Ultra-Low Phase Noise Frequency Division With Array of Direct Digital Synthesizers

In this article, we present a four-channel direct digital synthesis (DDS) design that operates with a common clock ranging from 500 MHz to 24 GHz and generates output frequencies up to 1.75 GHz. A key feature of this board is its custom field-programmable gate array (FPGA)-based synchronization method, which ensures alignment accuracy of 170 ps between the channels, enabling precise frequency and phase relationship settings. In addition, the DDS board incorporates a user-friendly web interface that allows for continuous control and monitoring capabilities over TCP/IP. Multiple synchronized channels can be power-combined to produce a low-phase noise output due to coherent addition of the common carriers and the noncoherent addition of the residual DDS noise. By exploiting these principles and combining eight parallel channels of two DDS boards, we achieve exceptional residual phase noise performance, with $\mathscr {L}(1 \text {Hz}) = -147 \text {dBc/Hz}$ and $\mathscr {L}(100 \text {kHz}) = -180 \text {dBc/Hz}$ for a 9.765625 MHz output signal. These noise levels surpass the previously reported results achieved with regenerative frequency dividers. We also present a method for obtaining accurate residual noise measurements using an absolute phase modulation (PM) noise and amplitude modulation (AM) noise analyzer. Furthermore, we analyze the phase alignment tolerances required to minimize the AM-to-PM and PM-to-AM conversion that commonly occurs in power-combined signals. Finally, we demonstrate the synthesis of a highly stable 9.765625 MHz signal obtained from a cavity-stabilized optical frequency comb (OFC), with an absolute white phase noise of ${-180} \text {dBc/Hz}$ .

I. Introduction

Low-noise frequency synthesis, which is the generation of new and different carrier frequencies from a set of existing fixed frequency sources or clocks, is a critical component of many applications such as telecommunication systems [1], [2], phase-locked loops (PLLs) [3], optical frequency tracking [4], optical transfer oscillators [5], and radar [6], [7], [8]. The three most important operations in frequency synthesis are frequency multiplication, division, and translation, which must be implemented in a low-noise fashion to not degrade the newly synthesized signals. Direct digital synthesis (DDS), which can act as a source and a programmable frequency divider, has gained widespread adoption in frequency synthesis, due to advancements in field-programmable gate array (FPGA) technology and the availability of compact, high-performance single-chip implementations. DDS offers fast and precise control of a signal’s frequency, amplitude, and phase, making it an ideal choice for frequency synthesis. Although DDS may not match analog electronics in terms of phase modulation (PM) and amplitude modulation (AM) noise [9], and spurious responses due to phase truncation [10], [11], its convenience and phase continuity with frequency changes make it an invaluable resource.