The anti-blocking amplifier, prized for its “high IP1 + low noise” performance, has become the go-to device for rejecting strong interferers in RF front ends. Yet its internal bias inductors—ranging from tens to hundreds of microhenries—can form a low-frequency series resonance with any mismatched trace or matching capacitor. Amplified by the device’s non-linearity, this resonance appears as modulation spurs alongside the desired signal. To prevent this, three design rules must be observed:
- Reproduce the exact component values given in the data sheet.
- Decouple the supply with “small + large parallel capacitors and a π-filter” to suppress ripple and resonance.
- Connect the ground pad through multiple vias directly to the ground plane to shorten return paths and lower Q.
If a narrow-band in-band filter follows, its out-of-band low-frequency region is highly reflective. Reflected energy can return to the amplifier, forming a second amplification loop that regenerates spurs. To break this loop, place a 1 µH shunt inductor to ground after the DC-blocking capacitor. Choose a part with SRF > 3 GHz and low DCR. This inductor shorts reflected energy below 1 GHz, drops loop gain by more than 20 dB, and the spurs disappear.