QO-100 Experiments – ADF4351 SynthShield

I’ve recently used a number of ADF4351 modules in various projects. The last two modules have been used to provide a single fixed frequency output. An Arduino Nano running the F1CJN single frequency sketch is used to control the ADF4351.

The unit above is the 2256MHz LO (Local Oscillator) currently used in my QO-100 transmit up-converter.

This second one provides 40MHz for the Adalm-Pluto and can be used instead of the upgraded TCXO when a 10MHz GPSDO is available.

A small number of components are needed to interface the Arduino and ADF4351. I’ve tended to use a piece of vero-board to “tie” the various parts together. This method of construction has worked well but has resulted in each one being built in a slightly different way, i.e. layout, size and wiring.

As my QO-100 experiments have developed I identified the need to be able to have the ADF4351 LO module produce various set frequencies, not just the single fixed frequency.

These requirements being possibly:

    • 2256MHz for QO-100 transmit up-converter (2256 + 144 = 2400MHz)
    • 2176MHz for 13cms terrestrial working (2176 + 144 = 2320MHz)
    • 595.500MHz for 144MHz RX IF (595.500 + 144 = 739.500MHz LNB output)
    • 307.500MHz for 432MHz RX IF (307.500 + 432 = 739.500MHz LNB output)

In addition to being able to select one or two frequencies I wanted an LED to confirm that the PLL was locked and if being used for the transmit side, the transmit should be inhibited if the PLL is not locked.

These enhancements were fairly straightforward to implement. I tested these changes using a spare set of boards and a solder-less prototype breadboard. I then learnt about the Arduino Pro Mini board which is even smaller than the Nano that I’d been using. This plus a recent interest in learning how to use KiCAD resulted in the development of a small board that would fit on top of the standard ADF4351 PCB.

 

A small modification to the ADF4351 board is needed (in addition to the usual disabling of the onboard Xtal unit when used with an external GPSDO reference). The standard power connector needs to be removed and replaced with two header pins (1 x 3, 2.54mm spacing, with centre pin removed). These new pins plus the existing 2 x 5 header pins enable the new board to control the ADF4351 and provide 5V power. The photo below shows the ADF4351 board after the power connection modification.

The new SynthShield board can be used with header sockets to connect to the Arduino and synthesizer boards.

For an even smaller overall height the header sockets can be dispensed with and the Arduino Shield board soldered directly to the header pins on the ADF4351 board.

The Synth Shield has header pins to support the programming of the Arduino, power (7-15V), frequency selection (default choice of two, but four is possible) and either an LED or relay used to indicate the PLL lock-state.

I’m currently using the first build of the SynthShield board to provide the LO for a new QO-100 down-converter project.

ADF4351 being used to provide LO for QO-100 down-converter

 

 

 

 

 

 

 

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