Synthesizer Merges Fast Switching With Low Phase Noise

Aug. 26, 2009
With an ultra-low-noise PLL mechanism called phase refining, this synthesizer brings output phase noise to almost ideally multiplied crystal reference behavior.

Frequency synthesizers come in many configurations but often with performance tradeoffs. Phase Matrix's QuickSyn challenges traditional performance expectations with frequency switching speed of 100 s and phase noise of 150 dBc/Hz at 10 kHz offset from the carrier. This synthesizer covers 0.1 to 10 GHz with resolution down to 0.001 Hz. It delivers output levels from 25 to +15 dBm.

Aside from frequency coverage and resolution, a synthesizer's spectral purity (i.e., phase noise and spurious content) is the primary evil that ultimately limits the performance of any system. More recently, however, switching speed has become a significant player in this game as well.1 Newer microwave systems require faster switching due to the ongoing increase of data flow. Unfortunately, phase noise and switching speed don't co-exist very well. As a result, any phaselocked- loop (PLL) synthesizer must balance switching speed with phase noise.

This balancing act has become a common industry tradeoff. If the engineer needs a clean, low-noise output, his or her switching speed will typically be in the millisecond range. If faster microsecond tuning is required, phase noise will probably be too far away from what is needed. Dr. Alexander Chenakin, Phase Matrix's synthesizer guru and the "father" of the QuickSyn, asks, "Why can't we do both? Do we break any fundamental laws of physics? Not at all. It is just a common, old mythamong many others."

The QuickSyn synthesizer combines microsecond-range tuning and low phase noise, which is somewhat comparable to top-rated signal generators (see the Figure). The idea is to wash out the freerunning voltage-controlled-oscillator (VCO) noise at all offsets to its thermal noise by widening the PLL to a few megahertz. This feat is accomplished with an ultra-low-noise PLL mechanism called phase refining.2 It brings the output phase noise to almost ideally multiplied crystal reference behavior.

Another myth or perhaps general consensus is that the main job of any synthesizer is to deliver a continuous-wave (CW) signal only. What if the designer needs modulation, frequency, or power sweeps? According to Chenakin, "Inside a synthesizer, there are always many devices that can carry these functions and be reused to increase the functionality without a significant cost penalty." In fact, the QuickSyn provides all major modulation capabilities (AM, FM, phase, and pulse), power leveling and control, frequency and power sweep, list mode, and many other functions that are usually found in complex benchtop instruments at a brick-module cost level. A QuickSyn evaluation kit is available. Phase Matrix, Inc., 109 Bonaventura Dr., San Jose, CA 95134; (408) 428-1000, FAX: (408) 428-1500, Internet:

1. A. Chenakin, "Frequency Synthesis: Current Solutions and New Trends," Microwave Journal, May 2007.
2. A. Chenakin, "Low Phase Noise PLL Synthesizer," patent pending.

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