Design of ADF4106 Phase-locked Frequency Synthesizer Based on Single-Chip Microcomputer Control

0 Introduction

In the design and development of modern electronic technology, especially in the fields of communication, radar, aviation, aerospace, and instrumentation, it is necessary to further improve the frequency accuracy of a series of high-precision and high-stability frequency sources. In this way, general oscillators can no longer meet the development requirements of various applications, and although the performance of crystal oscillators is relatively good, their frequency is single, or can only be fine-tuned within a very small range. Therefore, this paper proposes a design scheme for realizing a phase-locked frequency synthesizer based on the control of the single-chip microcomputer AT89C2051, using phase-locked technology, and taking the frequency synthesizer chip AD4106 produced by ADI as the core.

1 System structure principle

The specific implementation structure of the phase-locked frequency synthesizer is shown in Figure 1. This system is composed of the frequency synthesizer AD4106, loop filter, voltage-controlled oscillator, crystal oscillator, reference divider and program divider to form a phase-locked loop (PLL). The input of the loop is divided by a high-stability and high-accuracy crystal oscillator through a reference divider, and then the reference frequency is output. Then, the good narrowband tracking characteristics of the phase-locked loop are used to accurately stabilize the output frequency of the voltage-controlled oscillator at the reference frequency or a certain harmonic. The loop chip ADF4106 is equipped with a program divider and a reference divider, which can be programmed and controlled by the single-chip microcomputer AT89C2051. The high-performance output frequency f0 can be obtained by changing the single-chip microcomputer program and using the appropriate N and R values ​​provided. When the loop is locked, the output frequency f0 _can be formally calculated:



Where: N is the program division ratio, R is the reference division ratio, fr _is the fixed frequency of the crystal oscillator, and f0 _is the output frequency.



2 System Hardware Implementation

The following takes a 410 MHz phase-locked frequency synthesizer as an example to illustrate the system implementation process.

2.1 Structural characteristics of ADF4106

ADF4106 is an integrated digital phase-locked frequency synthesis chip produced by ADI, USA. It integrates a low-noise digital phase detector, a programmable frequency divider, a programmable A/B counter, a dual-mode pre-frequency divider (P/P+1), a precise charge pump and other modules. The biggest feature of ADF4106 is its extremely high operating frequency, which can simplify the system structure, reduce power consumption and equipment cost. In addition, the chip's working state can be easily controlled by controlling the registers in the chip through a simple 3-wire interface. The internal structure of ADF4106 is shown in Figure 2.



The technical parameters of ADF4106 are: the output frequency is less than 550 MHz; the dual-mode factor is 8/9 (P=8); the reference frequency is 5M＜fr _＜ 100M; the locking frequency f0 ₌ Nfr _/ R, where N=(BP+A); A, B, and R are internal programmable counters, A is 5 bits, and B and R are 14 bits.

2.2 System hardware circuit

The system hardware circuit connection diagram is shown in Figure 3. In the figure, the CP pin of ADF4106 is connected with external capacitors of 390 pF, 2700 pF and 39 pF and resistors of 10 kΩ and 27 kΩ to form a loop filter. The third-order loop filter is used in this system, which is mainly used to attenuate the high-frequency components in the error voltage and improve the anti-interference ability of the system. Its loop bandwidth is 20 kHz. In addition, since the input voltage of the voltage-controlled oscillator is large and the output voltage of the frequency synthesizer is small, the operational amplifier chip AD820 is selected in the design to amplify the error voltage signal.



Based on the premise that the frequency synthesizer meets other technical indicators, the circuit must be miniaturized. Therefore, the POS-535 produced by Mini-Circuit is selected in the design. The small-sized and highly reliable voltage-controlled oscillator.

The single-chip microcomputer in this design is the AT89C2051 chip of Atmel, which can be used to control ADF4106. AT89C2051 is an 8-bit single-chip microcomputer with 2 KB of programmable and erasable flash memory and 15 I/O programming ports. In addition, during the design, a 74L373 chip should be connected between the microcontroller and ADF4106 to buffer the stray interference in the microcontroller.
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3 System software implementation

The control bits of ADF4106 are 24 bits, of which the lower two bits are address bits, 00 represents "R counter", 01 represents "N counter", 10 represents "Function Latch", and 11 represents "Initialization Latch". Its crystal oscillator frequency is 10 MHz, and the output frequency is 410 MHz. Figure 4 is a timing diagram of the control relationship between the system CLK, DATA and LE. The configuration program of its single-chip microcomputer AT89C2051 is as follows:




4 Simulation test

The main technical indicators of the system are: output frequency 410 MHz, phase margin is 45°, amplitude margin is -10 dB, tuning sensitivity is 20 MHz/V, and frequency stability is ±1×10-6. The system is simulated using ADIsimPLL software from ADI, and the simulation results are shown in Figures 5 and 6, which fully meet the design requirements.



5 Conclusion

This paper introduces the design method of using the single-chip microcomputer AT89C2051 to control the frequency synthesizer chip AD4106 to realize the frequency synthesizer. Its circuit structure is simple, the performance is reliable, and a series of high-performance frequency signals can be obtained by changing the system software programming.