FAQ #9: My Bode plot doesn’t look right, what am I doing wrong?

My Bode plot doesn’t look right, what am I doing wrong?

The most common error made when measuring Bode plots is that the signal is too large.  Reduce the signal to the minimum level (-27dBm for the Bode-100).  The plot may still not seem exactly right.  It is often necessary to further attenuate the source signal in order to maintain small signal operation.  The poor measurement is NOT due to the analyzer, but due to the small signal limits of the circuit being measured.  In order to assure that level is sufficiently low, increase the signal level 2dBm and check that the results have not changed.  If they have, insert an attenuator, such as the J2140A between the source and the injection transformer and repeat these steps until increasing the source signal 2dBm does not change the results.

The Bode 100 offers shaped level in order to provide larger signals where possible in order to minimize the noise in the measurements.  Other noise management techniques the Bode-100 offers include trace averaging and selectable resolution bandwidth and also selectable attenuators.  See the Bode 100 user manual for additional information regarding these settings.

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