How To Balance Application Priorities For PLL And VCO Tradeoffs

Making application driven tradeoffs is a daily part of design work. With the overwhelming choices for phase locked-loops and voltage-controlled oscillators, optimizing a design for test and measurement or lab and field science equipment has become an increasing challenge.

In particular, phase-locked loop (PLLs) and voltage-controlled oscillators (VCOs) are used in instrumentation applications to generate signals from low-frequency RF to terahertz applications. An integrated PLL/VCO solution can meet the needs of low-cost and small-form-factor laboratory and field science equipment for students and hobbyists. Higher performance PLLs and VCOs, in contrast, are used in test and measurement instruments for high-speed production test environments for leading smartphones or in high-frequency semiconductor characterization.

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Figure 1: Test and measurement instruments such as a Spectrum Analyzers, require highly capable PLL-based synthesizers in order to tune across a wide frequency range with tight resolution. 

There are many factors to consider when pairing a VCO and PLL or when choosing an integrated option; knowledge of available components and efficient methods of analysis can save on number of design cycles, cost, and enhance end-product performance.

 
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Figure 2: A PLL consists of an error detector, loop filter, VCO, and feedback divider. PLLs are used in instrumentation and radio hardware to ensure tight frequency stability, enable high precision testing, and compliance with the latest modulation and communication standards.

What PLL Synthesizer And VCO Solutions Are Out There?

Typically, the phase noise, harmonics/spurs, tuning sensitivity, power, size, and RF bandwidth are the common parameters to consider when evaluating VCOs and PLLs for a particular set of requirements. Cost, size, and power consumption may be the limiting factors in VCO and PLL device options and can depend upon the end price of the instrumentation. Also, for characterization grade laboratory equipment, raw performance is typically the top consideration.

 For an example of top performance, Analog Devices HMC704 is an 8 GHz and 16-bit fractional-N PLL that exhibits phase noise and spurious response of less than -112 dBc/Hz at 8 GHz with a 50 kHz offset in fractional N mode. This device consists of a very low noise digital phase detector, reference divider, precision controlled charge pump, and a VCO divider. With such low spurious content and phase noise, the HMC704’s output spectral purity enables wide loop bandwidths in transmitters and receivers, which in turn enables faster frequency hopping and low microphonics. Additionally, for digital predistortion (DPD) systems, the 24-bit fractional modular in exact frequency mode virtually eliminates frequency error and allows for efficient amplification in communications power amplifier (PA) applications.

Some applications require greater levels of power efficiency and smaller footprints. A device that caters to these needs is the HMC586 VCO, which operates off a single positive supply of +5 V at only 55mA of current draw, ideal for battery operation. Additionally, the HMC586 incorporates both a built-in resonator and a buffer amplifier, so the footprint and bill-of-materials (BOM) are both reduced. To ensure that the device can fit many field and low power applications, the low-phase noise performance and wide tuning bandwidth can be leveraged to fit many frequency synthesizer and PLL design needs.

For field applications, an integrated PLL/VCO with sophisticated control and programming features may be advantageous for use in field instruments operating up to several GigaHertz. In these cases, a device such as the ADF4355 or HMC832A possesses many features that can be controlled through a digital interface to provide efficient operation from battery supplies, and also provide nimble programmability from a simple button or touch-screen interface. The integration of PLL synthesizer, VCO, and loop filter to complete a PLL can lead to a fully specified and tested solution that can easily be dropped into a design. Concerns over parasitics, process variations, and a multitude of failure modes is also reduced with integrated solutions; integrated solutions also lead to rugged field solutions compared to discrete designs.

In production environments, hardware footprint, programmability, and measurement speed are top priorities. The latest military radios and radar, and even commercial telecommunications equipment, require much higher frequencies than previous methods. In these cases, the ADF41020, an 18 GHz programmable frequency synthesizer, can be implemented as a local oscillator (LO) for up-conversion or down-conversion of high frequency microwave radios and radar. The 18 GHz of bandwidth allows for the design of a PLL circuit with the addition of a VCO and loop filter, which doesn’t require a frequency doubler.

Pocket-Change Instruments That Still Perform

The cost of LFS and T&M equipment can dramatically increase with design, development, testing, and calibration of these complex instruments. Leveraging integrated PLL/VCOs can provide a cost and performance edge that would be challenging to achieve with discrete devices. For instance, the ADF4350 or ADF4351, when paired with an external loop filter and frequency reference, can develop a fractional-N or integer-N PLL which operates from 35 MHz to 4.4 GHz.

 Hobbyists and students are predominantly interested in instruments that receive amplitude or frequency modulated (AM/FM) signals contained within sub-1 GHz, sub-5 GHz, and unlicensed or amateur radio bands. Due to the very small budgets of these users, they desire a single piece of equipment that provides as much flexibility as possible, oftentimes trading frequency coverage for sensitivity and phase noise performance. With cost being the number one limiting factor for this demographic, many of these instruments are made in high volume factories in China, or by small and developing startups, where overhead is low.

