How do I integrate a digital control interface into an RF module without causing EMI?

Integrating a digital control interface (SPI, I2C, UART, or parallel digital bus) into an RF module without causing EMI requires isolating the digital signals' high-frequency harmonics from the sensitive RF circuits. Digital signals contain harmonics extending to f_harmonic approximately 0.5/t_rise (for a 1 ns rise time: harmonics extend to approximately 500 MHz), which fall within the operating band of many RF circuits and can: raise the receiver noise floor, create spurious signals in the transmitter output, and modulate the local oscillator or synthesizer. The mitigation techniques are: physical separation (place the digital control circuitry as far as possible from the sensitive RF stages (LNA, mixer, VCO); a minimum of 10-20 mm separation is recommended; if in a compartmentalized module: place the digital circuits in a separate shielded compartment from the RF circuits), slow down the digital edges (use series resistors (33-100 ohms) on the digital signal lines to slow the rise time from 1-2 ns to 5-10 ns; this reduces the harmonic content above 100 MHz without affecting the digital communication speed; many digital protocols (SPI at 10 MHz, I2C at 400 kHz) work perfectly with 10 ns rise times), filter at the compartment boundary (every digital signal crossing into or out of the RF compartment should pass through a low-pass filter or ferrite bead: a ferrite bead (100-1000 ohms at 100 MHz) in series with each digital line provides 20-30 dB of rejection at RF frequencies while passing the DC and low-frequency digital signal; alternatively: a resistor-capacitor (RC) filter at each crossing point), use a clean ground strategy (connect the digital and RF ground planes at a single point (star ground) or use a solid continuous ground plane with no slots or gaps that would force digital return currents to flow through the RF circuit area; a slot in the ground plane under a digital trace creates a ground loop antenna that radiates into the RF circuit), and minimize clock radiation (digital clocks (SPI clock, FPGA clock, processor clock) are the strongest EMI sources because they are periodic and generate strong spectral lines at the clock frequency and its harmonics; route clock traces as short as possible, on inner layers (not surface), and with ground guard traces or ground pour on both sides).