Objective
To understand- Characterization of IQ modulator: the Carrier Leakage and Sideband Suppression
- The 1dB-Compression Point of a Transmitter.
- The output 3rd Order Intercept Point (OIP3) of a transmitter.
- Modulation Analysis: Channel Power, ACPR,EVM
Characterization of IQ modulator: the Carrier Leakage and Sideband Suppression
A common approach to characterize the I-Q modulator performance is to apply two signals V sin(wt) and Vcos(wt) to the I and Q input terminals and examine the spectrum produced at the RF output. In the ideal case, the output in the band of interest is simply given by;
- In practice, there are imbalances in the device (For example, gain and phase imbalance in the mixers and phase shifter network).
- Carrier leakage in a function of DC offset.
- It can be shown that the sideband suppression is a function of G & Φ.
1dB Compression Point
- The point at which the output power differs from the ideal transfer function by 1dB as the input power increases.
Dynamic Range
- The input power range over which the receiver provides a useful output. The low power limit is the sensitivity specification and upper limit is the input power at 1dB compression point.
Dynamic Range = Input 1dB Compression Point - Sensitivity Level
Output 3rd Order Intercept Point (OIP3)
IP3 can be calculated without extrapolation using above formula.
Modulation Analysis: Channel Power, ACPR, EVM, CCDF
Channel Power: Channel power is the average power in the frequency bandwidth of the signal of interest. The measurement is generally defined as power integrated over the frequency band of interest.
ACPR: The adjacent Channel Power Ratio (ACPR) is usually defined as the ratio of the average power in the adjacent frequency channel to the average power in the transmitted channel.
EVM: The error vector is the vector difference at a given time between the ideal reference signal and the measured signal. The error vector is a complex quantity that contains a magnitude and a phase component. Error Vector Magnitude (EVM) is the root-mean-square (RMS) value of the error vector over time at the instants of the symbol clock transitions.
CCDF: The CCDF curve shows the probability that the power is equal to or above a certain peak-to-average ratio. Figure A shows the power versus time plot. This plot represents the instantaneous envelope power of the waveform. Figure B displays the CCDF curve of the signal. Here the x-axis is scaled to dB above the average signal power, which means we are actually measuring the peak-to-average ratios as opposed to absolute power levels. The y-axis is the percent of time the signal spends at or above the power level specified by the x-axis. For example, at t=1% on the y-axis, the corresponding peak-to-average ratio is 7.5dB onthe x-axis.
To be continued....