In these applications, equipment such as spectrum analyzers/signal analyzers (SAs) generally operate with the maximum frequency of interest up to 8 GHz, though most cutoff around 4.4 GHz (which coincidentally matches the frequency coverage of the ADF4350/ADF4351). This is largely the same for arbitrary waveform generators (AWGs), scalar network analyzers, and vector network analyzers (VNAs).

When Power Savings And Performance Are A Must

There is a demand for portable spectrum analyzers to incorporate more features in smaller and higher performing form factors in combination with VNA features, power measurement accessories, and AC/DC measurements. The climb to higher frequencies—60 GHz and beyond—for microwave and millimeter-wave backhaul devices and base stations – is also causing customers to require ever higher frequency performance from these portable instruments. Consequently, maintaining low phase noise in integrated PLL/VCOs while keeping power usage to a minimum is increasingly important. Devices that meet these demands are the HMC832A and ADF4355 integrated PLL/VCOs.

For applications that are outside of a laboratory—field service, rugged production environments, or system maintenance/repair—high performance analysis must be performed. Still, these environments are not forgiving enough for delicate precision lab instruments. In these cases, ruggedized portable field service equipment must still provide high end performance on a small battery power supply with a rich set of time/cost saving features. For example, the Keysight Fieldfox Handheld RF and Microwave Analyzer, the R&S FSH Handheld Spectrum Analyzer, and the Anritsu Site Master Cable & Antenna Analyzer + Spectrum Analyzer, all combine many cable and spectrum analyzer features in a rugged and portable package that can perform for several hours at a time.

Devices such as these often take advantage of the efficiencies provided from highly integrated PLL/VCO solutions to measure several GigaHertz of frequency range with tens of MegaHertz of measurement bandwidth. The PLL/VCO devices that drive these instruments must have low phase noise performance to enable greater than 90 dB of dynamic range and low harmonic and spurious activity as large frequency sweeps are necessary for common field troubleshooting procedures. These instruments are regularly used to demodulate AM/FM/PM, QAM, BPSK, QPSK, OQPSK, and other modulation types for common telecommunication systems. Licenses to unlock different demodulation and analysis features can be an added revenue generator for device manufacturers.

When Top Performance And Speed Is Key

For instruments of high caliber, dynamic ranges well above 90 dB are expected while still operating at the extremes of frequency sweep speeds and wide measurement bandwidths. Very complex modulation schemes— OFDM, high level QAM, and pulsed signals—are often being measured by test and measurement equipment from 26.5 GHz, 50-60 GHz, and now to 80 GHz. Therefore, very low phase noise and extreme frequency stability are demanded of PLLs and VCOs in instruments for these applications. Otherwise, the phase noise, frequency instability, and harmonics/spurs could mask brief events that are vital in characterizing complex systems rapidly and efficiently. This is why devices, such as the HMC704, push the boundaries of low phase noise and spurious content.

 In order to characterize and optimize designs for high-speed digital or high-frequency RF/microwave applications, PLLs and VCOs that support benchtop and production environment equipment must match, and even exceed, current standards of performance. In these applications performance is optimized over cost, power consumption, footprint, and design complexity. Users that leverage equipment of this level are often designers and manufacturers of leading telecommunications equipment, satellite communications, military/defense communications, aerospace systems, radar, and microwave/millimeter-wave imaging systems.

There Are Tools Out There To Help You Evaluate PLLs And VCOs

As the margin of error is often extremely low for applications requiring PLLs and VCOs, any inaccuracies in the datasheet or SPICE models ported into traditional spreadsheet or modeling software can dramatically reduce end-design performance and increase design cycle iterations. For these reasons, ADI developed ADIsimRF and ADIsimPLL to aid with the signal chain design and performance budgeting for PLL/VCO applications. The tools and features provided by ADI reduce design time and ensure that the PLL/VCO devices integrate seamlessly into the final design.

ADIsimPLL, for example, offers a comprehensive and easy-to-learn platform that helps with design and simulation, including nonlinear effects not normally capture in a spreadsheet or SPICE model. This includes anti-backlash pulses, Fractional-N spurs, and phase noise. ADIsimPLL rapidly develops time and frequency domain plots that are automatically generated from component selections—all ADF4xxx family products are available in the built-in libraries—custom VCOs can also be added to the simulator’s component library. Future articles will provide greater detail on how ADI’s free tools, such as ADIsimRF, can be leveraged to cut design time and easily sort through massive amounts of datasheet and SPICE models to pick the right devices for a specific application.

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Figure 3: ADIsimPLL employs a rich set of analysis features and plots that are automatically populated and easily updated as design changes are implemented.

PLL and VCO device decisions can often be a limiting factor in high-speed digital and RF/microwave lab and field instruments, which are deployed in applications from a student's lab bench, to production test facilities, and even on the tops of radio towers. There are a wide range of features and performance variations that must be analyzed and compared to optimize an instrument's design and meet user expectations at the right price point. It is often powerful for LFS and test and measurement instrument designers to have access to a wide selection of frequency synthesizers, VCOs, and PLLs in discrete and integrated form from the same supplier. This way, the selection and ordering process can be streamlined. While choosing a PLL/VCO combination is not a simple task, there are software modeling tools that can speedup and ease the headaches associated with the selection process.


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