Direct to Microwave Signal Generation and Acquisition

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  • Author: Joan Mercade

WGs have been always part of any vector signal generation system as an internal or external component. Two channels are used to generate the I and Q components to feed an IQ modulator. The required sampling rate for the AWG depends on the modulation bandwidth, not in the carrier frequency. In b), a single channel AWG is used to generate a modulated IF signal to feed a mixer. The mixer upconverts the IF signal to the final frequency. In this case, sampling rate is mainly determined by the IF carrier frequency. In both cases an additional L.O. source is required. When sampling rate is enough (so it is at least twice the carrier frequency) the final RF signal can be generated directly as seen in c) without the need for any external mixer, modulator or L.O: source. It is even possible to generate RF signals beyond the Nyquist frequency by using images of the signal located in higher order Nyquist bands as seen in d). Usually, a BPF (Bandpass Filter) is required to get rid of the unwanted images.

Another way to generate RF modulated waveforms is by using an AWG to produce a fully modulated IF signal with one single channel and then upconvert it to the final carrier frequency using an external mixer.. This scheme requires an additional CW L.O. to feed the mixer. The advantage of this architecture is that the IQ modulation of the IF carrier is performed numerically when the waveform is calculated. As a result, there will not be I/Q alignment errors (amplitude, frequency response, skew, quadrature). The requirements for sampling rate will be higher, though. Sampling rate will have to be, at least, twice the maximum frequency component of the IF waveform. This means that the sampling frequency must be always higher than twice the carrier frequency and the modulation bandwidth.Both these techniques are discussed in this white paper.

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