DROs can be problematic in terms of their sensitivity to temperature variations and mechanical vibrations. When comparing different models, key specifications include frequency, frequency stability with temperature, output power, output-power variations, phase noise, spurious levels, harmonic levels, physical size, and power consumption. As a commercial example, model DRO100 from Synergy Microwave Corp. (Fig. 1) is a 10-GHz DRO supplied in a compact, lead-free, RoHS-compliant metal package with SMA connector. It provides at least +8 dBm output power at 10 GHz and tunes with voltages of +1 to +15 VDC, along with typical tuning sensitivity of 3 MHz/V.

1. Model DRO100 is a 10-GHz DRO supplied in a lead-free, RoHS-compliant metal package. (Photo courtesy of Synergy Microwave Corp.)

Harmonics are typically -40 dBc while phase noise is typically -81 dBc/Hz offset 1 kHz from the carrier, -111 dBc/Hz offset 10 kHz from the carrier, and -156 dBc/Hz offset 1 MHz from the carrier. The DRO draws 70 mA current from a bias supply of +7 to +10 VDC and is designed for operating temperatures from -15 to +75°C.

Of course, there are many alternative oscillator technologies available for providing a 10-GHz signal, including YIG oscillators and VCOs. Each approach is noteworthy for a particular performance benefit—e.g., YIG oscillators for their low phase noise and VCOs for their fast tuning and settling times. For an application where frequency agility is a key requirement, for example, a first step might involve a comparison of available VCOs across the frequency tuning range of interest, with that comparison also including critical specifications such as phase noise, spurious levels, and harmonic levels.

As an example, model DCO400800-5 from Synergy is a VCO housed in a tiny surface-mount package measuring just 0.3 x 0.3 in (Fig. 2). It tunes from 4000 to 8000 MHz and draws just 20 mA at +5 VDC. It delivers at least -3-dBm output power over the frequency range with typical tuning sensitivity of 120 to 620 MHz/V. The phase noise is typically -80 dBc/Hz offset 10 kHz from the carrier, -105 dBc/Hz offset 100 kHz from the carrier, and -125 dBc/Hz offset 1 MHz from the carrier. The VCO is designed for use from -40 to +85°C.

2. This VCO tunes from 4 to 8 GHz in a surface-mount package measuring just 0.3 x 0.3 in. (Photo courtesy of Synergy Microwave Corp.)

YIG oscillators are based on single-crystal YIG spheres which resonate within an applied magnetic field, allowing frequency tuning over wide ranges at microwave frequencies. Quite simply, the resonant frequency increases as the applied magnetic field strength increases. YIG oscillators have been formed with both permanent magnets and with electromagnetic designs, with the field strength of the electromagnet and its main coil winding a direct function of the applied current.

There are limits to the size of the magnetic field—and the highest tuning frequency—that can be achieved for a given YIG oscillator housing. Oftentimes, a secondary coil winding must be incorporated within the package to achieve frequency modulation (FM). In some cases, heating circuits may also be built into a YIG oscillator to stabilize frequency over a broad operating temperature range. Advances in YIG oscillator (and filter) technology in recent years have made possible compact YIG sources that can fit within TO-8 packages.

At the highest frequencies, specifiers usually reach for Gunn oscillators. These are based on a Gunn diode mounted inside a mechanical cavity, which functions as the resonator. Such sources are commonly found in millimeter-wave applications, with Gunn oscillators using GaAs semiconductor technology reaches to frequencies of 100 GHz and higher. Gunn oscillators with gallium nitride (GaN) and other semiconductor technologies reach well into the THz frequency range.