Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.

Clean, quiet frequency synthesizers are essential for modern communications systems. But the performance of RF/microwave frequency synthesizers is often tied to a lower-frequency reference oscillator, such as an oven-controlled crystal oscillator (OCXO), and great effort is often required to produce a microwave frequency synthesizer with low phase noise. To demonstrate, a 10.24-GHz frequency synthesizer with OCXO reference source was developed, and the design path to that synthesizer will be traced.

Phase noise is a vital parameter for oscillators and synthesizers in communications and other systems. It is measured as the ratio between the power density in one phase noise modulation sideband, per hertz, and total signal power.1 Typically, a wideband synthesizer will exhibit more noise than a single-frequency synthesizer. When low noise is required, a single-frequency synthesizer can be combined with frequency mixers to build a wideband synthesizer with low noise.

The phase noise of the signal produced by the synthesizer is determined by the performance of the oscillator used to build the synthesizer, the performance of the reference, and the transfer characteristic and intrinsic noise of the synchronization method. Major contributors to phase noise are the internal oscillator and the reference source (for noise offset far from and close to the carrier, respectively). An OCXO may be used as the reference for low-noise applications, possibly locked to a rubidium clock or a 1 pulse-per-second GPS source.

The phase noise performance of the oscillators has a fundamental limit imposed by the Johnson-Nyquist theory. A resistor at room temperature (300 K) produces about -173.82 dBm/Hz noise, with this power level equally split in two sidebands. A signal with 0-dBm power will have a lower phase-noise limit of -177 dBc/Hz, improving upon -177 dBc/Hz only if the signal carries more than 0-dBm power, as with some low-noise OXCOs.2,3 The best low-noise oscillators are capable of approaching this limit at far offset frequencies. For close-in frequency offsets the phase noise will be determined by the quality factor (Q) of the resonator.

Searching For Low-Phase-Noise Synthesizers, Fig. 1

The frequency offset at which the phase noise approaches the theoretical limit is roughly proportional to the frequency of the oscillator, increasing as the frequency increases. Figure 1 shows typical performance levels for different low-noise oscillators, including a 100-MHz OCXO, a 1-GHz surface-acoustic-wave (SAW) oscillator, a 10-GHz dielectric-resonator oscillator (DRO), a 10.24-GHz sapphire oscillator (SO), and a 10-GHz opto-electronic oscillator (OEO).4-10

In Fig. 1, the OCXO phase noise exceeds the -177 dBc limit, because its power is 13 dB higher than the 0-dBm limit. The lowest-frequency oscillators typically exhibit the lowest noise floors. They also show lower close-in phase noise, since the quality factor (Q) of lower-frequency resonators is higher, and because the flicker characteristics associated with these resonators is better. When using these oscillators as the reference of a synthesizer, their noise contribution is scaled by 20log(Fout/Fref). Thus, the noise contribution of a 100-MHz OCXO will be increased by 40 dB when producing a 10-GHz signal. Figure 2 shows the expected noise from these oscillators when used to generate a 10-GHz signal.

Searching For Low-Phase-Noise Synthesizers, Fig. 2

Searching For Low-Phase-Noise Synthesizers, Table 1

As that figure indicates, the lowest close-in phase noise is from the sapphire oscillator (SO), followed closely by the OCXO. The lowest phase noise far from the carrier is for the optoelectronic oscillator (OEO) and the dielectric-resonator oscillator (DRO). Combining the SO or OCXO with the OEO or DRO could achieve lower noise levels. Additional specifications are available in Table 1.

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.