For years, tunable oscillators based on yttrium- iron-garnet (YIG) resonators have been the first choice for many broadband systems and test instruments, valued for their low phase noise and excellent tuning linearity. The one drawback of the technology has been the size, since the oscillators have traditionally been housed in large cube-shaped or cylindrical packages with SMA connectors. But with refinements to YIG oscillator designs over the years, they have finally gotten small enough to pack into standard TO-8 packages, as evidenced by the introduction of the MLTO series of permanent-magnetic YIG-tuned oscillators from Micro Lambda Wireless, available in standard and custom frequency bands from 2 to 8 GHz. The oscillators are housed in cylindrical metal packages with 0.625-in. diameter and heights of 0.27, 0.35, and 0.50 in.
YIG oscillators are based on the principle that a sphere formed of YIG material will resonate within an electromagnetic (EM) field. The resonant frequency increases as the field strength increases, allowing frequency tuning by adjusting the current used to create the EM field. The YIG sphere is typically mounted on a ceramic rod as part of a YIG assembly, and placed within a pole gap of the electromagnetic field. The field is formed by winding copper wire to form a coil. The coil essentially forms an electromagnet with the oscillator's metal package. The field strength of the electromagnet is a linear function of the applied current. The wire coil couples energy from the electromagnet to the sphere; similarly, a coupling loop extracts resonant energy from the sphere to produce the output frequencies of the YIG oscillator.
As current flow increases through the wire windings, the EM field strength around the YIG sphere increases and its resonant frequency increases. There are practical limits to the size of the EM field that can be achieved for a given package size and when the electromagnet formed of the oscillator package will reach saturation, which will ultimately limit the highest fundamental frequency that can be attained with a given YIG oscillator design. A second, smaller coil with much fewer windings is also used within the YIG oscillator housing, to provide frequency modulation (FM) as well as a means of stabilizing the oscillator to an external phase-lock-loop (PLL) integrated circuit (IC). This secondary coil has much lower inductance and lower resistance than the main tuning coil, allowing for a much faster tuning rate than the main coil.
The energy coupled from a YIG oscillator's resonant tank circuit is boosted to usable output levels by means of an active device, a low-noise transistor or monolithic-microwave-integratedcircuit (MMIC) amplifier. In the past, low-noise silicon bipolar transistors were typically used for frequencies to about 15 GHz, with GaAs field-effect transistors (FETs) employed for higher frequencies. In the MLTO series YIGtuned oscillators, the active device is a discrete silicon germanium (SiGe) transistor. The device exhibits higher electron mobility, lower device thermal noise, and lower device shot noise compared to traditional silicon bipolar transistors operating through 8 GHz, helping the MLTO series oscillators achieve excellent spectral purity throughout all models and frequency ranges.
The MLTO YIG oscillators are supplied in cylindrical packages much like their predecessors, but considerably smaller, and with pin contacts rather than coaxial connectors. The pins provide connections for tuning the main and frequency-modulation (FM) coils, for supply voltages, for ground, and for RF output signals. One of the reasons that these oscillators can be made so much smaller than traditional YIG sources is the elimination of an internal heater. Conventional YIG oscillators incorporate heaters to stabilize the frequency of the resonant circuitry over temperature. Even without a heater, the MLTO YIG oscillators minimize frequency drift with temperature to a worst case of 10 MHz. Eliminating the heater means less power consumed and less heat generated. The MLTO oscillators require less power than larger YIG oscillators, consuming about 60 mA at +8 VDC and about 15 mA for the -5 VDC supply, compared to typical values of 200 mA at +12 VDC and 25 mA at -5 VDC for a larger cylindrical YIG oscillator covering the same frequency range. This does not include the bias for heater circuitry in a conventional YIG oscillator.
The MLTO series YIG oscillators can be mounted directly on printed circuit boards (PCBs), allowing designers to shrink the RF/microwave boards in receivers, spectrum analyzers, signal generators, and other applications. The oscillators are supplied in three different product families, each with diameter of 0.625 in. and with heights of 0.27, 0.35, and 0.5 in. Each package has eight pins, and each of the oscillators provides at least +10 dBm output power across the tuning range, with worst-case outputpower variations of 3 dB.
