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In this paper, the authors present the design and implementation of a microwave oscillator circuit with ultra-fast amplitude control for an inspection system appropriate for real-time measurements of continously streaming materials. The material passing through the resonator of the oscillator changes the resonance frequency and the quality factor of the resonator. This influences the oscillation frequency and the loop gain required for maintenance of stationary oscillation in the linear regime. From the measurement of the oscillator frequency and the gain, adjusted via an automatic gain control (AGC), resonance frequency and loaded quality factor of the resonator and therefrom the material parameters, i. e. material density and humidity, can be determined. For the automatic gain control circuit, a Gilbert-cell based variable gain amplifier (VGA) with high modulation bandwidth of 70 MHz and wide linear gain range is designed using SiGe HBTs. Cascaded variable gain amplifiers with a wide linear gain range of over 40 dB and an ultra-high control loop-bandwidth allow to measure the parameters of materials streaming at a velocity of around 10 m/s with an error in the order of 1 %.