A Compact , Agile , Low - Phase - Noise Frequency Source with Am , Fm and Pulse Modulation Capabilities

This paper presents a broadband frequency source featuring high technical characteristics and extended functionality. The source is a compact (5 by 7 by 1 inches) module that covers the 0.1-to-10 GHz frequency range with a 0.001 Hz step size. The module is based on a novel (patent pending) PLL design that combines fast switching speed, low phase noise, and low spurious characteristics. The measured phase noise at an output frequency of 10 GHz and 10 kHz offset is –122 dBc/Hz. For an output frequency of 0.1 GHz and 10 kHz offset the phase noise drops down to –150 dBc/Hz. The switching time is about 20 uSec to be within 1 MHz from the final frequency and less than 100 uSec to be within 50 kHz respectively. Spurs do not exceed the –70 dBc level. The output power is leveled between –25 and +15 dBm. The module can be controlled through either SPI or USB interface and also includes AM, FM and pulse modulation capabilities. The developed module can be used as a broadband, agile signal source in a variety of test-and-measurement, communication, and monitoring systems. INTRODUCTION The frequency synthesizer is a key element of virtually any RF/microwave system. It generates a stimulus signal or is used as a local oscillator in a variety of upand downconversion schemes. The industry feels persistent pressure to deliver higher-performance, higher-functionality, smaller-size, and lower-cost synthesizer designs. Aside from frequency coverage and resolution, phase noise and spurious content are critical parameters that impose the ultimate limit in the system’s ability to resolve signals of small amplitude [1]. Another key parameter of the synthesizer that impacts overall system performance is the frequency switching speed [2], [3]. The time spent by the synthesizer transitioning between frequencies becomes increasingly valuable since it cannot be used for data processing. While many systems still work adequately with millisecond switching speeds, newer requirements demand microsecond operation together with comparable spectral purity of the lower-speed designs. Another challenge is to increase the synthesizer functionality by implementing various functions such as output power control, output power mute, frequency and power sweep, and list mode. Many applications also require various modulation options such as amplitude, frequency, and pulse modulation. Inside a synthesizer, there are always many devices that can carry these functions and be reused to increase the functionality without a significant cost penalty.