add modulation

[hartytp]

If anyone actually needs this...

1. Step attenuator at output with (isolated?) SPI bus
2. 16 (18?) bit DAC that trims the 3V3 supply voltage for fine control

The DAC can also be used for digital feedback loops e.g. using a low-noise power detector, ADC and microprocessor. It's still nice to use this design even with feedback, as it has much better stability and noise than a synth etc. So, only need very low bandwidth. Doing modulation digitally, one can retain the excellent broad-band performance while still adding a slow feedback loop (high-order IIR with few Hz cut-off probably).