A Fundamental Wideband Low Noise VCO Series
Signal sources play a key role in many microwave systems, with the quality of the microwave signal being particularly critical as the complexity of modulation increases. During its operation over nearly five decades, Sivers IMA has been responsible for the production of voltage-controlled oscillators (VCO), dielectric resonator oscillators (DRO), yttrium iron garnets (YIG) and subsystems. Over the last decade, however, the emphasis has been on VCOs as the basis for signal source and frequency modulated continuous wave (FMCW) products, with fundamental wideband low noise VCOs steadily increasing in popularity.
The company’s latest introduction is the VO3280 series, which is a drop-in fundamental wideband low noise VCO intended for any application where size and performance is important. It has evolved from the company’s VO3260 series, with the new integrated microwave monolithic integrated circuit (MMIC) design enabling high repeatability and thus reducing the work of the system integrator. Although wideband and low noise are optimized, the series also achieves low current consumption and high output power.
A narrow band VCO can take advantage of high Q in the resonator circuit and thereby achieve a low phase noise. Normally, when designing a wideband VCO, compromises often need to be made between the tuning range and the phase noise capabilities. With the VO3280, however, Sivers IM A has designed a VCO series combining both low phase noise and a broad tuning range. Figure 1 shows the phase noise characteristics of the series.
The VCO series is designed for wire or ribbon bonding, but is also available in various packages with SMA connectors and solder pins. It weighs 2.5 g and has ruggedized design incorporating a mixture of ceramics and metal—the carrier and lid are made from Kovar, gold plated over a barrier of nickel, while the substrate is ceramic and the connections are gold plated on tantalum nitride. This robustness makes it suitable for tough environments such as in airborne, military and space applications.
With only six units, which are listed in Table 1, the VO3280 series covers the frequency range from 2 to 25.5 GHz, with an output power of 15 dBm, while the VO3262 series provides +25 dBm output power and is recommended for use in high level mixer applications. The VO3280 and VO3284 are just 2.5 g (0.035 oz) units. The VO4280 is a radiated and mechanical protected version in a SMA house and the VO4283 series is a basic variant of a SMA connected version.
Figure 2 is a block diagram of the VO3280. Even though it is fitted with an internal regulator, Sivers IMA recommends a low noise power supply and proper bias filtering to obtain a low noise RF signal. It is important for users to avoid using a switched (switch-mode) power supply directly on the bias input, or be extra careful with the bias filtering and a linear power supply. A 100 nF capacitor close to the bias connection is preferred.
The oscillator’s tuning input voltage is between 0 and +20 V and normally the tuning current is very low. In order to keep the slew rate (or tuning speed) as high as possible, the filtering is kept to a minimum. There can be some RF leakage on the tune and bias terminals. On demand the modulation bandwidth can be adjusted to > 100 MHz.
S-band and C-band oscillators are temperature-compensated bipolar oscillators that will not oscillate below 2 V tuning voltage. If the oscillator is required to run continuously, such as the idle in a phase-locked loop (PLL), the tuning voltage should be kept above 2 V in order to keep the PLL locked. Figure 3 shows the tuning interface.
The RF output is a 250 μm wide microstripline, where Zo = 50 Ω and is AC coupled with a 10 pF capacitor. A built-in low pass filter reduces the harmonic to better than –25 dBc and the VO3280 series provides up to +18 dBm output power, as shown in Figure 4.
Importantly too, a coplanar ground-signal-ground (GSG) output is available and since the VCOs are fundamental, any sub harmonics are eliminated. Harmonics are suppressed by a built-in low pass filter. The VCOs show very good phase noise performance, considering the tuning range for some models is as high as 50 percent. Bipolar oscillator transistors are used up to 10 GHz in order to obtain the best phase noise performance possible and above 10 GHz GaAs FET transistors are used.
Due to the advanced design, even the FET VCO is powered by a single +15 V power supply and the built-in linear voltage regulator ensures low pushing, making application design easier. The tuning voltage is between 0 and +20 V, and the hyperabrupt varactor oscillator design gives an excellent tuning ratio, as shown in Figure 5. Every oscillator is individually tuned and the performance is optimized in-house.
One of the main benefits of the VO3280 series is its wideband capabilities. Instead of designing an oscillator bank with control components and control logic, a wideband VCO can be used instead, simplifying the system design dramatically. These VCOs are used in countless applications. However, two are more common—digital tuned oscillators (DTO) and phased-locked loops.
DTOs are often used in applications where the speed of frequency shift is critical, such as a local oscillator (LO) in set on receivers (SOR) or frequency hopping radar receivers. The table of calibrated frequency values is stored in an EEPROM. Temperature drift is eliminated due to the fact that the VCO is temperature stabilized either with an oven set for 55°C, or a Peltier thermoelectric module set for 35°C.
As an alternative, the VCO can be calibrated at several temperatures and an interpolated calibration table is stored in the EEPROM that is used together with a microprocessor. A temperature sensor in the VCO assembly gives the microprocessor the current temperature. In order to cover multi-octave frequency bands, several VCOs can be stacked and selected by the microprocessor (see Figure 6).
PLLs are used in applications where the stability and accuracy is essential for the function of the system, that is, as an LO in a coherent radar jammer. In this example (see Figure 7), a sample of the output signal is divided digitally or down converted and fed into the phase/frequency detector. The detector compares the divided signal with the reference signal and a correction is given through the loop filter to the tuning circuit, adjusting the VCO until the divided and reference signal are in phase.
The VO3280 series features high bandwidth, low phase noise, high linearity, medium and high power, and an extended temperature range. It is a hermetically sealed, drop-in VCO available in standard and custom housings. Customized frequency ranges and power levels are offered and coplanar and stripline RF ports are available. Every unit is individually tested and its ruggedness means that it is suitable for tough environments. Additional information may be obtained via e-mail at firstname.lastname@example.org.
Sivers IMA AB
US office: (603) 878-4566
RS No. 301