RF Oscillators: VCO Integrated Circuits for Microwave/mmWave Products

The RF world can be daunting for the digital designer, and navigating the range of components for different systems can be difficult. So much analog signal processing is done at the hardware level, and getting the job done requires choosing the right analog components. One critical component is an RF oscillator, which can be used as a signal source or as a stable reference signal.

Different VCOs are rated for different signal levels, but they give you a convenient way to control the frequency of your reference oscillator. Some immediate applications include direct frequency modulation, frequency synthesis, and stable reference oscillators. Here’s what to watch for when selecting a VCO for these and other applications.

VCO Applications in RF Products

Many low power wireless and RF applications are moving towards SoCs, where a VCO, PLL, DSP components, CPU, ADC/DAC, and other functions are being integrated into a single unit. For other intermediate and high power output applications, you’ll likely need to rely on discrete components and a dedicated VCO IC for your application. In addition, if you’re innovating unique systems for use as stable oscillators or other applications at GHz frequencies, it helps to use a VCO IC as part of the overall system design before purchasing a more expensive SoC.

Important VCO Specifications

There are two primary types of VCOs for use as RG oscillators. A linear VCO is designed to produce a sinusoidal waveform with an integrated varactor in an LC tank circuit. Ideal varactors are nonlinear, but there is a range of voltage drops across the varactor that can be approximated as nearly linear, which will determine the tuning range in the circuit. Relaxation oscillators are designed to produce a square, triangular, or sawtooth waveform and are easily formed on an MMIC. An astable multivibrator VCO is one example of a relaxation oscillator that outputs a square wave, although this can be converted to a triangle wave with an integrator circuit.

An ideal VCO will output a perfectly sinusoidal/triangular/sawtooth tone with no phase noise, zero temperature stability, and with a linear frequency range. Real components do not operate in this way, but they can often come close. Here are some important specifications to consider when selecting a VCO for an RF oscillator:

• Tuning voltage range: This will determine the range of input DC voltages the device can accept, which will then be converted to the desired frequency.
• Frequency range: The frequency range you’ll find in a datasheet is normally a graph that shows a linear range. If you increase beyond the top end of the input voltage vs. output frequency graph, you’ll find that the output frequency saturates at some maximum value. Ideally, the frequency range should be a linear function of the tuning voltage, but this is not always the case.
• Tuning range: This is just the frequency range divided by the input voltage range (within the linear region). This is generally quoted as a constant value in units of MHz/V or GHz/V.
• Temperature stability: Different components will have different levels of stability. In general, the power output and output frequency will decrease as the temperature increases, but the exact amount of decrease varies by component. Be sure to check this as it will determine which thermal management measures might be needed in your board. Oscillator type: Be sure to check the type of oscillator used in the VCO (see above).
• Harmonic/sub-harmonic output: Some VCOs will allow you to directly access a higher harmonic or a lower harmonic on the output. Normally the first overtone (double the fundamental frequency) or the first sub-tone (half the fundamental frequency) can be accessed from the output.
• Phase noise: this specification is critical for applications that require precise timing, such as digital-to-time conversion in lidar and radar. This is also a critical specification when a VCO is used as a reference oscillator.

RF Oscillator VCO Component Options

Analog Devices, HMC739LP4E

After their acquisition of Hittite, Analog Devices offers the Hittite HMC739LP4E VCO as part of their portfolio. This VCO provides half frequency and divide-by-16 outputs in addition to the standard tunable frequency output. The standard frequency can be tuned linearly from 23.8 to 26.8 GHz at up to 14 dBm output, with slightly nonlinear tuning beyond this range. Although it has a narrow tuning range, it has relatively high power output and is ideal for point-to-point radio, point-to-multipoint radio, and VSAT applications.

This MMIC is built around a GaAs-InGaP heterojunction bipolar transistor, so it provides efficient power output throughout the tuning range. It also has very low phase noise, as shown in the graphs below.

SSB phase noise from the HMC739LP4E VCO. From the HMC739LP4E datasheet.

MACOM, MAOC-009268-PKG003

The MAOC-009268-PKG003 from MACOM is one option for an InGaP HBT-based VCO for GHz frequency generation. This VCO provides tunable output from 12.7 to 14.2 GHz at up to 9 dBm power output. This component also offers a half frequency output at up to 1 dBm power output. One advantage of this component is the power output is basically constant over the tuning voltage range, which is not the case with the previous component. It also has excellent temperature stability, as shown in the graphs below.

Temperature stability of the MAOC-009268-PKG003 VCO. From the MAOC-009268-PKG003 datasheet.

Texas Instruments, LMX2531LQE1415E

The LMX2531LQE1415E VCO from Texas Instruments is a complete frequency synthesizer solution that includes an integrated PLL. When combined with an external reference oscillator, this device generates low-noise local-oscillator signals for up and down conversion. It also includes a serial control interface for tuning with a fractional-N delta-sigma modulator (up to 4th order). Some ideal applications include satellite communications, cellular base stations, CATV, and clocking for data converters.

Open-loop and closed-loop phase noise. From the LMX2531LQE1415E datasheet. (Alt text: LMX2531LQE1415E phase noise plots) https://drive.google.com/open?id=17CohCZB3_NKahnxiUMZC3tmM7tJezroq

We’ve presented just a few of the many RF oscillator VCO components you can find on the market. There are other options for lower power applications, such as PLLs with integrated VCOs. These integrated components and many more discrete VCO ICs for RF products can be found in on Octopart.

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