This version is outdated by a newer approved version.
This version (10 Jun 2019 03:08) is a draft.
Approvals: 0/1
This version (10 Jun 2019 03:08) is a draft.Approvals: 0/1
This is an old revision of the document!
Table of Contents
ADALM-PLUTO Phase Noise and Frequency Accuracy
In any transmitter (or receiver) design, frequency stability is of critical importance. Many are interested in both long-term and short-term stability. Long-term frequency stability is concerned with how the output signal varies over a long period of time (minutes, hours, days, or months). It is usually specified as the ratio, Δf/f for a given period of time, expressed as a percentage or in dB. These changes can be the results of thermal, aging, or voltage variations.
Short-term stability, on the other hand, is concerned with variations that occur over a period of seconds or less. These variations can be random or periodic, and are normally referred to a phase noise, measured in dBc/Hz.
Frequency Stability
Phase Noise
The phase noise spectrum of an oscillator shows the noise power in a 1 Hz bandwidth as a function of frequency. Phase noise is defined as the ratio of the noise in a 1 Hz bandwidth at a specified frequency offset, fm, to the oscillator signal amplitude at frequency fo. It is customary to characterize an oscillator in terms of its single-sideband phase noise, where the phase noise in dBc/Hz is plotted as a function of frequency offset, fm, with the frequency axis on a log scale.
Note the curve is approximated by a number of regions, each having a slope of 1/fX, where x = 0 corresponds to the “white” phase noise region (slope = 0dB/decade), and x = 1, corresponds to the “flicker” phase noise region (slope = –20 dB/decade). There are also regions where x = 2, 3, 4, and these regions occur progressively closer to the carrier frequency.
In modern communications algorithms, the phase noise is much more important than frequency stability, as carrier offset and timing correction algorithms will eliminate most frequency accuracy/stability issues. This is why most people look at phase noise from 10kHz to 10 MHz. In the below tests, we can see things from 100 Hz to 100 MHz, which covers more than what most people are interested in.
However, at lower frequencies (100 mHz, or over 10 seconds) to 100 kHz, we can see that the noise, or in this case drift, can vary quite a bit. This sub 100 kHz offset/noise/drift can normally by resolved by signal processing algorithms where carrier offset correction algorithms have specifically been designed to reduce this drift.
More information
There are a few great tutorials and writeups includeing:
university/tools/pluto/users/phase_noise.1560128914.txt.gz · Last modified: 10 Jun 2019 03:08 by Robin Getz