Noise

 

 

 

 

Phase noise

 

The difference between absolute and residiual phase noise

Absolute phase noise is the phase noise measured on the output signal of a one port device. Generally, when phase noise plots or tables are shown on the data sheet of an oscillator or signal generator, it is the absolute phase noise measurement data.
Residual phase noise is a measure of the noise added to an input signal by a 2-port device. Some signal generators specify the residual phase noise in their data sheets. Think of the signal generator as a two port device (in this case, a frequency multiplier) where the “input port” is the external frequency reference (generally 10 MHz) and the “output port” is the RF output. 
Having residual phase noise data enables users to calculate the absolute noise of their signal generator at the RF output when using an external 10 MHz signal as the reference input.  Remember that the noise of the external frequency reference will be multiplied up to the output frequency and increase by 20 log(N) where N is the ratio Fout/ Fref. This noise will appear at offsets less than the phase lock loop bandwidth of the narrowest reference loop in the signal generator and should be added to the residual noise at these offsets to determine the absolute phase noise of the signal.  For example, in the case of the E8257D PSG analog signal generator, the loop bandwidth in the 100 MHz reference loop can be set from 25 Hz to 650 Hz.
Because the phase noise of the reference oscillator is multiplied up, users should exercise care when using an external reference. Sharing a common 10 MHz reference signal among several instruments is a good way to ensure that they won’t drift in frequency relative to each other. However, depending on the quality of that reference, it may also degrade phase noise performance.  

What is the difference between the PLL normalized phase noise floor and a normalized 1/f noise number

The normalized phase noise floor (PNSYNTH) (sometimes called the PLL Figure of Merit or FOM) is calculated as before:

PNSYNTH = PNmeas - 10log(PFD) - 20log(N)

The difference is that we now widen out the loop bandwidth to 500kHz or greater and measure the phase noise (PNmeas) at an offset of 100kHz

or more to accurately capture the PLL noise floor. This has meant that our PNSYNTH numbers that we specify have improved in many cases.

This is reflected in the PLL selection tables and is in the process of being updated in all the older data sheets.

the new 1/f spec (PN1_f) is calculated as

PN1_f = PNmeas - 10log(10kHz / foffset) - 20log(fRF / 1GHz)

where you input the measured phase noise (PNmeas) at an offset foffset. This formula just assumes a 10dB/decade 1/f noise slope and is normalized

to 1GHz as the 1/f noise scales with frequency.

For low PLL loop bandwidths (< 20kHz) the in-band phase noise will be entirely dominated by the 1/f noise. For newer PLL devices we are actively improving our 1/f performance through design - an example of this is the improvement in 1/f noise seen going from the ADF4350 to the ADF4351

(PN1_f goes from -111dBc/Hz to -116dBc/Hz). For wide loop bandwidths > 50kHz, the normalized noise floor becomes important.

ADIsimPLL V3.3 and higher now models both the noise floor and 1/f noise so you can accurately simulate PLL phase noise vs. PLL loop bandwidth.