FILTER SYNTHESIS TECHNIQUES.

Several active filter techniques for the reduction of receiver interference in the 225 to 400 MHz range are described. Positive feedback Q multiplier techniques were extended to include 1 the use of multiple feedback loops to achieve a high order of stable multiplication in each stage and 2 the use of cascaded stages of Q-multiplied resonators to obtain improved skirt selectivity. Negative resistance Q multiplication was achieved over a wide frequency range through the development of a common collector transistor amplifier that exhibits stable negative resistance properties in the UHF region. The negative resistance amplifier was incorporated into a breadboard model of a tunable filter which employs both active and passive stages to produce a high Q response characteristic with high skirt selectivity over the entire band. An AM cancellation filter that achieves suppression of an unwanted signal by cancellation via a synthesized replica of the signal was developed. The breadboard model demonstrated a suppression capability of 30-35 dB for AM signals and about 50 dB for CW signals. To enhance the capabilities and versatility of UHF active interference suppression filters, linearization techniques for broadband solid state amplifiers were investigated. The application of negative feedback and the use of the push-pull mode of parallel operation provided a significant reduction in harmonic generation while retaining good gain-band-width characteristics in amplifiers of one-watt power output capabilities. Author