| 摘要 |
<p>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 I 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).</p> |
