SBAA526 February 2022 AFE7950
TX modulators take complex I/Q data input and mix to the desired RF frequency. Similarly, RX demodulators mix down to quadrature baseband. In both cases imbalances within the I/Q baseband paths manifest to system level degradation. DC offset imbalance results in carrier leakage on the transmitter output or a DC component on the RX output. Amplitude and phase imbalance between the I and Q paths results in degraded sideband (i.e. image) suppression performance. Most of the imbalance comes from the modulator itself and tolerance variations in the low pass filters in the I/Q paths.
These impairments are not anything new. Typically, there are adjustment capabilities in the TX DAC or RX demodulator to minutely adjust DC offset, amplitude, and phase between the I and Q paths to improve the suppression of the unwanted components. These adjustments are valid at one temperature and at one frequency. Some kind of feedback loop is needed to monitor the performance and adjust parameters as frequency or environmental factors change. For the direct conversion K-band modulators/demodulators, the inherent balance performance is worse than in counterpart devices operating below 6 GHz. When the inherent performance starts out worse, it becomes harder to maintain optimum tuning to achieve best suppression.
Image suppression in particular is very problematic because the image is superimposed on the desired signal itself. There are no means to filter it. Even though it cannot be seen with typical RF equipment like a spectrum analyzer, it distorts the integrity of the signal. An Error Vector Magnitude (EVM%) measurement that demodulates the information of the signal reveals the degradation that the image introduces. The image performance may be responsible for up to 60% of the overall degradation that includes other impairments related to carrier feedthrough, intermodulation distortion, and thermal and phase noise. Similarly, the receiver has the same issues with the addition of a second order intermodulation distortion (IM2). Unlike third order intermodulation (IM3), the IM2 distortion comes from any wide bandwidth signal or jammer mixing with itself and falling at baseband where the desired signal resides. The article in the reference section provides more details on this phenomenon.
Figure 2-1 shows the expected impairments of the direct conversion architecture.