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InterModulation Distortion (IMD)

An important consideration for any broadband microwave converter is its ability to accurately re-create signals at the converted frequency. Amplifiers and mixers driven at high power levels can create unwanted in-band 'intermodulation' signals. Thus, the upper end of the Spurious Free Dynamic Range (SFDR) is often limited by unwanted IMD signal products, while the lower end of the SFDR is typically established by leakage spurious from Local Oscillator (LO) synthesizers.

As power levels increase, amplifiers and mixers begin to reach voltage rails where they can no longer provide distortion free signals. This finite output power level can be graphed relative to the input power so that the compression effect can be seen. If we put two signal tones into an amplifier, when the signal amplitude grows, it will ultimately hit a voltage or current limitation that will clip off a portion of the output signal. The Fourier transform of such a clipped signal into the frequency domain yields harmonic products of the two signals at twice the frequency of the fundamental input tones (A & B). These harmonic tones mix with the original tones to create third order intermodulation products very close in frequency to the original two tones.

IMD

The amplitude level of the intermodulation products can be predicted by a theoretical point called the Third Order Intecept (TOI) point or as it is sometimes called IP3. The higher the TOI/IP3, the lower the IMD products for a given power level.

Broadband converters are particularly susceptable to IMD products since they virtually always fall in-band. To prevent IMD products TOI/IP3 must be increased by selecting amplifiers with higher voltage and current capability or mixers that have higher diode barrier voltage with stronger LO drive levels.

Graph

To acheive 70 dB of IMD free performance at output power levels of 0 dBm, enough to drive most high power amplifiers, we use amplifiers with our high power option having TOI/IP3 greater than 40 dBm. Another benefit of using these more capable components is that the in band harmonic level drops reducing or eliminating another source of undesirable signals that can be created in the frequency converter.