A Synthesizer/Mixer Board

At the February, 2012 50 MHz and Up meeting, Lars AA6IW did a presentation on a synthesizer and mixer he had made for down conversion from P-Com modules to make a 24 GHz radio that connects to a 144 MHz IF radio. Members expressed interest in making this a club project and having more of these boards made. I volunteered to help facilitate this project.

I think that many microwave hams have at least one pair of 23 GHz P-Com Up and Down Converter modules that they have gathered in the last decade which could be used for a radio on the 24 GHz band but were not completed with the other parts necessary to interface with a standard radio (usually a 2 meter multimode).

Lars AA6IW and Dmitri AF6IF designed a small board 3.8 x 1.8 inches. It takes an accurate 10 MHz signal as a reference input, synthesizes a 3600 MHz LO and mixes the 3744 MHz IF of a P-Com module down to the standard ham frequency of 144 MHz. The board can be powered from a 12 V battery.

unit picture

Synth Board picture

P-Com Modules

In the early 2000's a number of Up and Down Converter modules became available on the surplus market from P-Com 23 GHz ODU's. These are usable for operation at the 24192 MHz ham frequency. Al Ward, W5LUA, characterized these modules in his paper Using Surplus 23 GHz Modules.

P-Com Modules picture

These modules take an LO frequency that is in the 10 GHz range which is doubled in the module and mixed with the 24 GHz signal to convert to an IF, typically in the 3 GHz range. One handy approach is to use an LO frequency of 10224 MHz which is the commonly used frequency for converting the 10368 ham frequency to 144 MHz. The frequency scheme used for 24 GHz is as follows

 10224 x 2     = 20488 (Doubled in P-Com module)
 24192 - 20448 =  3744 (IF input/output for modules)
  3744 - 3600  =   144 (Intermediate LO of 3600 converts to 144 MHz)

So the conversion Synthesizer/Mixer board for these modules generates the 3600 MHz LO and mixes with the IF from the P-Com modules to convert to 144 MHz. The synthesizer on this board is set up to use a 10 MHz reference frequency to lock the 3600 MHz LO signal.

Board Technology

The Synth/Mixer board is based on the TI-National LMX2541 chip family described here LMX2541 . Specifically, the 3740 version of the chip is used which is capable of generating the 3600 MHz frequency we need. TI - National has design software for selecting chips and creating designs for many of their clock chips including the LMX2541 family. This software is available here: Clock Design Tool . The tool is compatible with designs like the board for the P-Com application and a 3600 MHz LO.

The board has a signal conditioning SY88893VKG chip for the 10 MHz reference input, it has a Mini-Circuits GALI-5 amp for driving the LO signal from the synthesizer. This drives a Mini-Circuits SIM-43MH+ mixer for converting the 3744 MHz from a P-Com module down to 144 MHz. The LMX2541 is loaded with operational parameters from a small AVR ATINY microprocessor on the board.

3600 Block Diagram

In theory it should be possible to use the board with different programmed parameters and loop filter parts to get other LO frequency options. For example, it seems that 106.5 MHz or 1278 MHz should be achievable options although, for now, the P-Com 3600 and mixer conversion is all the board has been used for. For some other application, it should be possible to eliminate and bypass the mixer to use the board just as an LO source. Although theoretically possible to use the board to synthesize different frequencies than 3600 MHz of this application, specific frequencies might require a different version of the LMX2541 than the 3740 version used for this application.

Additional Documentation

Below are some links to files containing some additional details related to the project. If clicking on one of the links doe not give you an option to save or open the file, you may need to right-click on the link and select 'save link as...' or something similar per your browser.

Some technical details about the board including a schematic and board layout.
Some possible options for a box to house the board.
SMA Conn.zip
Some possible options for SMA edge connectors that could be used on the board.
A copy of the Project article published in MUD 2012.