Alexander Chenakin, Suresh Ojha, and Sadashiv Phadnis, Anritsu
Anritsu's new Rubidium™ signal generators meet today's demand for high-performance microwave signal sources between 20 and 43,5 GHz. Built on innovation and quality, Rubidium signal generators exceed performance expectations with atomic level frequency stability and very clean phase noise of –140 dBc/Hz with 10 kHz offset for a 10 GHz carrier.
Signal generators are a must; modern technologies could not exist without them. [1,2] In order to meet today's market requirements, Anritsu has introduced the Rubidium series, a new generation of microwave signal generators based on innovative technology  that combines wide frequency coverage, excellent resolution and high output power together with low phase noise and atomic level stability. The synthesizer core is based on a 2 to 20 GHz YIG oscillator anchored to a direct analog extracted and distributed internal reference as shown in Figure 1. The native frequency coverage of the YIG is extended by means of a frequency multiplier and a crossover (followed by a high power amplifier, amplitude control, and harmonic filtering) to achieve coverage from 9 kHz to 20 GHz or 43,5 GHz. The YIG output signal is downscaled using a direct analog converter that removes any crossover and therefore phase noise degradation in the PLL (phase lock loop). A switched frequency multiplier is inserted into the PLL which (a) decreases the number of frequencies generated by the direct analog distributor and (b) suppresses residual PLL noise at low frequencies.
As a result, the presented architecture basically offers a noiseless PLL mechanism, which means that it translates the synthesizer reference noise without adding phase noise degradation above the 20logN fundamental. A reference that combines three sources is used to provide the lowest possible phase noise for any frequency offset. Furthermore, this combined reference is controlled by a rubidium atomic clock which adds a much higher degree of stability compared to a conventional OCXO-based reference. The operation of the rubidium clock is based on fundamental constants and not on physical dimensions, making it extremely stable. Several reference frequencies are available for timing the instrument, including a high-frequency 1,6 GHz output for maximum fidelity.
The architecture of Rubidium signal generators provides high performance in terms of spectral purity and stability. Phase noise is always a key specification for signal generators. Rubidium signal generators offer four exceptional noise levels of -140 dBc/Hz at 10 GHz and 10 kHz offset with enhanced noise option as shown in Figure 2. Another important aspect is frequency stability. Conventional signal generators typically incorporate 10 MHz OCXO oscillators, whose performance is relatively stable. However, the oscillation frequency of an OCXO oscillator depends on the mechanical resonance of the crystal or, in other words, on the dimensions of the crystal. It is evident that, before a change in temperature, the dimensions of the crystal also change and this results in small variations in frequency. In addition, the crystal material itself evaporates over time and this affects the frequency. Hence introducing an atomic standard greatly improves the stability of the internal time base, not by several multiples but by several orders of magnitude.
Accuracy and stability of frequency and output levels are essential in a microwave signal generator. Hence, traditional signal generators require periodic calibration, which however represents a certain complexity. Whether the calibration lab is close by or across the country, shipping an instrument for calibration at a critical time is an expensive and time consuming option.
To reduce total cost of ownership, Rubidium signal generators incorporate a calibration feature that adjusts their internal time base and power output in situ. One of the main tasks when calibrating a signal source is to adjust the time base that ultimately defines the frequency accuracy of the instrument. Fortunately, Rubidium signal generators are supplied with a Rubidium time base considered a frequency standard in its own right. As a result, it is not necessary to calibrate the frequency in most cases in practice, which is very important in sensitive applications such as aerospace and defense.
Rubidium signal generators incorporate a global navigation system receiver that receives a signal extracted from a high-precision atomic source installed aboard a satellite orbiting the Earth. The output of the receiver is a stream of 1 pps pulses that can be used to adjust the instrument's internal time base to synchronize it with any frequency standard offered by global navigation systems. This adjustment is carried out simply by clicking the mouse, thus eliminating the need to send the signal generator for factory calibration. Another valuable feature of this signal generator is that it allows you to connect a USB power sensor (see Figure 3). The power sensor allows power to be measured directly in the plane of the device under test, thus taking into account any added loss from external cables or other devices. The precision power sensor also calibrates the output power of the instrument, within certain limits, depending on the needs and without sending it to the factory.
In short, the new Rubidium technology offers superior performance when compared to traditional instruments. With its excellent levels of spectral purity and signal stability, the Rubidium series is an ideal signal source for design and manufacturing test of components and systems in a wide variety of applications, including wireless communications, aerospace, defense, and consumer electronics.