Amazing transmitter that uses an ATMega16 sampling audio in real time and instructing an AD9851 DDS to FM

The arrival of the WIA's "Amateur Radio" magazine is always a pleasure but as a home brew enthusiast I find myself flipping through looking for circuits (often from the fabulous Drew Diamond).

This month an article really caught my eye...



While I'm up to speed with the use of microprocessors I don't welcome seeing projects in many magazines that are just a Pic or Atmel chip with a bit of I/O and nothing else. What's more annoying is that the interesting part - the bit that shows how it works - the source code, isn't printed; so the article is not very useful on it's own.

Promoted strangely on the cover of AR as "A transceiver for an interesting and exciting band", Dale Hughes, VK1DSH presents his design for a 10m FM transceiver where the transmitter's microphone audio is sampled at 14kHz using the on-chip analog to digital converter. The chip converts the audio samples into the data stream needed to tell the AD9851 DDS chip to frequency modulate at the output frequency.

Bill wrote to me "Basically, during transmit the code enters a tight loop which does an auto-trigger ADC conversion then uses the previously converted value to adjust the current DDS frequency, so the output is always one sample behind. This happens at about 14k Hz and each DDS value is sent via SPI to the DDS chip which then updates its operating frequency. It's quite simple really and I was very pleased with the result."

It never dawned on me that these little chips had the grunt for real time audio sampling or that the DDS chip could be FMd like this.

The receiver is elegant too, based on a MC3357 FM receiver chip with the local oscillator also coming from the DDS.

Dale's email address wasn't easy to find so I wrote to the credited reviewer, Bill Maxwell, VK7MX who put me in touch. Dale quickly sent through the assembly source and also a slide show he presented at Gippstech last year about DSP with AVRs.

In that slide show Dale describes both FM generation and AM demodulation (the latter sampled at 7kHz), he mentions that SSB is not possible with the AVR. Mind you he's running the chip at 16Mhz and I'm sure I've seen some overclocking experiments.

Thanks Dale!