The oscillator series with the lowestprofile package (Fig. 1) currently has three standard units, models MLTO- 20204, MLTO-20406, and MLTO- 20608, covering 2 to 4 GHz, 4 to 6 GHz, and 6 to 8 GHz, respectively (see Table 1). The three oscillators have respective center or free-running frequencies (the operating frequency when 0 mA tuning current is applied to the main coil) of 3, 5, and 7 GHz, each with free-running frequency settability of 20 MHz. For all three models, harmonics are held to -15 dBc or better while spurious content is -70 dBc or better. The phase noise for all three models is typically -128 dBc/ Hz offset 100 kHz from the carrier. As measurements with a model E5052B signal source analyzer and model E5053A microwave downconverter from Agilent Technologies show (Fig. 2), the phase noise far from the carrier drops below -146 dBc/Hz offset 1 MHz from a 3.5-GHz carrier. Even for MLTO series oscillators designed for higher frequencies, such as 8.5 GHz, phase noise remains low at all offset frequencies (Fig. 3).
Each of these lowest-profile MLTO series TO-8 YIG oscillators tune with main coil sensitivity of 6 MHz/mA (every additional 1 mA of current results in typically a 6-MHz increase in frequency). The linearity of that tuning curve is within 2 MHz with typical hysteresis of 3 MHz. The FM coil has a typical tuning sensitivity of 300 kHz/mA for all three models, with typical 3-dB bandwidth of 400 kHz. Minimum FM deviations of 50 MHz are possible. The lowest profile MLTO YIG oscillators require bias of 60 mA at +8 VDC and 15 mA at -5 VDC.
The tallest of the three MLTO series YIG oscillators, with a height of 0.5 in., is also available in three standard models with tuning ranges of 2 to 4 GHz, 3 to 6 GHz, and 3 to 8 GHz (Fig. 4). The performance of these larger oscillators is similar to that of the lowest-profile MLTO sources, with differences in maincoil tuning sensitivity (because of the larger package) and in frequency range (see Table 2). The two higher-frequency oscillators in this series cover 3 to 6 GHz and 3 to 8 GHz, with the lowestfrequency model still operating from 2 to 4 GHz. The three sources have freetuning (zero-current) frequencies of 3.0, 4.5, and 5.5 GHz, respectively, with the same spurious, harmonic, and phasenoise performance as the lowest-profile MLTO models. The main difference between these different-height oscillators is in main coil tuning sensitivity, with typical tuning sensitivity of 9.7 MHz/ mA for these tallest TO-8 YIG oscillators, with somewhat greater hysteresis (typically 10 MHz) in the tuning than with the lower-profile oscillators. The FM coil sensitivity remains at the 300 kHz/mA of the lowest-profile models, and the bias requirements remain the same as the shorter models.
In the middle, the MLTO series YIG oscillators with 0.35-in. height (Fig. 5) also includes three standard models, MLTO-30204, MLTO-30306, and MLTO-30308, with tuning ranges of 2 to 4 GHz, 3 to 6 GHz, and 3 to 8 GHz, respectively, with similar performance to the tallest and shortest of the TO-8-packaged oscillators. The output power, output-power deviations, spurious, harmonics, phase noise, pulling, and pushing figures are identical to the other oscillators, with main coil tuning sensitivity of 7.5 MHz/mA and maincoil tuning hysteresis of 5 MHz for all three models. The YIG oscillators in the middle maintain the 300-kHz/mA FM coil tuning sensitivity of the taller and shorter oscillators, with the same bias requirements.
The MLTO series TO-8 tunable YIG oscillators are designed for commercial operating temperatures from 0 to +65C, although units are available for extended operating temperatures to +85C. The oscillators are designed for minimal frequency drift at room temperature, with drift down and up in frequency with temperatures lower or higher than room temperature, respectively (Fig. 6). P&A: 4 wks. Micro Lambda Wireless, Inc., 46515 Landing Pkwy., Fremont, CA 94538; (510) 770-9221, FAX: (510) 770-9213, Internet: www.microlambdawireless.com.