rotated like in a finished product (like a radio...)
Can you confirm there is no easy way to phase shift/multiply/add signals which are >100Mhz?
On a 16MHz processor! Do you need to ask?
How can I have a variable capacitor (<20pF /30pF) that can be manually rotated like in a finished product (like a radio...)
Yes there are lots of them. However using a free running oscillator controlled by a variable capacitor is never going to be stable enough for a transmitter.
Also transmitting in the broadcast FM bands is illegal in many countries no matter what the range.
Where in the world are you.
FM modulator chips are all over the place -- you will have no trouble finding a chip or module that works in the 88-106 MHz band and does conventional broadcast FM modulation.
A variable capacitor could be a Varactor or Varicap --
Its capacitance is varied by changing a bias voltage, which could come from a potentiometer.
Grumpy_Mike:
On a 16MHz processor! Do you need to ask?
Actually I was talking about Op-amps when I said AO sorry for that mistake, AO is the common name we use in France. The arduino is obviously not going to receive the modulated signal.
Grumpy_Mike:
Yes there are lots of them.
Can you be more precise about it?
Moreover I don't want to make a 'real' transmitter that is going to be used, something that works well enough is ok. Unless you tell me that using a variable capacitor, I won't ever be able to send a modulated signal that can be demodulated afterwards I think it perfectly suits my needs.
Finally talking about legislation, there are quite a lot of small fm transmitter designed for smartphones being sold in physical shops. I'm aware this is not authorized but it is tolerated here in France. And as I said, the transmitter is not going to be used more than one or two minute and will be used underground.
FM modulator chips are all over the place -- you will have no trouble finding a chip or module that works in the 88-106 MHz band and does conventional broadcast FM modulation.
The goal of that project is not the result itself but more how I manage to have a working transmitter. So I'd prefer to make it by myself.
gardner:
A variable capacitor could be a Varactor or Varicap -- Varicap - Wikipedia
Its capacitance is varied by changing a bias voltage, which could come from a potentiometer.
I've heard about it but, correct me if I'm wrong, I don't think I can change the value of the capacitance on one side and then use that specified capacitance in the oscillator.
If I replace the variable capacitor by a varicap the value of the capacitance is going to change because of the AC that is going through and I won't be able to control it, isn't it?
It is also quite similar to the first circuit posted here, and that is exactly what I need for a transmitter.
About the question I still have
ā¢ Can an OP-amp handle 0->5V at 100Mhz without having a slew rate.
That would mean a minimum slew rate sigma of 5V100MHz2=1GV/S=1000V/Āµs which is huge
ā¢ Any idea about a variable capacitor you could manually adjust like a potentiometer (no screwdriver)
I found 'air variable capacitor' that could suit my needs but they are overpriced (50$)
ā¢ Is the capacitor replacable by a varicap, and how?
Anyway, such a diagram will need AO to phase shift or add them and I'm not sure an AO can handle 100Mhz without significantly changing the signal. Am I right?
Assuming you mean the Op Amp Summing Amplifier (labeled "Additioneur")(presumably French for "Adder")
You'll need an Op Amp with a high GBP (Gain Bandwidth Product).
The Linear LT1226 has a 1 Ghz GBP.
That is not a very specific term, what is good enough?
I'm aware this is not authorized but it is tolerated here in France.
That sounds like a typical French thing. There are laws and you ignore them. That is why the UK hate the EC they are the only ones that stick to the rules.
