National Semiconductor Corporation

Daniel E Fague

Gerard G Socci

Benny Madsen

Dennis M Rose

The preferred embodiment improves the modulation accuracy of a communication system having a surface acoustic wave (SAW) device filtering a modulated signal. An equalizer filter is used to predistort the signal to be filtered so as to equalize the SAW filter response. In the preferred approach, the equalizer also provides baseband filtering. In this particular embodiment, the original baseband filter impulse response is convolved with the inverse of the actual SAW filter impulse response. The resulting values provide the coefficients for a digital filter (i.e., a modified baseband filter) forming part of a DQPSK modem. When the signal filtered by the modified baseband filter is then up-converted and filtered by the SAW filter, the predistortion caused by the modified baseband filter equalizes the SAW filter distortion, and only the modulation errors of the original baseband filter occur. In other embodiments, the equalizer filter characteristics are derived solely from the SAW filter impulse response or frequency response to equalize any distortion resulting from the SAW filter. In the preferred embodiment, the coefficients for the digital filter are stored in a ROM lookup table.

Equalization filter compensating for distortion in a surface acoustic

A radio frequency (RF) transceiver includes a direct modulation transmitter and single down-conversion receiver for operation in a time-division-duplex (TDD) telecommunications environment. A single RF signal source, in the form of a phase-lock-loop (PLL), is used on a time-shared basis to provide both the carrier signal for the transmitter and the local oscillator (LO) signal for the receiver. In the transmitter, direct modulation is effected by modulating a voltage-controlled oscillator (VCO) in the PLL with a burst of the transmit data while opening the loop and holding the loop feedback tuning voltage constant. In the receiver, a self-adjusting comparator threshold is provided for automatically setting and adjusting a demodulated signal comparison threshold used in retrieving the data and data clock from the demodulated receive signal. The interface between the transmitter and receiver and the host controller provides the control signals needed for the time-sharing of the single RF signal source, the proper programming of the PLL for the different transmitter carrier and receiver LO frequencies, the PLL loop control for the direct modulation of the VCO, and the enablement, or powering down, of the transmitter and receiver sections to minimize transceiver power consumption.

Radio frequency telecommunications transceiver

The preferred embodiment modem is an all hardware modulator which receives as its input two baseband drive signals, I and Q, which can come from a ROM filter or any other digital filter. Instead of storing modem responses in a ROM, multiplexers for the I and Q channels are used in combination with a control circuit to essentially perform as sine/cosine modulators for the baseband signal. The inputs into the I channel multiplexer include a non-inverted I signal, an inverted I signal, and a mid-value signal. The Q channel multiplexer has applied to it a non-inverted Q signal, an inverted Q signal, and a mid-value Q signal. Two-bit counters are applied to the control terminals of the I and Q multiplexers, where the 2-bit counters are clocked by a sample frequency. In one embodiment, the sample frequency is chosen to be four times the carrier frequency. The counter for the Q channel begins one count behind the I channel counter, thus giving a 90.degree. phase shift for the Q channel. The outputs of the two multiplexers are added to provide the output of the digital modem. The resulting word is then applied to a digital-to-analog converter and subsequently up-converted, filtered, and amplified for transmission.

Digital oversampled quadrature modulator

A ROM filter includes a number of ROMs, each of which is programmed to hold data relating to several pulse response curves during only a particular time interval. As data are read into the filter, data pertaining to a particular pulse response curve are addressed in each of the ROMs. The outputs of the ROMs are connected to an adder, which adds the data read from the ROMs and passes it through a digital-to-analog converter. In this arrangement, the ROMs are required to store only the data relating to the pulse response curves during a particular interval. These data are added together in real time in an adder which is external to the ROM. With this structure the area of the ROM can be significantly reduced, as compared with ROM filters in which the addition is programmed into the ROM.

Growing community of inventors

Sunnyvale, CA, United States of America

Daniel E Fague