The synthesizer gains more bandwidth

Prowling through the various surplus shops on eBay, I spotted an old YIG oscillator for which I managed to find a datasheet (which with YIG devices is very rare). YIG (Yttrium-Iron-Garnet) devices are pretty exotic and expensive devices that take advantage of the unusual magnetic properties of the crystal structure of the YIG material. The crystal has a very high Q factor on microwave frequencies and a very narrow resonance point which is very strongly and linearly dependent on the magnetic field. Using an YIG crystal in the feedback loop of an oscillator or as a filter element allows extremely wide tunability with a very good phase noise and with a precise frequency control. The linearity error of frequency vs current throughout the tuning bandwidth may be in the order of 0.1%.

Most modern YIG oscillators contain a permanent magnet to set a base frequency and a magnet coil is used adjust the frequency to either side of the starting point. Common cheap Stellex-made modern YIG oscillators that can be found on eBay have a zero-current frequency around 9GHz and tune +-1GHz. Most YIG devices without a permanent magnet tune by an octave or two.

Above is a picture of two YIG oscillators. The smaller, golden colored is a Stellex-manufactured permanent magnet device with a tuning range of 8-10GHz, while the big and battered device bolted on the heat sink has an internal 12bit DAC and main tuning coil driver and is digitally adjustable for an output of 4-18GHz.

Most YIG devices are designed for a tuning sensitivity of 20MHz/mA, as is this one. It requires +15V for the main supply, -15V for the driver biasing, +5V for the driver logic supply and +28V for a heater that stabilizes the YIG crystal by heating it to around 80°C. I'm not currently using the heater though. So, ignoring the other supplies, the oscillator draws around 100mA from the 15V for the oscillator itself and the current required to set the frequency with the magnet coil. As the tuning range goes from 4GHz to 18GHz, the coil requires a minimum of 200mA for the lowest frequency, going up to 900mA at 18GHz. This means the oscillator will be dissipating around 15W of heat, so without a heat sink it will be running pretty hot.

Here's a snap of the driver block inside the cubical part of the YIG oscillator. I'm not going to open the RF part, it is usually wire bonded and very sensitive. The driver electronics are lacquer coated and pretty robustly built. The device is marked as MIL specified, it probably originates from some obsolete ECM system.

To control the oscillator with the Beagleboard, a control board is required. The above board contains a switch mode regulator that generates the -15V required by the oscillator, an I2C port expander connected to a number of MOSFET's to switch different parts of the system on and off, an I2C bus multiplexer to expand the system control I2C bus, a number of headers to connect to various other modules, and two additional I2C port expanders that feed the 12bit parallel tuning word to the YIG driver DAC. 

Integrated together, the signal generator now had two frequency ranges (but no RF switch between them), covering a total frequency range from 137.5MHz to 18GHz. The YIG device only has a resolution of about 3.4MHz and an accuracy of perhaps 40MHz.