AM Transmitter circuit.

Thread Starter

MerryGR

Joined Jul 21, 2019
25
Hello, I've decided to design myself a AM transmitter circuit. It's operating at around 600 kHz. The main problem is that I don't know how to properly make and connect this Colpitts oscillator to the AM modulator. I have set the Q point of the first transistor in the middle of the load line.. and oscillations are pretty good. But one thing I don't understand is to how properly pick the correct coupling capacitors. This is what I have designed:

pullup.png

Now, in general, what I need to know is this:
  1. How to properly pick C5 and C4
  2. How should the AM transmitter circuit look like connected to the Colpitts oscillator (ideally using transistors .. if you would be so kind you can also try to calculate it for me and then explain, I'd be much appriciated to you for this)

I'm aware of the fact that using such transmitters is illegal. I just wanna try it here in my room. I also know it's not recommended building it on the breadboard.

Would you help me, guys?
Thanks in advance!
 

Alec_t

Joined Sep 17, 2013
10,920
I'm aware of the fact that using such transmitters is illegal. I just wanna try it here in my room.
The problem is that radio waves won't recognise the walls of your room as boundaries (unless your room is built like a Faraday cage), so will get out and can cause interference to sensitive receivers belonging to legitimate users of the radio spectrum.
Forum rules.
 

dendad

Joined Feb 20, 2016
3,330
Even a few mW can cause problems. You never know what you may be interfering with.
Years ago, friends of mine did just that and unknowing jammed the local fire brigade. They were fortunate they just got a smack on the wrist, as well as loosing all their electronic equipment!
If you want to play with transmitters, do a bit of study and go for your ham radio license.

As for helping, this is help.
We cannot assist in illegal activity, no matter what you may want.
 

Thread Starter

MerryGR

Joined Jul 21, 2019
25
I do not live in US, I live in a country where there are almost no AM stations.. I rarely hear something on one frequency.
 

DickCappels

Joined Aug 21, 2008
6,199
We are not allowed to encourage illegal activities on this forum. After a lot of looking I was unable to find any reference to unlicensed low power radio transmitters in your country (according to your IP address). Can you confirm the country in which you live so we can understand whether there is a problem or not? The moderation staff will discuss the question of whether or not discussion of this transmitter is allowed. Don't go away!
 

WBahn

Joined Mar 31, 2012
25,299
I'm aware of the fact that using such transmitters is illegal.
And herein lies the problem. You are asking us to assist you with something that YOU are claiming is illegal where you live. That is a direct violation of forum rules.

Before this discussion can continue, YOU need to explore the actual laws where you live and determine what you need to do to stay within them. Most likely there are spectrum bands and power levels that will let you do what you are trying to do. Once you find out the laws and post them, we have members that can help you ensure that your circuit doesn't go afoul of them. In the meantime, we don't want to help you get yourself in trouble or cause problems, potentially serious ones, for other people using the spectrum.
 

dendad

Joined Feb 20, 2016
3,330
Yes, you do. For the US and Canada, 600 kHz is squarely within the AM broadcast band. Plenty of websites can tell you the stations and frequencies in your area.
What about harmonics? A band pass filter on the output is a good idea.
If you do want to make an AM transmitter, it is best to not directly modulate the oscillator. Add an extra buffer stage so the frequency is not pulled by the modulation.
 

MrAl

Joined Jun 17, 2014
7,189
Hi,

Just to add a little here...

Another issues is carrier harmonic content. Your output needs to be fairly pure so that you dont put out signals on other frequencies you dont intend to.
For an ideal pure sine wave carrier, you can transmit on one frequency so that normally the signal can only be received on one frequency. But if you put out a square wave, there are multiple frequencies that can receive the signal. As an experiment, i was able to transmit on at least three different frequencies using a square wave and receiver about 30 feed away. Luckily the signals diminish as the harmonic number goes up, but you see the point. Try to get a pure signal if you do anything at all. That means a sinusoid as carrier.
A square wave at 600kHz will transmit on 600kHz, 1800kHz, 3000kHz, 3600kHz, etc., but a sine wave will only transmit on 600kHz plus or minus a small amount. A clipped sine wave is also rich in harmonics unfortunately, so you need to limit the clipping action.
You can test your circuit with a short wave receiver. See what frequency settings pick up the carrier.
 
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