Laboratory instruments, such as network and spectrum analyzers, often include signal sources to electrically excite a system being measured. These sources can usually provide a variety of signals, such as sinusoids, chirps, noise, etc, at a desired frequency of interest.
In the early prior art, these signals were generated by analog mixers which upconverted a baseband analog waveform, such as a chirp, to a desired frequency of interest by mixing with a signal from an analog local oscillator. The resulting product included both sum and difference terms, one of which was undesired and required filtering. The filtering task was difficult due to the wide range of different signals the instrument could produce.
More recently, designers of some instruments have adopted digital techniques for signal generation. Digital signal generation offers several advantages, one of which is the ability to simply and precisely set signal frequencies under microprocessor control. It also permits use of digital filters that can be reconfigured as necessary to eliminate spurious products from the mixing process.
While advantageous in many respects, digital signal generation as presently practiced has certain drawbacks. It is an object of the present invention to overcome these drawback to provide a highly accurate signal source that can be economically implemented and provide great flexibility in the number of different waveforms that can be generated.
According to the present invention, a digital mixer is supplied with two digitized input signals in complex form. The product of the complex input signals is converted to analog form and provides the desired output signal without any alias terms.
In one embodiment of the invention, the first input signal is stored in a first memory as a sequence of data points corresponding to a complex exponential sinusoid, or a representative portion thereof. The second input signal is a baseband excitation waveform that is downloaded in complex form into a random access memory. Operands from these two sources are provided periodically to the digital mixer for complex multiplication. The multiplication of the digital representation of the complex sinusoid with the digital representation of a baseband excitation waveform yields a resultant digital data stream that corresponds to the baseband waveform shifted up in frequency. The process is analogous to single sideband modulation of analog signals.
The sequence of sinusoid operands provided from the first memory is determined by a phase counter that indexes through the sinusoid memory by a .DELTA..theta. term. .DELTA..theta. is selected so that the data indexed thereby corresponds to samples of a sine wave of the desired frequency. If .DELTA..theta. indexes a signal phase intermediate two complex data in the memory, an interpolated value is provided to the mixer.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.