The calibrator is based upon the Analogue Devices ADF 4351 Wideband Synthesizer and the Arduino project by F1CJN. This project utilizes an ADF4351 module available on internet auction sites, such as ebay, for around £20. The ADF4351 is capable of operating between 35MHz and 4400MHz in 10KHz steps at a maximum output level of 5dBm. The ADF4351 has a SPI interface, which in this case interfaces with the Arduino.
The design by F1CJN utilises a LCD Button Shield to display the frequency and the buttons are used to set the required frequency and mode. I have modified the code to allow the frequency to be set via the serial port which is connected to the host computer via USB. Additionally the USB powers the Arduino and the ADF4351 board. A separate 5V power supply may improve spectral purity, however, I have not found this a problem. The RSPduoEME application controls the ADF4351 by sending the required calibration frequency to the Arduino serial port. The calibration is effectively turned off by setting the frequency out of band to 4000MHz. The schematic below shows the interconnections, the 560/1K resistors are used to convert the Arduino 5V to 3.3V logic levels for the ADF4351. Full constructional details of the Arduino project are provided in the F1CJN web article.
The modified code allows the setting of the external reference to 24MHz, convenient as the RSPduo external reference is also 24MHz. If the external reference is used, the resistor connecting to the XTAL oscillator needs removing on the ADF4351 module. The calibrator reference input mode is set using the push buttons on the shield as described in the F1CJN article. Note, the LCD reference indicator does not work correctly and shows the lock state even when the reference is not applied. This seems to be a ADF4351 hardware issue as the software correctly displays the state of the Mux output. The modified Arduino Sketch is available from the RSPduoEME download zip file.