Ultra Low Phase Noise Stratum-3 TCXO with High Output Power

Abstract

Modern wireless communication equipment, point-to-point and point-to-multipoint microwave digital radios, radar equipment, microwave sources require high stability, low power consumption, and very low phase noise frequency sources in UHF band. Existing TCXO solutions can achieve -150 dBc/Hz phase noise floor at the carrier frequency of 100 MHz. The goal of this work was to create a 500 MHz TCXO with Stratum-3 stability performance (0.28 ppm over temperature, 4.6 ppm over all conditions), which can reach -155 dBc/Hz noise floor, -120 dBc/Hz at 1 KHz offset from the carrier, while providing +7 dBm of output power. The goal was accomplished by integrating in a small SMD package high performance, low frequency, Stratum-3 digitally compensated reference TCXO, low noise off-the-shelf phase locked loop (PLL) IC, and ultra low noise 500 MHz VCXO. The key solutions for this development besides solid digitally compensated TCXO design were optimization of the PLL circuit, and design of a VCXO. The VCXO is based on a 3rd overtone 100 MHz AT-cut crystal resonator with relatively high Q, passive band-pass filter, tuned on the fifth harmonic of the 100 MHz VCXO, and a free-running L-C oscillator, which is injection locked to the above mentioned fifth harmonic of the VCXO. The resulting device performance met all the stated goals and even somewhat exceeded the phase noise expectations. The sub-harmonic suppression was better than 50 dBc