| 摘要 |
An RF transmitter employs negative feedback to reduce non-linearities in power amplifying means 18 to 21 and, since the transmitter received as input I and Q components, a Cartesian Loop is provided, by means of which a signal derived from the output of the amplifier by a probe 22 is split into de-modulated and Q components in order to provide feedback signals at 27 and 28. Unfortunately the negative feedback cannot correct for errors in the feedback loop itself, and component imperfections introduce a DC offset into the feedback loop. The modulation is suppressed carrier but the DC offset re-introduces the carrier. The invention provides a method of reducing the DC offset to reduce the carrier breakthrough by injecting known DC offsets into the I and Q inputs and measuring the resulting ripple in the output of the power amplifier means corresponding to the re-introduced carrier, enabling calibration offset DC levels to be calculated and injected into inputs 27, 28. |
