Reference signal sources set the spectral standards for a wide range of phase-lockedloop (PLL) applications, from clock translators in measurement instruments to ground-based and airborne radar systems. Traditional references sources, such as crystal, ceramic, or surfaceacoustic- wave (SAW) oscillators, are based on high-quality-factor (high-Q) resonators. They are generally limited to fundamental frequencies of about 1 GHz and must be multiplied to provide higher reference frequencies. And with multiplication comes degradation in phase-noise and harmonic performance. Fortunately, Synergy Microwave Corp. has developed miniature reference oscillators that use degenerated mode-coupling and regenerative noise filtering techniques to achieve excellent spectral performance at fundamental frequencies past 2.4 GHz.1-18
The new HFSO free-running and FCTS phase-locked voltage-controlledoscillator (VCO) reference sources use a patented degenerated mode-coupling mechanism to achieve outstanding spectral purity and low jitter through 4 GHz, in surface-mount packages measuring just 0.5 x 0.5 x 0.18 in (Fig. 1).
The typical size of the FCTS model is 0.9 x 0.9 x 0.22 in (Fig. 1); it can also be supplied in larger packages (2.0 x 2.0 x 1.0 in.) with SMA connectors.
In contrast to other high-frequency reference-source solutions, such as crystal-oscillator-based references that employ oscillators, gain blocks, and filtering networks that add to high development costs and reliability concerns, the HFSO and FCST sources provide simpler, more reliable solutions that save space and power consumption. The patent-pending frequency-generation approach used in the HFSO and FCST sources includes a methodology to enhance the mode-injection locking range, and to reduce or eliminate the amount of filtering needed to suppress subharmonics and higher-order harmonic products. The approach also reduces the susceptibility to microphonics while retaining low phase noise and a moderate tuning range to compensate for shortterm and long-term effects of thermal drift and aging.3,9,12 The new reference sources are ideal for use as clock translators for high-speed analog-to-digital converters (ADCs) and direct-digitalsynthesizer (DDS) clocks.
The HFSO and FCTS series of VCOs break with tradition and overcome the long-time hurdle of achieving cost-effective, low phase noise at 300 MHz and above. These fundamental-frequency sources reduce the phase-noise floor to -170 dBm, with typical output power of +2 dBm and 100-kHz tuning range. They are available in standard and custom frequencies, with low non-recurringengineering (NRE) costs for custom solutions compared to multiplied reference sources.
Model HFSO1000-12 is an example of a free-running reference source (Fig. 2). It is designed for use at 1 GHz, housed in a tiny surface-mount package measuring 0.5 x 0.5 x 0.18 in., and can be phase locked to an external crystal oscillator source for the required frequency stability. It can also be supplied in an SMA-connectorized package with dual outputs, and can include the internal temperature-controlled crystal oscillator (TCXO) or ovencontrolled crystal oscillator OCXO reference source.
Phase-noise measurements for the HFSO and FCTS reference sources were made with both the Agilent E5052A Signal Source from Agilent Technologies and the R&S FSUP Signal Source Analyzer from Rohde& Schwarz. The phase noise for the HFSO1000-12 reference oscillator (Fig. 3) was measured with an Agilent E5052A Signal Source. The measurements, with the source running from a +12 VDC, 30 mA supply, and show typical tuning range of 100 kHz and phase noise of -153 dBc/Hz offset 10 kHz from the carrier. The phase noise for the HFSO1000-12 over temperature (-40 to +85ºC is fairly consistent (Fig. 4). The table provides a summary of the HFSO1000-12’s performance.
The compact HFSO and FCTS sources were developed as lower-cost alternatives to more expensive crystalresonator- based free-running VCOs and phase-locked sources operating at 300 MHz and above. Unlike the multiplied crystal oscillator solutions, the degenerated mode coupling approach lends itself to L-, S-, C-, and X-band frequencies using the N-push techniques common to radar applications.2 So far, HFSO and FCTS units have been fabricated for use at 433.42, 433.92, 435.72, 434.42, 622.080, 745.795, 776.795, 800, 915, 869, 1000, 2000, and 2400 MHz.
Numerous factors can impact the phase noise of a free-running source such as the HFSO VCOs, notably groupdelay effects and insufficient suppression of unwanted modes. Work at Synergy Microwave on the various coupling mechanisms led to an effective implementation of a degenerated mode-coupling mechanism in the HFSO and FCTS series that can also be fabricated by means of semiconductor processes.1-6
Measurements of an HFSO 2000-12 source, operating at 2 GHz and with +7.53-dBm output power, revealed typically 30-dB harmonic rejection. For a 2-Push mutually coupled oscillator, the typical phase noise is -139.5 dBc/Hz offset 10 kHz from the carrier (Fig. 5). Such a low-noise reference source requires less than 60 mA current at +12 VDC, and can cover its tuning range (about 100 kHz) with control voltages of 0 to 12 V. The phase-noise performance was characterized with evanescent-mode injection techniques in optimally 2-Push coupled states with better than 30 dB subharmonic suppression.
A model HFSO2400-12 reference oscillator operating at 2400 MHz was also evaluated, characterized with phaseinjection techniques in Class-C states using an Agilent E5052A Signal Source (Fig. 6). It was operated with 30 mA current at +12 VDC and yielded -4.07 dBm output power with 15-dB harmonic rejection. For the self-injection locked multiplier techniques used with this source, the typical phase noise at 10 kHz offset from the carrier is –136 dBc/ Hz.14-18 The reference oscillator features a tuning voltage range of 0 to 12 V to correct for any frequency shift due to aging and temperature effects.
The HFSO and FCTS topology is not limited to these frequencies, but can be extended to considerably higher frequencies for any other fixed frequency with minor modifications in size to accommodate circuit topologies such as 3-push and 4-push configurations for enabling fast adaptability for free running and phase locked low-noise signal sources for up to C-band and X-band frequencies.2-6 The phase-noise performance of these reference sources compared well to multiplied crystaloscillator solutions (Fig. 7).
The free-running HFSO reference sources and the phase-locked FCTS reference sources provide outstanding phase-noise performance with low power consumption in surface-mountpackages that are a fraction of the size of traditional reference oscillators. They also offer high reference frequencies without multiplication, saving the complexity and the compromised reliability of multipled reference sources. With their performance levels and small size, they are suitable for a wide range of commercial, industrial, and military applications, wherever a stable signal source is needed as a reference for a tunable or fixed-frequency RF or microwave oscillator in a phase-locked system.
The free-running HFSO oscillators and phase-locked FCTS sources can be specified in standard and custom frequencies, and in coaxial or surfacemount packages. Synergy Microwave Corp., 202 McLean Boulevard, Paterson, NJ 07504; +1-973-881-8800; FAX: +1-973-881-8361, e-mail: email@example.com, Internet: www.synergymwave.com.