next topic »

[on:]

Hello mates,
i need to send RTTY or CW on 27MHz - Amplitude Modulation.
Can you give me an advice, how to do that?

I made my oscilator and amplifyier, but i cant make the AM from the arduino.
The schema is:

arduino ==== 27MHz oscilator ==== amp ==== antenna

The circuit is working perfect, but i cant make the AM

[Reply #1 on:]

If you've made the transmitter don't you just need a mixer to add your arduino output to the carrier?

[Reply #2 on:]

Probably, but i don`t know how?

[Reply #3 on:]

Can some one help?

[Reply #4 on:]

Modulate the oscillator like it shows you here:-
http://scitoys.com/scitoys/scitoys/radio/am_transmitter.html

[Reply #5 on:]

If you provide the schematic of your oscillator, I or someone else can probably advise on how to get an Arduino to key it on/off.

[Reply #6 on:]

I attach the schematic.
I use TTL oscilator, but this works too.

[Reply #7 on:]

So, fond of crucifying crystals are we? What an appalling circuit.

Still you can modulate it by varying the power signal to the chip. As it is a HC chip it will go higher than 6V, look up and see how high it will go. Then use a transistor to convert the output of an arduino pin to switch between this high voltage and 6V. This will AM modulate it.

Not sure where in the world you are but in most places this is illegal.

[Reply #8 on:]

27MHz is CB, and its free to use - Bulgaria.

The 72HC240 can go to 7V max.
What will be the schematic of your circuit?

[Reply #9 on:]

27MHz is CB, and its free to use - Bulgaria.

It is free to use but you are not free to make transmitters. All transmitters must be "type approved", which means the design must be submitted to the relevant licensing authorities and the actual transmitter tested for emissions. Then you have to make only that circuit that you have had approved, in your factory. When you have your approval certificate you can go ahead and make them.

Still:-
Find out the minimum value it will still oscillate at because you don't want to stop it all together. Then use a potential divider in the collector of a transistor, emitter to ground, base through a resistor to arduino output.
The center point of this potential divider goes to the supply of your chip and the other end to a 7V supply.
Calculate the values of your resistors so that when the transistor is on the potential divider delvers the minimum voltage you found before. When the transistor is off then the potential divider does not come into play and your oscillator supply is connected to the 7V.

[Reply #10 on:]

Thank you so much!

[Reply #11 on:]

btw that oscillator will create substantial interference at 3 * 27MHz, 5 * 27MHz (which is in the airband), 7 * 27MHz etc. and (probably to a lesser extent) at even multiples of 27MHz too. So you won't be very popular with the users of those frequencies.

[Reply #12 on:]

dc42, thank you for that informacion.
What you suggest to disable this interferences? Did you think, that interferences will have little power becouse of width of antenna(2.65m of wire)?

[Reply #13 on:]

The interference comes from the fact that you have a square wave, with all it's associated harmonics. If you have a 27MHz LC resonant circuit between the oscillator ant the antena that will help reduce them.

[Reply #14 on:]

I make it very simple, I use  analogWrite(pin, pulsewidth);

So i make this, arduino pin 9 makes 2.5V for 500ms, then up to 5V for another 500ms, then back to 2.5V.
Is that ok?
Here is video: