Precautions When Measuring IMD Products

Background:

1. Whenever measuring distortion products, remember that the instrumentation contains analog components.
2. This analog elements are subject to produce the exact same symptoms that we are trying to measure (harmonics and IMD products) when they are driven into their nonlinear regions of operation.
   
3. The IMD generated by the instruments will add to the measured response from our device and give us an incorrect reading.

A simple step to check whether the instrument is being overdriven are:

1. Manually increase the input attenuation and observe the levels of the intermodulation products.
   
2. If the intermodulation products do not change, then the instrument was not being overdriven and we can restore the attenuation to the previous setting.
   
3. If the instrument was generating intermodulation products before we raised the attenuation, then we would observe that the intermodulation products are reduced after we change the attenuation.
   
4. Measurements should be taken with the attenuation set so that the intermodulation products do not change when we increase it.