Friday, September 11, 2015

10GHz Station Upgrades

With the 2015 September 10GHz and up contest approaching fast, I am in process of making some performance improvements to my 10GHz station. I had already missed the first round of the August 10GHz and up, so I wanted to make sure to complete a few necessary items before the 2nd round is here.

First on the agenda is to upgrade my PLL frequency synth that runs my LO to use low-noise regulators. This was a design error on my part, when I originally designed these freq synths, they had less than ideal phase noise as a result. The noise itself was determined to be coming from the 78nn series of linear regulators I had originally spec'd out for the design for the 5v vco, and dual 3.3v analog and digital supplies. A test of an unused synth I had built (for a beacon project) proved to show quite an impressive improvement once I replaced the regulators:

ADF4107 Phase Noise
Phase noise is shown here on the left with the original noise regulators, and a much cleaner output is shown on the right after the low-noise regulator replacement.

Next item on my list was to add some additional attenuation into the LO path that drives the mixers. Originally when I had a phased lock PLL brick, LO drive power through both outputs was approximately 12dBm. This was ideal for the two Magnum Microwave mixers I was using. After switching to the PLL synth and x4 multiplier, my output to the LO's through a splitter was a little hot at 15dBm. This was the upper max limit of power in the datasheet. This as a result was causing some additional spurs on the RF side, which while immediately filtered was still resulting in some spur leakage. A 3dB pad in line with the output of the x4 multiplier cleaned the excessive spurs up nicely.

The final improvement is in regards to the actual frequency reference itself for the LO PLL freq synth. I had designed my synth to use an on board TCXO protected by a shielded enclosure for both rf shielding and hopefully help stabilize the temperature. Temperature drift is critical in any design, this one was particularly touchy to temperature. When the synth was running and locked to my 2556GHz output frequency, simply blowing air across the TCXO caused the output frequency to start drifting. The specs for the particular crystal I had chosen were not that great at ±2.5ppm over the specified temperature range, I would definitely need to do better for good stability.

I have a rubidium 10MHz reference I use at my bench. I didn't want to devote this just for my mobile station as I use the reference for all of my test equipment. I would also need to frequency double it to work with my ADF4107 PLL as it requires a minimum 20MHz reference. As a compromise, I decided to go with a crystal oven oscillator (OCXO). I had a 20MHz version available that runs off of 12V which will be perfect for this mobile 10GHz station. Details of this will probably be in a separate post due to the difference being pretty interesting.