Anritsu Series MS278XB - Manual (Page 185)

Measurements RF Measurements
MS278XB OM 5-29
Third Order Intercept Measurement
The Third Order Intercept (TOI) measurement computes and displays the output intercept point and places
markers on the trace to indicate the measured signals and their third-order products.
For odd orders, the measurement is done by:
1. Applying 2 tones with a small separation (usually < 1 MHz) and identical amplitudes to the DUT. Note
that the signal generator must generate very low levels of interfering signals at the expected location of
the distortion components.
2. Measuring the amplitude difference between the applied tones and the distortion components. The
distortion components will be at known frequencies, based on the separation of the 2 tones and the order
of the distortion.
3. Computing the intercept point. This is the theoretical level where the distortion component would be the
same level as the input signal. Since the distortion changes by a known amount with signal level, based
on the order, this is a simple computation.
Figure 5-9. Third Order Intercept Measurement