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Understanding Phase Noise
Needs and Choices in
Signal Generation
Application Note
The phase noise performance of an RF or microwave signal generator
is often a key factor in determining how well it fits an application. For
example, phase noise performance is especially important in the testing
of high-performance systems such as Doppler radars and cognitive or
software-defined radios (SDRs). Excellent phase noise specifications
are also important when using a signal generator for oscillator
substitution or analog-to-digital converter (ADC) testing.
When evaluating an RF or microwave signal generator for use in such
applications, several performance factors are worth a close look:
spurious, harmonics, broadband noise, AM noise and phase noise.
Looking specifically at phase noise performance, it is affected by the
internal architecture of the instrument and the features and capabilities
layered on top of that architecture. The most common architectures
are single-loop and double-loop, and these will be explored later in this
note. Available features include digital modulation capabilities, pulse
capabilities and multi-unit synchronization--and the presence of these
can affect phase noise performance.
In both the design and evaluation of a signal generator, phase noise
performance includes tradeoffs such as cost, switching speed and
optimization at various frequency offsets from the carrier signal. To
address a variety of requirements, some signal generators offer two or
more levels of phase noise performance (e.g., standard and optional
capabilities). Others allow optimization of phase noise performance at
wide or narrow offsets. Still others may allow the user to selectively
degrade phase noise performance and observe the effects on the
device-under-test (DUT).
To provide context, this note first discusses the fundamentals of
phase noise before moving on to a closer look at architectural choices
and the effects of various functionality alternatives. The discussion
then moves on to an overview of Agilent signal generators and the
built-in capabilities they offer to enhance or selectively degrade
phase noise performance.
The foundation: stability and noise
Any discussion of phase noise is mostly concerned with the frequency stability
of a signal. Long-term stability, perhaps of an oscillator, may be characterized
in terms of hours, days, months, or even years. Short-term stability refers to
frequency changes that occur over a period of a few seconds or less. These
short-cycle variations have a much greater effect on systems that rely on
extreme processing to extract more information from a signal. For that reason,
this discussion will focus on short-term stability.
Short-term stability can be described in many ways but the most common is
single-sideband (SSB) phase noise. The US National Institute of Standards and
Technology (NIST) defines SSB phase noise as the ratio of two power quanti-
ties: the power density at a specific frequency offset from the carrier and the
total power of the carrier signal. This is most commonly measured in a 1-Hz
bandwidth at a frequency "f" away from the carrier and the units are dBc/Hz or
"decibels below carrier frequency power over a 1-Hz bandwidth."
The level of phase noise is deterministically related to the carrier frequency,
increasing by 6 dB for every doubling in frequency. When characterizing the
performance of components integrated into advanced radar and communication
systems, measurements of phase noise for a 1 GHz carrier may extend from
roughly -40 dBc/Hz at "close to the carrier" offsets (1 Hz or less) down to -150
dBc/Hz at "far from the carrier" offsets (10 kHz or more). These measurements
will be about 18 dB higher with a carrier frequency of 8 GHz. At such low levels,
the measurement noise floor is affected by two microscopic electronic effects:
thermal noise from passive devices, which is broad and flat (white noise);
and flicker noise from active devices, which has a 1/f shape (pink noise) that
emerges from the thermal noise at lower offsets. Both of these contributors are
unavoidable because they are present all along the signal chain: in the measur-
ing instrument, in the device that produces the signal-under-test (SUT), and even
in the cables that connect the two.
Another sometimes overlooked source of noise is any type of amplifier in the
signal chain. While the main purpose is to increase the power level of a weak
carrier signal, the amplifier adds its own noise and also boosts any input noise.
The net effect: amplifier, thermal and flicker noise combine to give any phase
dBc/Hz
noise plot a characteristic shape and, more significantly, reduce the theoretical
lower limit of any phase noise measurement (Figure 1).
flicker
noise
amplifier noise
thermal noise
Log frequency
Figure 1. The three main contributors to noise create a theoretical lower limit for phase noise measurements.
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These effects all show up in the phase noise characteristics of a high-
performance signal generator. For example, the underlying sources of noise
can be traced back to the major sections of the instrument block diagram
(Figure 2). For offsets below 1 kHz, the noise is dominated by the performance
of the reference oscillator, which is multiplied up to the carrier frequency. The
other major contributors are the synthesizer at offsets of 1 kHz to roughly
100 kHz, the yttrium-iron-garnet (YIG) oscillator from 100 kHz to 2 MHz, and
the output amplifier at offsets above 2 MHz. When these effects are well
understood, they can be minimized and optimized within a system design to
ensure maximum performance.
Agilent E8257D SSB Phase Noise at 10 GHz
Measured with Agilent E5500 Phase Noise Test System (no spurs)
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