For a digitally controlled YIG oscillator and filter, a driver is needed that can convert serial data from a microcontroller to a well defined, stable, and low noise current.
Bandwidth of the circuit should be a few 100 Hz, and maximum current in the 300 mA range, so it needs to run of a reasonably high supply voltage, otherwise, the inductance of the coil will limit the slew rate. The YTO needs about 120 mA full scale, the YTF about 260 mA.
I might do some fine tuning on the DACs later or change the current sense resistors for a 2.5 V drop at close to max current, for best signal to noise ratio, but for the test circuit, 10 Ohm RH-25 resistors will be used. The current sense resistors are a very critical part – they need to be low drift, over time, and over temperature, regular resistors, with 100 ppm/K or more will only cause drifting frequencies, and trouble.
That’s the test setup, with +20 V and -10 V power supply, for the YIG. In the final setup, there will be independent, filtered and regulated supplies for low phase noise.
The circuit is driven by a HP 8904A signal generator, with independent adjustment of offset and voltage. Here, the output at 70 mA current, with a +-1 mA amplitude variation:
So, about +-40 MHz – close to expected +-35 MHz.
Bandwidth analysis will follow.
Here a quick calculation of the DAC resolution, 1 LSB will be about 0.13 MHz, more than sufficient for the DAC tune. The DAC used, a DAC8830ICD has typical +-0.5 LSB non-linearity, max +-1 LSB. Additional tuning will be easily accomplished by the FM coil, using a PLL.