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Gain, LO and phase compensation in a single sideband transmitter using the AD9788 TxDAC and ADL5372 quadrature modulator

Posted: 26 May 2008 ?? ?Print Version ?Bookmark and Share

Keywords:AD9788 application note? phase compensation? transmitter?

In a traditional radio transmit (Tx) architecture, a baseband signal (voice or data), is modulated to an intermediate frequency (IF) and then modulated again to the transmit RF. Traditionally, inexpensive upconverting modulators only provide good performance over roughly one decade of frequency conversion, so it is often necessary to execute the complete upconversion in two or more frequency hops. Note that with two hops, a second spectral image is produced that has no value for the transmitter. In fact, to meet spectral mask specifications (so as not to interfere with other transmitters and receivers), this image must often be filtered, a task which is not always an easy or inexpensive thing to do at the RF.

A single sideband transmitter, on the other hand, uses complex mathematical techniques to suppress the unwanted negative frequency image. In addition, by using these mathematical techniques rather than filtering, two channels of data can be transmitted within the same bandwidth, thus doubling spectral efficiency. This is done by placing one channel of data on a cosinusoidal carrier (the I data) and the other channel of data on a sinusoidal carrier (the Q data), with both carriers set to the same frequency. Modern receivers have the ability to separate signals on these quadrature carriers, and to separate the I data from the Q data.

This application note describes how to use the AD9788 TxDAC from Analog Devices as a key component in a single sideband transmitter where a high dynamic range IF signal is required at the DAC output. The complex numerically controlled oscillator (NCO) in the AD9788 enables this architecture, and the programmable gain and offset matching, as well as phase compensation in the AD9788 allow the user to calibrate out unwanted artifacts of the complex modulation. This optimizes performance and lessens the need for additional filtering and/or more rigorous frequency planning to reduce the effects of these artifacts.

View the PDF document for more information.





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