HSM Series & HS9000 Series RF Synthesizers
Holzworth's proprietary non-PLL based RF synthesis architectures offer many unique performance advantages over more traditional PLL based synthesizer designs. One of the many important performance advantages is known as Phase Coherency, which is directly related to the incredible, relative channel-to-channel phase/frequency stability that the Holzworth synthesizers provide in a multi-channel configuration.
"Phase Coherency" is a term that is commonly used to define how direct analog synthesizers operate. Due to the nature of the direct analog architecture, the term has also been adopted to define a synthesizer's ability to maintain the phase of a frequency after a frequency change so that upon returning to said frequency, its original phase trajectory is unaltered. This is very useful for radar frequency hopping and is more universally referred to as "phase memory". Phase coherency in synthesizers defines a precise phase relationship between multiple frequency sources while being clocked by the same reference signal. This absolutely holds true for direct analog designs as all frequencies are ever present and precisely related to the reference clock. The direct analog system simply switches/routes the desired frequency to the RF output port. The phase coherent definition also holds true for Holzworth's ultra stable, non-PLL designs when multiple synthesizers are clocked by one reference signal.
Holzworth non-PLL Design Overview
Holzworth synthesizers are designed with a digital front end and a proprietary, direct analog back end. The proprietary architecture maintains low spurious response while also providing industry leading phase noise performance, exhibiting signal jitter performance of far less than 100fs.
The PLL was originally excluded from the Holzworth designs for optimal signal stability and fast switching speeds. Unlike PLL designs, there is no post switch settling time to reach the new frequency. PLL based designs exhibit switching speeds similar to the Holzworth designs, but also include a post switch frequency variance that is often as great as ±100kHz. This should be specified as settling time. Holzworth synthesizers exhibit little or no settling time under most switching scenarios.
The phase coherent nature of the non-PLL synthesizer creates very precise frequency relationships between multiple channels. The integer related outputs demonstrate a relative channel-channel phase drift of less than ±0.5ps over a one hour time frame. While operating in a controlled environment (±1C) to minimize thermal effects, the design can maintain this sub-picosecond channel-to-channel phase drift indefinitely.