1. Field of the Invention
The present invention generally relates to processing systems for testing products including a modulator. More particularly, the present invention relates to systems for testing products that manipulate digital data and that include an I/Q modulator.
2. Description of Related Art
As is well known to those skilled in the art, data may be transmitted using a carrier signal via a modulation process. Modulation processes involve the encoding of source data onto a carrier signal. During modulation, one or more of three fundamental frequency-domain parameters--that is, amplitude, frequency, and phase--is or are operated upon.
A distinction must be made between analog and digital data and analog and digital signals. Viewing modulation processes broadly, four different combinations are possible: (1) digital data may be encoded into a digital signal; (2) digital data may be encoded into an analog signal; (3) analog data may be encoded into a digital signal; and (4) analog data may be encoded into an analog signal. There are various reasons why one combination may be preferred over the others in a particular situation. For example, with respect to the combination most relevant to the discussion herein, the combination of encoding digital data into an analog signal, it is often used because certain equipment processes digital data but some transmission media, such as optical fiber and the unguided media, will only propagate analog signals. One of the most familiar uses of the digital data to analog signal transformation revolves around the transmission of digital data through the public telephone network, which was designed to receive, switch, and transmit analog signals in the voice-frequency range of about 300 to 3400 Hz.
As previously mentioned, modulation involves operation on one or more of three characteristics of a carrier signal. Those three characteristics are amplitude, frequency and phase. Not surprisingly, therefore, there are three basic modulation techniques for encoding digital data into analog signals: amplitude-shift keying (ASK) hereinafter; frequency-shift keying (FSK) hereinafter; and phase-shift keying (PSK) hereinafter.
The FSK modulation technique, the one most immediately relevant, involves representing the two binary values by two different frequencies near the carrier frequency. Typically, the two different frequencies are offset from the carrier frequency by equal but opposite amounts. FSK is commonly used on voice grade lines up to 1200 bps. FSK is also often used for high-frequency (e.g., 3 to 30 MHz) radio transmission.
Another concept relevant to the present invention is quadrature modulation. Quadrature modulation, which is a process well known to those skilled in the art, is the modulation of two carrier components 90 degrees apart in phase by separate modulating functions. Quadrature modulation has become a popular technique for more efficiently using bandwidth by having each signalling element represent more than one bit.
Still yet another concept relevant to the present invention is the concept of ensuring output signal quality of a particular signal processing product is high. Although the desirability of having high output signal quality of such a product is self-evident, there has not heretofore been developed a commercially viable, virtually foolproof method of verifying output signal quality that can be built into such products themselves. Stated another way, heretofore transmitters and the like have not been enabled to produce certain patterns designed to facilitate testing at the system level. Having a build in "tester", if you will, is desirable as it is readily accessible. However, there has not yet been developed such a "tester" simple and inexpensive enough to allow it to be incorporated into a commercial product. It should be readily apparent based upon the foregoing that the lack of such an output signal content verifier or tester is a shortcoming and deficiency of the prior art.