VCO driving high voltage phase locked loop frequency synthesizer circuit

“A phase loop (PLL) circuit is a feedback system consisting of a voltage controlled oscillator (VCO) and a phase detector, and the oscillator signal tracks whether the applied frequency or phase modulated signal has the correct frequency and phase. A PLL can be used when a stable high output frequency needs to be generated from a fixed low frequency signal, or when the frequency needs to change rapidly. Typical applications include filtering, modulation and demodulation, and frequency synthesis using high frequency, telecom and measurement technologies.

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A phase loop (PLL) circuit is a feedback system consisting of a voltage controlled oscillator (VCO) and a phase detector, and the oscillator signal tracks whether the applied frequency or phase modulated signal has the correct frequency and phase. A PLL can be used when a stable high output frequency needs to be generated from a fixed low frequency signal, or when the frequency needs to change rapidly. Typical applications include filtering, modulation and demodulation, and frequency synthesis using high frequency, telecom and measurement technologies.

Figure 1 shows a block diagram of a PLL-based frequency synthesizer. The VCO generates the output signal. It is held at the set frequency by the PLL and locked to the reference frequency. The reference frequency is usually provided by a very precise quartz oscillator. In the feedback path part of the phase-locked loop circuit, an adjustable VCO frequency division ratio is provided by a frequency divider before the phase detector.

Figure 1. Phase-locked loop block diagram.

Contains adjustable tuning elements such as varactors whose capacitance varies with input voltage. Therefore, the PLL circuit can be regarded as a VCO feedback control system. The input or control voltage required by the VCO is usually higher than the supply voltage supplied to the PLL circuit. The supply voltage is typically 3.3 V or 5 V, while the VCO may require more than 20 V depending on frequency requirements. To generate a wider range of frequencies, a VCO with a wider tuning range can be used. Figure 2 shows an example of a simple circuit that supports a gigahertz range VCO.

Figure 2. High-voltage charge pump power supply simplified circuit for the ADF4150HV.

DCYS100200-12 from Synergy Microwave Corporation can be used. The product produces a 2 GHz frequency at 28 V (VTUNE) as shown in Figure 3.

Figure 3. Control voltage versus frequency for DCYS100200-12.

There are several possible options for generating high control voltages. One is to use an active loop filter, which basically consists of a high-speed amplifier and a low-pass filter that converts the output pulse from the phase detector (CPOUT) into a clean DC voltage. Alternatively, a PLL frequency synthesizer with an integrated charge pump, such as ADI’s ADF4150HV, can be used. This device does not require an additional active loop filter. While both solutions require high voltage power supplies, using the ADF4150HV reduces the number of components required. Distortion and phase noise caused by active filter amplifiers are also avoided. In addition, the ADF4150HV allows the implementation of fractional-N or integer-N phase-locked loop frequency synthesizers. The frequency of the final VCO can be divided by 1, 2, 4, 8, or 16, allowing the output frequency to reach 31.25 MHz.

The high voltages required for the integrated charge pump of the PLL can be generated using the ADP1613 DC-DC boost converter without degrading the PLL performance. The ADP1613 is a high-efficiency switching regulator with integrated power transistors that can easily achieve an output voltage of 20V. Higher output voltages can also be achieved using additional external components, especially through external power transistors. The switching frequency of the ADP1613 is adjustable from 650 kHz to 1.3 MHz. This allows for better transient response and simple noise filtering. In general, a switching frequency higher than 1 MHz is recommended to reduce switching noise through the PLL loop filter.

The phase-locked loop frequency synthesizer circuit using the ADF4150HV provides ultra-wideband PLL functionality by using an integrated RF divider. The operating frequency range is 62.5 MHz to 2 Ghz. By using the same PLL hardware design, different frequencies can be generated for multiple different hardware platforms in the system. However, if a design is required for different VCO types, the corresponding loop filter needs to be integrated into the design. Only in this way can the reliable operation of the phase-locked loop be ensured. To achieve a relatively wide range of output frequency regulation, and the associated higher output power, small filters are also required at each RF output of the ADF4150HV. A parallel connection of a 27 nH Inductor and a 50 Ω resistor can effectively regulate frequencies up to 3 GHz. This resistor provides a defined output impedance lower inductance will cause the frequency band to extend into the lower range.

Today, all-in-one integrated solutions for larger frequency ranges (ie for PLLs, filters, and VCOs) are also available, but the close proximity of different components can lead to unwanted coupling. The discrete design and resulting physical separation can substantially reduce this risk.

The frequency synthesizer simulation tool ADIsimPLL? can also provide effective assistance for HF functional block development and HF signal chain device modeling. Using this tool, designers can more easily simulate all important nonlinear effects that affect PLL performance; for example, unwanted spurs (spurious frequencies) that occur during frequency synthesis.