This is a signal generator based on the SI570 chip.  This oscillator has very low composite noise.  The output of the generator tunes from 0.5 MHz to 128 MHz in 10 Hz steps.  The output ranges from 0 dBm to -70 dBm in 1 dB steps.  With the external 60 dB attenuator, the output can be as low as -130 dBm.  The generator block diagram and schematic are shown below.

SI570 Generator Block Diagram.

Generator Connection Diagram.

The hardware concept is fairly simple.  Refering to the Block Diagram and the Generator Top View below, the Arduino is in the upper left, beneath the shield that contains the SI570 oscillator chip.  The selected frequency goes to the selected Filter shown on the large board on the right.  That signal goes to the 96 dB Attenuator enclosed in a brass shield on the lower left.  From there, the signal goes to the RF output connector.  On the front panel, the switches are on the left, and the diplay is on the right.

Generator Top View.

Generator Front View.

RF Shielding Cover.

Generator Back View.

The bulk of the work is done in software which can be seen HERE.

The HP 8640B is known for its low composite noise.  I have two HP 8640B signal generators.  The picture below shows three spectral plots.  The spectrum in the middle is the SI570 generator, and the spectral plots on either side are the HP 8640B generators.  Notice the noise floor of the SI570 generator appears cleaner than the two HP generators.

SI570 Generator spectrum in the middle compared to two HP 8640B Generators.