QPSK: I/Q modulator impairments and their effect on the receiver.

This is a short post that follows on to the last post about QPSK modulation techniques using a I/Q modulator. The description was based on idealized operation of the modulator in an effort to explain the operation. The real world effects were not explored. The fact is that I/Q modulators have at least two impairments that affect the accuracy of the transmitted signal. In order to explain this, note that the phase shifts in QPSK are also realized through the gain of the I/Q channels. Thus if there is an imbalance in the gain of the two channels then an error in the transmitted constellation will occur. If either the I or Q channel is imbalanced with respect to the other, the constellation will change from a square shape to a rectangular shape. If there are also phase errors then the constellation will change to a parallelogram and so on. These are the two most important impairments in the I/Q modulation scheme. Another impairment in the overall transmitter is the non-linearity of the final power amplifier. This will affect the constellation too. The white additive noise in the transmitter and the receiver will cause the received constellation to be spread out in the phase/gain space. Under worst case conditions the constellation may cease to exist and extreme loss of bit error rate will result. However, as far as impairments in the I/Q modulator are concerned, it may be a relief to know that modern day I/Q modulators have very low gain and phase errors. For example a well known device has a gain error of 0.12 dB and a phase error of 0.05 degree. In addition trimming of these errors can also be achieved using digital to analog converters and non volatile memory storage. The errors are trimmed out in the factory before final delivery.

We design and deliver analog and RF/wireless ASICs and modules using state of the art semiconductor, PCB and assembly technologies. Please contact us at spg@signalpro.biz for a quote and a proposal.

Leave a Reply