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Calibration is often required for precision measurement instruments like synthetic aperture radar (SAR) such that an image pixel intensity is directly expressed in terms of the surface backscatter coefficient. An active reflector named as transponder is an effective SAR calibration device which yields a larger radar cross-section than a corner reflector. Likewise, encoding the retransmitted signal can decouple the transponder from the background scatterers, hence, it can be used to calibrate SAR for radiometry, polarimetry and interferometry. However, as this transponder acts as an amplitude modulator, any signal errors in the transponder will result in degraded performance of radiation calibration. Literature search reveals that little work has been reported. As such, various kinds of signal errors and their effects on radiometric calibration are investigated. The system configuration and corresponding signal processing algorithms are detailed. Starting with an investigation of different errors, closed analytic expressions for calibration performance are derived. Furthermore, the most essential element of this transponder, one X-band wideband voltage-controlled attenuator, is developed. Experimental results show that, because of its low cost and wideband modulation technique, this transponder has extensive applications, not just for radiometric calibration.