High-Gain Amplifier Measurements
When measuring High-Gain Amplifiers, errors in measuring any of the S-parameters during calibration can result in error in the S21 measurement. This is because all the S-parameters are used in the error correction math.
A particular problem occurs with high gain amplifiers because the source power is set very low. Thus, when making reverse measurements (S22, S12) the signal-to-noise is poor and the raw measurements can be dominated by noise. This noise in the raw measurements will result in a noisy trace appearing for corrected S21 or S11.
If you are using a large attenuator on port 2 (which improves output match), perform an Enhanced Response Calibration as follows. This corrects for the same errors as the full 2-port correction EXCEPT the interaction between the raw load match and the DUT output match.
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There is NO need to Uncouple the port powers.
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Set port powers to an acceptable level. Do NOT overpower the test port.
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Perform Enhanced Response Cal. Learn how. (Does not measure or correct for S12 or S22 port match).
If you want to do a full correction (for example, when your amplifier output match is poor so the Enhanced Response Cal above is not adequate), then...
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Uncouple the port powers. Learn how.
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Set input (port 1) power to approximately the output power of the amplifier up to 0 dBm
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Set reverse (port 2) power to the same power (for measuring isolation and S22)
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Perform a Full 2-port Cal.
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Re-set the input power (port 1) to a lower power level appropriate for driving the amplifier.
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Additional Error due to Mismatch of DUT Output Match and Raw Load Match |
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Last Modified:
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23-Sep-2009 |
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