Lambda SR1, Operation Manual, Page: 161 / 317

SR1 Operation 161
© 2014 Stanford Research Systems
2.4.6 IMD Analyzer
The IMD Analyzer works in concert with the IMD generator to make three classic audio intermodulation
distortion measurements. The SMPTE (Society of Motion Picture and Television Engineers standard RP-
120-1994) standard (also similar to the German DIN standard 45403) uses a signal composed of a low
frequency sine wave and a higher frequency sinewave at 1/4 (or equal) amplitude. The measured
distortion products are the sidebands of the high frequency sine at multiples of the low frequency.
Difference frequency distortion (DFD), the subject of an old standard by the CCIF, the predecessor of the
ITU-R, consists of applying two equal amplitude high-frequency sines separated by a small frequency
difference to the device under test and measuring the nonlinear distortion products at the difference
frequency and higher-order combinations of the two frequencies. The modern standard for DFD is IEC
60268-3. Finally, DIM (Dynamic Transient Intermodulation distortion) also known as TIM (Transient
Intermodulation Distortion) uses a square wave near 3 kHz and a sine wave near 15 kHz and examines
the distortion products at various combinations of the two frequencies.
All of these measurements have a long history and many pages have been written for and against each
of them. SR1 can perform all these measurements by first generating the appropriate stimulus using the
generator IMD waveform and then selecting the IMD analyzer to perform the measurement.
Summary of IMD Analyzer Outputs
Measurement Description
Ratio (Selected Ch) The relative IMD distortion for the selected measurement type and distortion
products. Displayed in either percent or dB.
Level A, B Peak-based level computation for both input channels. Because the IMD
Analyzer is internally based on FFT analysis, levels displayed are peak levels.
Frequency is calculated for analog, but not digital audio, inputs. Phase is not
calculated with this analyzer.