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Characterizing phase-locked-loop
signal transition behaviors of
Microphonic/Phase-hits
Application Note
Digital communication systems often have unique carrier synchronization and syn-
Introduction thesizer requirements such as rapid carrier acquisition/lock, and stable operation in
demanding environments. The performance of these requirements is affected by not
only oscillator characteristics themselves but also the design of phase-locked loop cir-
cuits interacting with unwanted stimulus such as microphonics. This paper discusses
how Agilent's Signal Source Analyzer helps you to identify unwanted phase-locked loop
transition "phase-hits", and achieve easy, comprehensive and accurate phase-locked
loop characterization in both linear and nonlinear regions.
2
Characterizing PLL Linear Starting with test parameters in characterizing phase locked loop linear and non-linear
behaviors; there are three different stages of test usage, component evaluation, oscil-
and Nonlinear Behaviors lator/phase-locked-loop circuit design, and verification/test at operating conditions.
The PLL controls the phase of the output signal to be N times the phase of the refer-
ence as shown in figure 1. Similarly, the frequency of the output signal is then N times
the reference frequency. Like all linear feedback control systems, PLLs have dynamic
behavior characteristics. The linear behavior described by the transfer function
method is useful to analyze loop operation in the vicinity of lock. Non-linear effects
dominate the behavior of the PLL in unlocked conditions such as when the frequency
divider ratio is changed to switch frequencies over a large range.
Component Oscillator/PLL Circuit Verification/Test
Evaluation Design at Operating Conditions
` Reference Source ` Loop Filter (PLL Response) ` Microphonic
` Phase Noise ` Phase Noise
` RF Transient ` Phase-hits
` VCO
` Phase Noise ` Spurs
` AM Noise ` Harmonics
` Tuning Sensitivity
Figure 1. Phase locked loop test parameters and challenges
The building blocks common to most phase locked loops are the phase detector, the loop
filter, the voltage controlled oscillator (VCO), and the frequency divider. Each of these
building blocks has both linear and non-linear attributes to its operation.
Oscillator/PLL Circuit
Design Test Challenges
` Loop Filter (PLL Response) ` Estimate PLL response on phase noise quickly
Phase Noise ` Fast sampling rate with sufficient frequency/
RF Transient phase resolution are required for fast switching
` Spurs PLL synthesizer lockup time test
` Harmonics ` Long jump measurement (i.e. 500 MHz)
Figure 2. Test parameters and challenges in the circuit design stage
3
Characterizing PLL Linear PLL response in the PLL linear operating region can be characterized through phase
noise measurement. Once both VCO and reference phase noises are known, output
and Nonlinear Behaviors phase noise of the PLL can be estimated based on the designed loop filter.
(cont'd.)
An RF transient characteristic known as lockup time is usually tested with the frequency
and phase guard band. Modern digital communication systems often require fast fre-
quency switching synthesizer because the available time slot of the carrier acquisition
is getting smaller in order to communicate multiple channels in a very short time period.
Typical requirement of lock up time is about 10 ms down to a single micro second order
level. So we need faster sampling rate measurement while we need to maintain suffi-
cient frequency or phase resolution.
Also, the frequency synthesizer needs to cover a wider frequency bandwidth due to
increasing bandwidth of the communication system. For example, testing of a long jump
frequency synthesizer as wide as 500 MHz is needed.
Verification/Test
at Operating Conditions Test Challenges
Microphonic Difficult to capture an event that occurs randomly
Phase-hits and unpredictably.
Not sure to define test threshold (frequency
resolution, time period)
Figure 3. Test parameters and challenges in the verification/operating test stage
When the designed PLL is sensitive to unwanted external physical stimulus, the system
may become unstable because the PLL oscillation may be modulating internally. This
non-linear behavior is likely to occur at various operating conditions, such as tempera-
ture changes, vibration, and shock.
Microphonic and phase-hits are known major problems and stability of the PLL needs to
be tested at operating conditions to address sources of unwanted behaviors.
The test challenges here are how to capture such events with an instrument and how
to analyze the signal behavior to address sources of these types of modulation, because
it is hard to predict when a phase-hit event occurs. Another challenge is how to define
the test limit to capture such an event in terms of the frequency or phase variation along
with duration of the event.
4
PLL Linear Behavior D
Loop VCO
V
K Filter K f KV
Analysis
R F(s) OUT