The frequency synthesizer is a critical component in both transmitting and receiving systems. It utilizes phase-negative feedback frequency control technology, offering excellent narrowband carrier tracking performance as well as bandwidth modulation tracking capabilities. It provides the local oscillator signal for up and down conversion, effectively suppressing phase noise and spurs. The frequency source implemented through phase-locked loop (PLL) frequency synthesis technology has been widely applied in fields such as radar, communication, and electronics.
This paper introduces the design of a GPS signal source as a reference, focusing on the typical application of Analog Devices' frequency synthesizer ADF4360-4 in GPS signal source development.
**1 Signal Source System Composition**
**1.1 System Design**
According to existing literature, the structure of GPS signals is well understood. The goal of designing a GPS signal source is to simulate the signals transmitted by satellites, which involves baseband modulation of GPS signals and generating a 1,575.42 MHz RF signal. In the overall system design, a superheterodyne up-conversion approach is adopted. The system is divided into digital and analog sections. The digital part follows the software-defined radio concept, using a GPS chip to perform baseband and intermediate frequency (IF) modulation, outputting an 8-bit digital IF signal. This signal is converted to analog using a D/A converter and sent to the analog section. The analog circuit is the core of the system, handling RF circuit design, including the frequency synthesizer, mixer, filters, and power control, to complete the up-conversion process.
**1.2 Module Design**
**(1) Digital Circuit:**
The digital circuit is responsible for the baseband/IF module design. It follows the software-defined radio idea, using a GPS chip to generate the navigation message (D code), C/A code, and a digital IF carrier. These are then baseband modulated and spread-spectrum modulated. The system includes a C/A code module, D code module, DDS module, and a modulation module. The C/A code is generated at 1.023 MHz with a 1 ms duration, while the D code is generated at 50 Hz. The DDS module generates a 12.5 MHz carrier, and the modulation module performs BPSK and spread spectrum modulation, producing a 12.5 MHz GPS digital IF signal.
**(2) Analog Circuit:**
The analog circuit handles the RF module design. It uses a frequency synthesizer, mixer, filter, and attenuator to convert the IF signal to RF. The signal is filtered and adjusted in power before being output as the final GPS RF signal.
**2 Frequency Synthesizer ADF4360-4**
**2.1 Working Principle and Performance**
The main function of a frequency synthesizer is to provide local oscillator signals for up-conversion. It typically consists of a digital phase detector (PD), loop filter (LF), voltage-controlled oscillator (VCO), and programmable counters. The PD compares the output of the R counter and N counter, generating an error voltage that is filtered and used to control the VCO, producing the desired output frequency.
The ADF4360-4 is a high-performance phase-locked frequency synthesizer from Analog Devices. It is a dual-mode pre-divided single-loop synthesizer that improves frequency resolution without changing the output frequency. Its key specifications include an output frequency range of 1450–1750 MHz, with a second harmonic output of 725–875 MHz. It operates at 3–3.6 V, with an output power range of -13 to -4 dBm. It features a programmable dual-mode prescaler (8/9, 16/17, 32/33), analog and digital lock detection, and an integrated VCO.
**2.2 Application Circuit Design**
In the analog RF module, the ADF4360-4 provides the local oscillator signal for the mixer. The application circuit includes an external temperature-compensated crystal oscillator connected to the REFin pin. The output pins RFoutA and RFoutB provide differential high-frequency signals, which are fed into the mixer via matching and resonant networks. Control pins CLK, DATA, and LE are connected to the FPGA for initialization, while other pins handle power supply, decoupling, and grounding.
**2.3 Initial Design**
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