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Pathology: Parameter Drift or Jump
Synonyms: frequency
drift, non-stationarity
Found by: Statistic Tracking, Voltage Statistic Tracking, RjDj Tracking
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Contents:
Summary Diagnosis and
Suggested Solution Path
 HAL has observed that a statistical waveform parameter, such as Mean or Peak-to-Peak (the exact parameter is noted in HAL's description of the problem), has either been drifting over some number of acquisitions, or has had a sudden significant change in value.
Parameter drift can be associated with a changing operating temperature of either the circuit or the test equipment. Changes in period, frequency and amplitude have been observed over time to directly track the operating temperature of some oscilloscopes. For example, a scope placed under the output of an air-conditioning vent. You may wish to ensure that the temperature of your measurement environment is sufficiently stable.
A sudden change in value may be expected due to an intentional change in signal conditions (e.g. a change in input voltage). If this is the case, you can usually safely ignore this alert, and may wish to mute the Agent if you will be making more intentional changes to the signal.
The other major cause of a jump in value is non-stationarity. This can have any number of causes, and it is beyond the scope of this entry to attempt to enumerate them all.
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Description
The Parameter Drift/Jump Agents detect when a statistical parameter, such as Mean or Peak-to-Peak, changes significantly. This change can occur either slowly (i.e. drifting) or all at once (i.e. jumping). These Agents will not create any responses until at least 10 acquisitions have been taken, in order to provide the algorithm with a baseline of data from which to look for an anomaly.
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Presentation
The parameter will be seen to either change slowly vs. time, or vs. acquisition for drift. A sudden displacement of the parameter will occur for a jump. Note that this is for parameters that are "directly measured" on the waveform. For parameters that are accumulated, or computed across many events (typically statistical measurements) that might span a long time interval, the presentation can be quite small or displaced in time. Further, it is possible for the dynamics of the way the waveform is changing to interact with the dynamics in the way the waveform is being sampled, measured or displayed to exaggerate the degree of drift or jumping.
When a Tracking Agent detects either drifting or a jump in a parameter being tracked, a new View will be opened in which the value of the parameter will be plotted versus acquisition to provide the user with a visual indication of the behavior. In the case of a drift, the linear best-fit line will also be plotted.
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Causes
Drift:
- The most common
cause of a parameter drift is a temperature drift in the signal's
environment
- Changes in the noise being carried in the measurement power environment can cause some measurements like jitter to increase or decrease. Many labs are "hot" from 10am to 2pm and have a level of noise before and after that time that result in observably quieter measurements.
Jump:
- This Agent can be
triggered if you have intentionally changed signal parameters since the
last acquisition (e.g. if you are sweeping Vdd), but did not clear Agent
history by clicking on 'Clear History' on the HAL tab. Using 'Clear History' will restart the
10 acquisition learning period before drifts or jumps will be noted.
- The most common
cause of an actual sudden change in a parameter is non-stationarity in the
signal. Since non-stationarity is
by its nature transitory, another Jump will quite likely be detected on
the next acquisition, as the signal returns to its normal state.
- Even very clean signal sources can exhibit periods of non-stationarity. They can be brief and infrequent, or consistent.
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Impact
In devices that regenerate a clock (e.g. PLL's, DLL's, etc), if
a drift is sufficiently slow, the device will generally be able to track along
with the change, and there may be no noticeable impact.
In parallel regenerative systems (e.g. a PLL type device in at least one branch of a clock-powering tree), the direction of drift along one leg of the tree can be retrograde to a clock emerging from a parallel branch. When those clocks create synchronization with parts of the circuit that communicate (such as the up and downstream state devices of a logic segment), a synchronization failure is possible.
Non-stationarity
is a particular problem when attempting to make RjDj measurements, since the
math behind RjDj separation does not allow for the existence of
non-stationarity. The non-stationarity will often manifest as "BER blooming".
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Related Pathologies
Other anomalies with similar presentations.
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Non-Stationarity
- A Jump in a parameter may be associated with non-stationarity, an abrupt (and
often short-lived) change in some fundamental characteristic of the
signal. Non-stationarity may also create
effects such as a change in the number or ordering of peaks in a histogram
display. Such a change will be flagged
by the Non-Stationarity Agent, and therefore may point to the same underlying
issue as a Jump condition.
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Solutions
Maintaining your measurement environment to within a small temperature swing is important to making accurate and repeatable measurements. You should avoid placing the scope/DUT in the direct path of heating/cooling vents or in direct sunlight, as such areas will usually exhibit a much larger temperature swing than other areas.
When making measurements across very long timescales, it is also possible for the measurement power environment to change through the course of a day. ASA has been called out on some consults to help reduce sudden increases in jitter that ultimately were caused by: a vacuum cleaner in the room next to the lab in Texas, a freight elevator motor on the other side of the wall from the power distribution point for a lab in Oregon, cell phone induced failures in Oregon, and test equipment adjacent to the DUT and it's own test gear being turned on or off or changed on numerous occasions.
Non-stationarity has a large number of possible causes, both within the measurement environment and within the DUT itself. We will not attempt to list all of the possibilities here.
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Related Topics
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Sample Files
There are no files available demonstrating Parameter Drift Or Jump. If you would like to contribute some sample data, please click "contribute" below.
If you don't have M1, you can download
a trial copy here .
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