Ferrite bias lines for use with GaAs IMPATT diodes

Lowpass ferrite-bead bias filters are used to suppress high-frequency bias oscillations and enhanced noise components obtained when high-efficiency GaAs IMPATT diodes (uniformly doped n-type and Read) are operated as free-running oscillators and saturated amplifiers in broadband (low Q factor) r.f. coaxial circuits with distributed-line bias arms. The impedance properties of the ferrite beads have been measured up to a frequency of 2 GHz, which has resulted in a 4-stage bias-filter prototype being realised. Also incorporated is a simple r.f. low-shunt-capacitive choke that reduces the loss on the main coaxial r.f. cavity to less than 0.2 dB over the frequency range 7¿12 GHz.