After the Circus

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Casino de Hull, Wednesdays of Fire 2007-08-04

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jean at geemoo dot ca
HF LO Module

[HF LO Module]       [HF LO Module]


I started this project because I wanted a module that could be used for another of different tasks. The board is built around an Analog Devices AD9834 DDS chip, and an Atmel AVR ATTINY48 MCU. The board is remarkably flexible and can be used for a number of different tasks. You could use the module as:
  • a 0 to 37.5MHz LO: the board has precise tuning making it extremely suitable for driving a mixer,
  • a CW transmitter: the MCU has control over the DDS's output, allowing you to enable and disable tone generation at will
  • a FSK transmitter: the DDS has two frequency registers, and allows you to choose which is active, making it trivial to switch between them
  • a PSK transmitter: the DDS also has two phase registers, which alllows you to instantly change phase at the toggle of a pin.
      As a proof of concept for the board, I wrote a QRSS beacon using a library I wrote for designing the board and a bit of C code.       The HF LO module is a DDS controlled frequency generator with a tuning range of 0 to 37.5MHz and a tuning resolution of 1 Hz. The PCB is a 4 layer board with a total size of 1.7x1.4". The DDS is driven by a 75MHz oscillator with a +-25ppm accuracy.

Caveats (AKA "Things I screwed up")

  1. When I first designed the filters, I searched on digikey and chose values for them that were actually purchasable. I forgot, however, to see the package sizes of the components, and as a result, it's not feasable to get inductors for two of the filter chains in an 0603 format. I was able to source parts for the 38MHz filter and of course the no filter paths, but the 10MHz and 1MHz filters don't seem do-able. I haven't populated the 38MHz filter yet, so these values could require some tinkering as well. I'd strongly consider either just jumpering over the switchs and having the 38MHz filter hardcoded in.. or jumper off board to the other two filters. It's up to you.
  2. Some tinkering might be needed with the resistors (R4 and R5) at the output of the DDS. This is still an area that I don't excel in and I could easily have come up with these values wrong. Again.. tinker, test and send me some results. I'd be happy for some input here.

Schematic       Layout       Gerber Files