Building an AM Transmitter

Sine wave AMIt’s been a little quiet lately here on the blog! It’s because I’ve been “heads down” in the process of building an AM transmitter for Richmond Radio.

After a lot of reading the FCC rules and thinking about the characteristics of AM and FM signals, I’ve decided to start Richmond Radio on AM.

I was confronted with the classic build vs. buy decision. On the “buy” side, there are some very nice, capable Part 15 certified AM transmitters available from folks like Hamilton and Chez Radio. There is a lot to putting even a small radio station on the air in a professional manner and having a ready made, well built transmitter is very attractive. However, these transmitters are not inexpensive given I have a budget somewhere between zero and not a lot.

On the build side of the decision, I am confident that I could design my own transmitter for not very much money. Although since it is DIY, it will take me more time to put a professional quality signal on the air.

At this point, I have the carrier generator and modulator built up.

For the carrier generator, I found that I could get inexpensive DDS (direct digital synthesis) modules for about $5. These units need to be “programmed” to the desired frequency each time they are powered up. This is done with a $2.95 ATTiny84 microcontroller.  So even at $8 for the parts, it is still half the cost of a single AM broadcast band crystal.

Here is the carrier generator and modulator built up on a prototyping board.

Carrier Generator & Modulator

The DDS gives the stability of a crystal oscillator but with frequency agility. It’s amazing how just a few years ago the capability of the DDS and micro-controller would have cost hundreds of dollars and take dozens of components. Now it’s tiny and costs less than lunch.

The modulation is accomplished an older but quite capable (and still available) MC 1496 Balanced Modulator/Demodulator IC. For AM operation we don’t want a balanced condition so the chip is biased such that the carrier is not nulled out.

Here is a video of the carrier generator and modulator in action while it was still on the breadboard. The output of buffer amplifier is coupled to the receiver by simply draping the wire over the radio.

This low level modulation is sort of a compromise. I avoid the need to develop more audio power and use a modulation transformer, but on the other hand I need to follow it with a linear amplifier. The amplifier will need to be class AB which is less efficient than the class C final with high level modulation, so I’ll be getting less output for my 100 mw DC input to the final amplifier.  The antenna match will have to be excellent.

So that’s what’s happening here at Richmond Radio. I’ll post updates on this AM transmitter build as needed.