FM Noise Degradation of A MMIC Based ILO for Ka-Band Phased Array Antennas

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A viable realization of the future generation of the multi-beam communication satellites is based on the fiberoptic feeds for direct radiating phased array antennas. The fiberoptic feed architectures based on the T/R level data mixing is recognized as the most viable. This architecture requires a stabilized local oscillators (LO) signal at each phased my element to perform mixing of RF/IF signals with the a stabilized LO, generating IF/RF signals for the Receive/Transmit mode of operation. For coherent communications, all the LOs should be synchronized to the same frequency referencc. Since efficient transmission of the frequency reference at Ka-band is not feasible, subharmonic synchronization methods are used for frequency stabilization of the LO. Among the four different methods of subharmonic frequency synchonization-frequency multiplication, subharmonic injection locking (IL), subharmonic phase lock loop (PLL), and subharmonic injection locking with phase lock loop (ILPLL), the ILPLL is the most efficient technique. This technique is a combination of the other three methods and is demonstrated as the most efficient method of generating a frequency and phase stable LO. Design of a MMIC based oscillator at Ka-band is shown in Fig. 1, as a building block to realize ILPLL. The MMIC LO circuit is laid out and fabricated using 0.3 μm HEMT foundry service. A varactor diode is implemented by single ending a T-gate HEMT as a Schottky diode for the VCO operation of LO. The measured characteristics of the MMIC VCO are +4 dBm at oscillation frequency of ≒19 GHz for varactor bias of -0.2 V. A tuning sensitivity of ≒720 MHz/V is measured at the reversed bias of 0.2 V. Output power variation is less than ±1 dB over the tuning range of 600 MHz. Both locking range and the SSB phase noise of the VCO under fundamental and sub-harmonic injection locking are studied. Considering the measured locking range versus the locking gain, the external quality factor of ≒10 is measured. The achieved normalized fundamental and 4th subharmonic injection locking range are quite comparable for injected power of P_i≧-10 dBm. This significant enhancement is due to the nonlinear multiplication and gain in the 25 dB buffer amplifier. The SSB phase noise comparison of the injection locked oscillator to the 4th subharmonic frequency is presented in Table for P_i≧-10 dBm. The FM noise level of the 4th subharmonic as a function of frequency detuning is also presented in the table. A minimum of 12dB degradation on the FM noise of the 4th harmonic of the frequency reference is theoretically expected. The FM noise degradation of the IL oscillator with respect to the 4th harmonic of the frequency references as high as 2 dB for Ω≦500 Hz and 6 dB for Ω≧50 kHz. As the |f_<LO>-4f_i|≒f_m (i.e., at the frequencies close to the edge of locking range, 2f_m) an Editional 4 dB FM noise degradation is experienced for Ω≦500 Hz. This FM noise degradation could be avoided with the ILPLL approach. No FM noise degradation is observed at the offset frequencies of Ω≧50 kHz, indicating an AM noise dominanted case.
1997-03-06

FM Noise Degradation of A MMIC Based ILO for Ka-Band Phased Array Antennas

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