Phase locking the YIG

While researching the use of the 4-18GHz YIG oscillator, I opened the driver case to make certain which wire goes where. On the oscillator cylinder side, there were two pins without wires connected. The oscillator assembly datasheet mentioned that the device only had a main tuning coil but no FM coil that would allow modulation or more precise frequency control. However, it seems the oscillator itself is a standard part with the FM coil integrated, while the driver block does not connect it to the external connectors FM coil pins. A very lucky find, as this allows phase locking the oscillator!

With a phase lock, the frequency resolution can be improved to a few Hz and frequency precision even better, while the frequency setting speed will improve a lot. As the frequency of a YIG oscillator is set by adjusting the main tuning coil current, it takes up to 10ms before the frequency will have stabilized to the new value. With the smaller and faster FM coil, the output frequency can be locked to the desired frequency in around 100µs, even if the main coil current is still changing (as long as the frequency step is inside the PLL lock range).

After some work, the above PLL block was born. The casing is milled from aluminium. The electronics contains a 4-18GHz prescaler (ADF5002), a fractional-N synthesizer (ADF4157), an active loop filter and a voltage-to-current converter with a differential output to drive the FM coil. With an external 10MHz reference frequency, the system  has a resolution of about 5Hz at 18GHz.

While pretty, the device did not work right away. Not having analyzed the entire system together, I had not included the coil driver and coil itself in the loop coefficients, and the loop was highly unstable. Some considerable rewiring and loop filter recalculations later, the loop stabilized nicely. However, the synthesizer output was rather disappointing. Testing with a Stellex YIG, the phase noise is very nice, hardly measurable with the down conversion setup and spectrum analyzer I used, but connecting the wideband device gave such a noisy signal that it was at least clear that this synthesizer would not be used for a spectrum analyzer preconverter.

YIG oscillators below 8GHz or so are usually bipolar transistor based, but above it GaAs-FETs have traditionally been used, except perhaps in the most modern devices. GaAs devices have far higher 1/f-noise, which translates directly into phase noise, and an ultra wide band device such as the one I'm using will have some compromises made to enable such a bandwidth.

Here's a pic of the modules bolted together.