V. V. Grinev, V. M. Ustimenko

Digital frequency modulation doesn-t require high amplifier linearity. Nonlinear amplifier can be used for its implementation. But Frequency shift keying supports relatively low data transmission rate because of the using high-Q filters in radio circuits for tuning to specified frequencies. The speed of frequency switching in FSK determines quick-action of transmission link. Increasing data rate on FSK base is possible with application of low persistence elements. Gunn diode is one of these elements. Gunn diode low persistence can be used on low-Q relaxational mode of operation in modulator with continuous Fast Frequency Shift Keying (FFSK). Recently detectors of such signals weren-t considered practically. Short-circuited or open-ended transmission line of proper length is suggested as a frequency-sensitive element of FFSK detector. There is an amplitude detector, which is placed into section removed from the end of line thus much, that various frequencies creates in its considerably different levels of standing wave electrical field. The voltage node of standing wave on top frequency and voltage antinode of standing wave on bottom frequency must be in section with detector diode to achieve maximal detector frequency selectivity. Decrease of detector dimensions can be attained by providing open-ended mode of transmission line. In this case minimal length from open end of line is equal to quarter wavelength of bottom modulation frequency. The selectivity growth those detectors can be obtained by including to nodes of received frequencies of transmission line two detector diodes, which output signals are subtracted. There is a goal of choosing length of transmission line, transmission line mode of operation (short-circuited or open-ended), length of detector lines, wherethrough diodes are connected. The top frequency signal level analysis in the detector diode installation section showed, that growth transmission line length allowed to reduce frequency band FFSK signal provided maximal detector frequency selectivity is obtained. It was revealed in the course of analysis of detector line input resistance with load, that line with λ/8 length of received frequency is of great interest. There is the special detector FFSK schematic on connected diodes, which is suggested for speed growth of frequency switching and providing high input resistance of amplitude detector with large time constant of load. Performed simulation of introduced schematic with shorted-circuited transmission line on microstrips showed max data rate two-position FFSK signal with continuous phase on 9,75 GHz and 11,25 GHz, on which detecting is provided, is about 2 Gbps.