Generating desired frequencies is a critical stage in nearly all RF and wireless devices, be it for communication, broadcast, or sensing. RF oscillators are the key to generating RF frequencies within a device, circuit, or system. However, RF oscillators benefit from additional circuits to ensure that the frequency content that is output remains stable and is within acceptable ranges. There are relatively simple feedback circuits that can achieve this and aid in a single stable frequency output from a given type of oscillator. However, there is often the need to generate a wider range of stable frequencies than a single tone. This is the task of RF Frequency Synthesizers, which are circuits with a preprogrammed set of output frequencies that can be used to adjust the output of an RF oscillator to deliver the desired tones while minimizing phase noise at the output.
A variation of the RF frequency synthesizer is the USB Controlled Frequency Synthesizers, which uses a standard USB computer interface (ideally VISA compliant), so that the frequency synthesizer circuit can be readily controlled by a computer. This is an extremely useful feature in automated testing where there may be a need to have a computer program/algorithm control the testing parameters. USB adds the benefit of plug-and-play connectivity for a variety of different computer platforms and operating systems, while allowing for a variety of other test equipment to be connected to the same control computer. Some USB frequency synthesizers may also accept SCPI compliant VISA commands, which enable control by a variety of test equipment platforms and automation software such as LabVIEW®. In this way, a USB controlled frequency synthesizer can be readily integrated into a larger automated test system or added as a convenience without substantial setup.
In general, USB controlled frequency synthesizers are phase-locked loop (PLL) synthesizers. A PLL synthesizer is able to generate a range of output frequencies that are a multiple of a single reference frequency. Hence, PLL synthesizers with one quality oscillator signal source, a local oscillator (LO), are able to generate a controlled range of frequencies much higher than the original oscillating frequency. The basic function of a PLL synthesizer is that the output voltage-controlled oscillator (VCO) is driven by a feedback loop controlled by the filtered and integrated output pulses of a phase-frequency detector (PFD) that compares the feedback frequency with a divided reference frequency output. The frequency of a PLL synthesizer is scaled using counters that reduce or increase the reference frequency input into the PFD.