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High Voltage Phase-Locked Loop (PLL) Synthesizer: Description and Design

Circuit Function & Benefits

The high performance PLL synthesizer circuit in Figure 1 requires 28 V for the tuning voltage of an octave range (1 GHz to two GHz) voltage controlled oscillator (VCO). A competent boost converter provides this voltage, and also the circuit runs using just one 5 V supply without any measurable degradation around the phase noise due to the dc-to-dc boost converter.

Figure 1. Boost Converter Employed for our prime Voltage Charge Pump Supply of the ADF4150HV (Simplified Schematic: All Connections Not Shown)

The circuit is optimized for PLLs which use octave range VCOs to provide a wide range of output frequencies. This type of VCO requires a high tuning voltage that won't be accessible in most systems that operate on relatively low supply voltages.

For example, the VCO within the circuit (Synergy DCYS100200-12) covers the fir GHz to 2 GHz frequency band; however, to use the full octave range available, a tuning voltage of 0 V to 28 V is needed.

There are two methods to supply this tuning voltage. The traditional technique uses an active loop filter with the amplifiers given by our prime voltage supply. The optimum technique, however, utilizes a high voltage PLL synthesizer, such as the ADF4150HV, where the PLL offers the tuning voltage without the need for an active loop filter.

Although both solutions need a high voltage supply, the ADF4150HV eliminates the need for an active loop filter, which not just reduces component count and costs, but additionally reduces the distortion and phase noise linked to the amplifiers within the active filter. Our prime voltage supply is attached to the VP pin of the ADF4150HV charge pump, and then any ripple around the supply is isolated from the VCO input through the passive loop filter.

The decreased sensitivity to distortion and ripple allows a competent dc-to-dc boost converter to create the 28 V supply for the charge pump supply pin (VP) of the ADF4150HV from a 5 V supply. See “Power Management Design for PLLs,” Analog Dialogue, 45-09, for an entire discussion on powering PLLs.

Figure 1 shows an industry-leading solution while using ADF4150HV and the ADP1613 boost converter. Total board area for the boost regulator is only 43 mm2, and also the ADP1613 will come in an 8-lead MSOP package.

Circuit Description

The circuit shown in Figure 1 uses the ADF4150HV, a high voltage fractional-N PLL IC, and also the ADP1613, a step-up switching converter.

The circuit is driven from a single 5 V supply connected to the input of the ADP1613. The ADP150, an ultralow noise LDO offers the 3.3 V VDD voltage connected to the AVDD, DVDD, and SDVDD pins of the ADF4150HV.

Note that the AVDD pins of the ADF4150HV share common decoupling, the availability for that digital block (DVDD and SDVDD) needs a separate group of decoupling capacitors.

The ADP1613 is a dc-to-dc boost converter by having an integrated power switch as well as an allowable output voltage of up to 20 V. Higher voltages are possible using additional external components. The pin selectable switching frequency of 650 kHz or 1.3 MHz allow excellent transient response and simple noise filtering.

The ADIsimPower design tool provides an smart way for designers to look for the appropriate components according to input and output requirements. The design of the ADP1613 circuit shown in Figure 1 uses the ADP161x downloadable boost designer tool. The inputs towards the tool would be the highest efficiency option together with the advanced settings of VOUT ripple = 0.2%, VOUT step error = 1%, and also the noise filter option. The input voltage is placed to VINMIN = 4.5 V, VINMAX = 5.5 V, the output voltage = 28 V, the output current = 40 mA, and also the ambient temperature = 25°C. The design file is included as part of the CN-0228 Design Support package at www.analog.com/CN0228-DesignSupport.

The ADIsimPower design file includes the balance of material, detailed schematic, bode plots, efficiency plots, transient response, and a suggested board layout.

The ADF4150HV is a 3.0 GHz, fractional-N or integer-N frequency synthesizer with an integrated hollywood charge pump. The ADP1613 provides the 28 V necessary for our prime voltage, integrated charge pump supply of the ADF4150HV.

The synthesizer directly drives the external wideband VCOs, thereby eliminating the requirement for operational amplifiers to achieve higher tuning voltages. This simplifies design and reduces cost, while improving phase noise, as opposed to the active filter topologies, which tend to degrade phase noise when compared to passive filter topologies.

When combined with an octave tuning range VCO, the ADF4150HV offers an ultra-wideband PLL function using the on-board RF dividers. By having an octave tuning range in the fundamental frequency, the RF dividers provide full continuous frequency coverage right down to reduced frequencies. For example, utilizing a 1 GHz to two GHz octave range VCO (such as the Synergy DCYS100200-12), the consumer can acquire continuous output frequencies from 62.5 MHz to two GHz in the ADF4150HV RF outputs, as shown in Figure 2. A broadband output match is achieved utilizing a 27 nH inductor in parallel having a 50 Ω resistor (to learn more, see the ADF4150HV data sheet). With such a wide output range, exactly the same PLL hardware design can generate different frequencies for several different hardware platforms within the system.

Figure 2. Ultrawideband PLL Using the ADF4150HV as well as an Octave Range VCO (Simplified Schematic: All Connections and Decoupling Not Shown)

Figure 3. Frequency Spectrum for a 1.5 GHz Output

Common Variations

The hollywood supply within this circuit is wonderful for microwave VCOs that need a large tuning voltage range.

The 3.3 V supply can be based on a 5 V supply while using ADP150 or ADM7150 low noise LDOs. Many microwave VCOs can operate on the 5 V supply.

When designing with various VCOs, design another loop filter to make sure that the phase-locked loop works well. The ADIsimPLL design tool offers an smart way for designers to look for the appropriate components according to VCO sensitivity (KV), PFD frequency, channel spacing, along with other requirements.

Alternate dc-to-dc switching converters can be used, however, for PLL synthesizers charge pump supply, you should select a boost converter with a high switching frequency (more than 1 MHz). The loop filter of the phase-locked loop can reduce/ suppress the spurs caused by the switching frequency of the boost converter.

Circuit Evaluation & Test

The circuit shown in Figure 1 belongs to the EV-ADF4150HVEB2Z evaluation board. A complete schematic, layout, and list of materials for that evaluation board can be found at www.analog.com/CN0228-DesignSupport.

The Synergy DCYS100200-12 VCO used on the evaluation board requires a clean 12 V supply, that is provided from the creation of an ADP7104 adjustable low noise LDO. However, to achieve good output voltage accuracy, the input voltage of the adjustable LDO should be at least 1 V greater than the output voltage, thus the availability from the board is set to the closest standard supply value over the necessary minimum (13 V), that's, to 15 V.

The input voltage to the ADP1613 dc-to-dc boost converter can accept 5.5 V maximum; therefore, on the evaluation board, it's limited to 5.1 V. This is achieved with a 5.1 V Zener diode biased having a 300 Ω resistor attached to the 15 V supply. The Zener diode then drives the input towards the ADP1613.

The 28 V supply on the VP pin of ADF4150HV is generated while using circuit shown in Figure 1. Results measured using the EV-ADF4150HVEB2Z evaluation board while using ADP1613 boost converter are provided in Figure 3. The evaluation software, ADF4150HV family software, controls all the features available on the ADF4150HV evaluation boards. A description of the EV-ADF4150HVEB2Z board is available in the UG-483 User Guide.