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A transfer measurement involves applying a stimulus signal to the input of a DUT and then measuring the response on the output. The transfer characteristic of the DUT determines how the output changes for different stimulus signals. For noise (or wander) transfer, the stimulus signal has specified characteristics: frequency and pk-pk amplitude. The pk-pk amplitude is then measured on the output of the DUT. The transfer characteristic for the specified frequency is given as gain (in dB) and is calculated from: gain (dB) = 20.log10 (Amplout / Amplin) |
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To be able to measure DUT performance to the level specified in the ITU-T standards, the test equipment must be able to ensure that it can accurately measure the difference in amplitude between the applied stimulus and the measured response. This requires that the amplitude of the generated stimulus is accurate and that the response from the DUT can be accurately measured relative to that input. This can be achieved in two ways:
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Calibration Changes in X.10.40
In Paragon-X software version prior to X.10.40, calibration was mandatory step prior to performing the transfer test. This results in very long test times because each frequency point is effectively tested twice: once for calibration and once with the DUT in place.
In X.10.40, calibration has been made an optional step. The absolute accuracy of Paragon-X generation and measurement is such that the G.8262 tests can be made accurately without calibration.
For the highest accuracy possible, calibration may still be performed. This may be useful in the case where there are marginal failures. In addition, calibration values are now stored. This allows multiple wander transfer tests to be performed with only one calibration run (compared to previous versions where calibration had to be performed each time).
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In Paragon-X software version prior to X.10.40, calibration was mandatory step prior to performing the transfer test. This results in very long test times because each frequency point is effectively tested twice: once for calibration and once with the DUT in place. In X.10.40, calibration has been made an optional step. The absolute accuracy of Paragon-X generation and measurement is such that the G.8262 tests can be made accurately without calibration. For the highest accuracy possible, calibration may still be performed. This may be useful in the case where there are marginal failures. In addition, calibration values are now stored. This allows multiple wander transfer tests to be performed with only one calibration run (compared to previous versions where calibration had to be performed each time). |
Testing using a Single Frequency
Using Paragon-X, a wander transfer measurement can be made using either a single frequency or a table of frequencies. For a single frequency:
The frequency and pk-pk amplitude are specified in the UI. Clicking Generate Wander starts the generation of this tone; the measurement then runs for 10 seconds. The pk-pk output amplitude is measured across the full 10 second capture. The gain is then calculated and the result presented on the UI.
The measurement method for single sine has not changed in X.10.40
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Using Paragon-X, a wander transfer measurement can be made using either a single frequency or a table of frequencies. For a single frequency: The frequency and pk-pk amplitude are specified in the UI. Clicking Generate Wander starts the generation of this tone; the measurement then runs for 10 seconds. The pk-pk output amplitude is measured across the full 10 second capture. The gain is then calculated and the result presented on the UI. The measurement method for single sine has not changed in X.10.,40 |
Testing using a Table of Frequencies
Using Paragon-X, a wander transfer measurement can be made using either a single frequency or a table of frequencies. Details about table operation is given below.
The main changes in X.10.40 are:
New Enhanced Defaults for Option 1 clocks with optimised dwell times
New Lower Limit, with both Upper Limit and Lower Limit checking now optional
A Recovery Time has been added to each step
Enhanced Defaults
A new set of defaults has been added to provide a quick way to get extended frequency coverage and optimised test time. These defaults are recommended for testing Option 1 clocks to G.8262.
Limits
G.8262 does not completely specify the filter to be implemented. In order to establish performance limits, the following assumptions have been made:
The minimum gain in the pass-band is -3dB
The filter is first-order with 20dB/decade roll-off
These assumptions then allow us to set lower and upper limits as shown in the diagram below:
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These limits are made optional so as to preserve backwards compatibility with previous software versions - the Lower Limit is disabled by default.
Recovery Time
Some DUTs will have a transient response caused by the frequency and amplitude changes between steps in the table. This response will result in incorrect pk-pk amplitude measurements and so should be ignored when making measurements.
In X.10.40, recovery times have been added as shown in the table below:
Frequency (Hz) | Recovery Time (s) |
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>35Hz | 0s |
15Hz to 35Hz | 5s |
4Hz to 15Hz | 20s |
<4Hz | 50s |
For example, for any frequency less than 4Hz, the first 50 seconds of the measurement will be ignored and the pk-pk amplitude will be calculated over the remaining capture period (as defined by the Dwell Time)
Since each step has a recovery time, this also changes the minimum allowed Dwell Time for a step.
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Using Paragon-X, a wander transfer measurement can be made using either a single frequency or a table of frequencies. Details about table operation is given below. The main changes in X.10.40 are:
Enhanced DefaultsA new set of defaults has been added to provide a quick way to get extended frequency coverage and optimised test time. These defaults are recommended for testing Option 1 clocks to G.8262. LimitsG.8262 does not completely specify the filter to be implemented. In order to establish performance limits, the following assumptions have been made:
These assumptions then allow us to set lower and upper limits as shown in the diagram below: |
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These limits are made optional so as to preserve backwards compatibility with previous software versions - the Lower Limit is disabled by default. Recovery TimeSome DUTs will have a transient response caused by the frequency and amplitude changes between steps in the table. This response will result in incorrect pk-pk amplitude measurements and so should be ignored when making measurements. In X.10.40, recovery times have been added as shown in the table below:
For example, for any frequency less than 4Hz, the first 50 seconds of the measurement will be ignored and the pk-pk amplitude will be calculated over the remaining capture period (as defined by the Dwell Time) Since each step has a recovery time, this also changes the minimum allowed Dwell Time for a step. |
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