1. Field of the Invention
The present invention relates to communication systems. In particular, the present invention relates to estimation of frequency and phase of analog signals.
2. Description of Related Art
Many modern day communications systems modulate phase and/or frequency of a carrier signal in order to convey information. In a communication receiver or a signal measurement instrument, a typical objective is to reverse the modulation operation on the received signal and determine the phase and/or frequency of the carrier signal.
Processing techniques used in a communications receiver may be analog or digital. In many modern day communication systems, signal processing is conducted digitally. In traditional receivers for reception of phase/angle modulated signals, analog-to-digital (A/D) conversion, down conversion to baseband, and, phase/frequency extraction are conducted separately. Typically the task of phase/frequency extraction is conducted after A/D conversion and the down conversion may be performed before or after A/D conversion depending on the system design preferences.
In such designs, the A/D conversion sub-system is optimized to get a high quality digital representation of the analog input signal. The design criterion for such systems has been based on minimizing the analog-to-digital converter (ADC) output signal amplitude quantization error. However, this is not necessarily optimal if the parameter of interest is actually the frequency and/or phase of the input signal. A system based on reducing the distortion in the frequency/phase of the input signal due to A/D conversion is likely to yield higher performance.
Accordingly, it is desirable to have a method and apparatus to efficiently and accurately estimate and/or digitize the instantaneous frequency and/or phase of an analog signal.
The present invention provides a method and apparatus to detect the instantaneous frequency of a signal. A sampler is used to sample the input signal. A metric generator uses the samples of the input signal to generate a metric signal whose magnitude is related to the instantaneous frequency of the input signal. With a minor modification to the frequency detector architecture, a power detector is obtained.
System, method and apparatus that are used for detecting and digitizing the instantaneous frequency of a signal are described. These utilize the frequency and power detectors. However, their use is not limited to frequency and power detectors presented with this patent application. These can be implemented with other types of frequency and power detectors. In one embodiment of the invention, the apparatus that is used to digitize the instantaneous frequency of a signal comprises a feedback circuit and a decimator. The feedback circuit generates a sequence signal from the input signal. The sequence signal contains the necessary information to detect and digitize the instantaneous frequency of the input signal. The decimator is coupled to the feedback circuit to decimate the frequency signal to generate a frequency signal. The frequency signal providing the instantaneous frequency of the input signal. Various embodiments of the feedback circuit are disclosed including an interpolative circuit and a predictive circuit.