When choosing a PLL frequency synthesizer, there are several key parameters to consider. Frequency range specifies the device RF output frequency range where the output is derived directly from the VCO or the divided output. Phase noise is a measure of short term frequency deviation from the ideal frequency due to random phase fluctuation. It is defined as the ratio of power measured in a 1Hz bandwidth to the total carrier power and expressed as dBc per Hz. Phase noise is specified at frequencies offset from the carrier. Common offset frequencies are 10kHz, 100kHz, 1MHz and 5MHz. It is common to measure single sideband phase noise. Fractional and Reference spurs are artifacts observed at the device RF output at an offset from the carrier frequency. Spurs are measured relative to the carrier. In an RF receiver down converter the spurious tones and phase noise will fold into the band of interest resulting in lower receiver sensitivity. An example will be shown later.