Extending useable frequency span of 1:1 wideband transformers used for distortion measurements

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With the continued migration of signal path components to differential stages, it is sometime necessary to convert that differential signal to single ended for distortion measurement purposes. Also, when a four-port network analyzer is not available, using 1:1 transformers is a typical way to map a differential I/O to single ended for more typically available network/spectrum analyzers. 

While the transformer response can be calibrated out for response shape measurements, it is more useful to have the differential to single ended path giving a flat gain response for distortion measurements. In the course of testing a 4GHz Fully Differential Amplifier (FDA) for OIP3, no flux-coupled transformer could be found that held 0.5dB flatness above 200Mhz. A simple technique of shifting the flat region up will be shown with resulting OIP3 data through 300Mhz for the ISL55210 wideband FDA.

Typical 1:1 transformers used in device characterizations and their bandwidth limits.
While various 1:1 wideband transformers are available, holding better than 0.5dB flatness above 200Mhz seems particularly challenging for those devices that use the winding structure of figure 1.  For the best OIP3 measurements, the test signals passing through the transformer should also be well separated from its rolloff regions to maintain adequate linearity in the transformer itself.


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Fig. 1 is the classic “balun” configuration where a balanced input signal on the left (in this case from the output of a Fully Differential Amplifier- FDA) is going to an unbalanced output – typically the single ended input of 50Ω measurement instrument. All transformers are specified from a particular source and load configuration (those would be equal in the 1:1 turns ratio case) with some midband insertion loss. Typically they report both the high and low -3dB  frequencies for these bandpass devices using a selected source impedance where the load is assumed to be matched to that. Table 1 summarizes a small selection from the 1:1 balun universe with the relevant specifications pulled from the vendor data sheets.


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