A kind of feedback control system for PLL, VCO
Used for frequency synthesis and high frequency measurement, communications, etc.
ADF4150HV, RF Splitter Supports Wider Bandwidth


A phase-locked loop (PLL) circuit is a feedback system that combines a voltage controlled oscillator (VCO) and a phase detector, which follows the method of tracking an applied frequency or phase modulated signal with the correct frequency and phase of the oscillator signal.

PLLs are used when it is desired to generate a higher stable output frequency from a fixed low frequency signal or when rapid frequency changes are required. Typical applications include frequency synthesis, measurement for filters, modulation, and demodulation at high frequencies, and communications.

Figure 1 is a block diagram of a PLL-based frequency synthesizer. The generated signal is synchronized to a reference frequency, maintained at a frequency set by the PLL, and an output signal is output by the VCO.

This reference frequency is provided using a high-accuracy quartz oscillator. A frequency divider is used in front of the phase detector in the feedback path of the PLL circuit to lower the frequency of the VCO by a variable factor.

A VCO contains a variable element such as a varactor diode, which uses a capacitance that varies with the input voltage. Therefore, a PLL is a type of feedback control system for a VCO. Usually, the input or control voltage required for a VCO is higher than the power supply voltage available to the PLL circuit.

The supply voltage is usually 3.3 V or 5 V, but the VCO may require more than 20 V depending on the required frequency. To generate a wider range of frequencies, a VCO with a wider tuning range can be used. Figure 2 shows an example circuit when using a GHz-range VCO.

Synergy Microwave's DCYS100200-12 can be used as a VCO. As shown in the graph in Figure 3, this product is capable of a frequency of 2GHz at 28V (V _TUNE ).

There are several ways to generate a high control voltage. One way is to use an active loop filter. This basically consists of a high-speed amplifier and a low-pass filter, which shapes the output pulse (CP _OUT ) from the phase detector into a clean DC voltage. Another way is to use a PLL frequency synthesizer, such as the ADF4150HV from Analog Devices (ADI), which integrates a charge pump and does not require an additional active loop filter.

Both methods require a high voltage power supply, but the ADF4150HV can reduce the number of components required. It can also avoid distortion and phase noise caused by active filter amplifiers. In addition, it can implement fractional-N or integer-N PLL frequency synthesizers. Therefore, the VCO frequency can be divided by 1, 2, 4, 8, or 16, and the output frequency can be as low as 31.25MHz.

The ADP1613 DC-DC boost converter can be used to generate the high voltage required for the internal charge pump of the ADF4150HV without compromising PLL performance. The ADP1613 is a switching regulator with integrated power transistors and can achieve an output voltage of up to 20 V. Higher output voltages can be achieved by using additional components such as external power transistors.

The switching frequency of the ADP1613 can be adjusted from 650kHz to 1.3MHz, which can achieve excellent transient response and simplify noise filtering. It is generally recommended to select a switching frequency of 1MHz or higher. This is to reduce switching noise through a PLL loop filter.

The PLL frequency synthesizer circuit using ADF4150HV can support a wide bandwidth (62.5MHz to 2GHz) by using RF divider. With the same PLL hardware design, different frequencies can be generated for different hardware platforms in the system. However, if the design requires various VCO types, it is recommended to include a loop filter corresponding to them. This can ensure reliable operation of the PLL.

When the output frequency adjustment range is relatively wide and therefore higher output power is required, a small filter is required for each RF output of the ADF4150HV. The frequency can be adjusted up to 3 GHz by using a 27 nH inductor and a 50 Ω resistor together. This resistor provides a fixed output impedance. The frequency range can be extended lower by using a lower inductance.

Recently, integrated solutions (integrated PLL, filter, VCO) that can cover a wide frequency range in a single housing have also been released. However, these solutions have the risk of unwanted coupling because each component is placed close together. Discrete designs minimize this risk because they have a spaced layout.

ADI offers ADIsimPLL™ as a PLL frequency synthesizer simulation tool to support HF functional block development and HF signal chain simulation. Using this tool, designers can simulate all the major nonlinear effects that can affect PLL performance. Unwanted spurious frequencies arising during the frequency synthesis process are one such example.



This article is a summary of an article titled "Driving the VCO of a High Voltage Phase-Locked Loop Frequency Synthesizer Circuit" by Thomas Brand, a field applications engineer at ADI.