High Frequency Low Phase Noise TCXO
By Sergio San Pedro, Raltron Electronics Corporation

TX6100 Raltron new TCXO series combine high output frequency, high stability, low power consumption, and very low phase noise. Current TCXO products can achieve -150 dBc/Hz phase noise floor at the carrier frequency of 100 MHz. This new TCXO can go up to 500 MHz 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 +3 dBm of output power. TX6100 is presented in a small SMD package high performance, low frequency, Stratum-3 digitally compensated reference TCXO, low noise Phase Lock Loop (PLL) IC, and ultra low noise VCXO. Solid digitally compensated TCXO design, optimization of the PLL circuit. The VCXO is based on a 3rd overtone AT-cut crystal resonator with relatively high Q, passive band-pass filter, tuned on the fifth harmonic of the 1 VCXO, and a free-running L-C oscillator, which is injection locked to the above mentioned fifth harmonic of the VCXO. The sub-harmonic suppression is better than 50 dBc.

The reference oscillator is a digitally compensated TCXO at standard telecom frequency. It has a DIP14 compatible footprint. DTCXO performance satisfies Stratum 3 requirements both for 24 hours holdover - 0.37 ppm (0.28 ppm peak-to-peak over full industrial temperature range -40 to 85º(C), and long-term over all conditions stability of ± 4.6 ppm.

The PLL circuit is implemented with PLL IC, and a loop filter. It is controlled by a microcontroller, which sets internal to the IC divider coefficients, as well as providing reliable start-up operation during power-on, power-off transitions. Loop filter bandwidth is determined by the phase noise requirements and the quality of the reference oscillator.

VCXO
A simplified VCXO circuit with corresponding spectrum of each stage is shown on Figure 2.
Since the reference oscillator provides frequency stability over all conditions of less than ± 5 ppm, worst case, the absolute pull range (APR) of the VCXO can be just a few ppm wider than that value. It allows the use of 3rd overtone crystal, which leads to improved phase noise performance, especially at frequency offsets closer to the carrier. The sustaining stage is realized as a CMOS gate Pierce oscillator with relatively high crystal drive level to attain low phase noise level on the noise floor. The second stage is a CMOS gate buffer-driver, which produces sufficient harmonic power, especially on the odd harmonics without adding detectable noise. The next stage is a third order Chebyshev band-pass filter, which provides the suppression of unwanted sub-harmonics and its multiples to below -50 dBc, compared to the level of the harmonic selected and matches the impedances of the stages. The signal from the filter output is fed into the next stage, which is a free running LC oscillator, tuned on the frequency close to that of the selected harmonic. When power of the desired frequency injected into free-running oscillator is sufficient, and the free-running frequency is within the pulling range from the desired frequency, injection locking occurs. After this stage, the signal is amplified by the integrated amplifier and matched with the load by a low pass filter. The achievable level of RF output power is +6 dBm with 5 V voltage supply. The spurious produced by the comparison frequency is below -120 dBc, while spurious related to the reference oscillator is less than -80 dBc.

The spectral purity plot for TX6100 at 400.000 MHz is shown on Figure 4. The suppression of sub-harmonics and their multiples is better than -50 dBc/Hz, while the spurious generated by comparison frequency and reference frequency are below the noise floor of the instrument.
The construction of the device is modular, which affords versatility. The resulting phase noise on the floor ranges from below -160 dBc/Hz at 300 MHz carrier frequency to -155 dBc/Hz at 500 MHz. Medium and long term stability is determined by the reference oscillator employed, which with flexibility provided by design, could range from Stratum 3 (DTCXO) to Stratum 3E (SC-cut OCXO) compatible performance.

References
R. Boroditsky, D.V. Bogomolov. “Ultra Low Noise UHF TCXO,” Proc. of 2005 IEEE International Frequency Control Symposium, J. Gomez., S. San Pedro.

RALTRON ELECTRONICS
www.raltron.com
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