As in past years, these challenges are not a competition, although we do get together to compare what everyone came up with.
My reasons for suggesting this project are:
- 7.125 is very active with AM activity here in NSW
- AM sounds lovely on air
- There are quite a few ways to generate AM
Crystals are available for 7.122 from Expanded Spectrum Systems for $2.55 ($2.15 in lots of 10). I used a technique suggested by Peter, VK3YE, to pull the crystal to 7.124 - which is near enough for AM.
Note that I couldn't get Peter's design to oscillate (I had substituted a 2N2222a I had at hand) but values from this page worked fine.
Ways to make AM
- Series modulation, either with a modulation transformer or heising modulation which passes the audio through an RF choke.
- Make DSB and re-insert the carrier - there's a bit on this in EMRFD.
- Pulse Width Modulation
- Software defined radio
The idea is that we'll build our transmitters, have a few contacts, and get together for a show and tell before the end of this year.
There seemed to be a good amount of interest at Sunday's meeting, I hope you'll join me.