The acoustic charge transport (ACT) device is an integrated, high speed, GaAs charge transfer device in which electron packets are transported in the traveling wave potential wells of a single frequency surface acoustic wave (SAW) generated in the GaAs. The ACT device has been disclosed previously in U.S. Pat. No. 4,633,285 and other published documents.
Transversal filters have been implemented using charge coupled devices (CCD), acoustic charge transport (ACT) devices, and other charge transfer devices. These implementations have employed direct connections between the charge detection electrode and summing or signal modification circuits.
Prior art ACT integrated circuits incorporate passive charge detection and summing preceding any active circuits.
An important feature of the ACT device is non-destructive detection of the propagating charge signal. This feature makes the device useful as a delay line and transversal filter, and for other signal processing. The sensitivity of the charge detection process usually establishes the dynamic range of the signal processing system. The ACT programmable transversal filter (PTF) disclosed previously in copending U.S. patent application Ser. No. 07/707,147 employs divider networks connected between the non-destructive charge sensing electrodes and common signal summing nodes. These divider networks and their associated interconnect wiring decrease the impedance of the sensing electrodes which reduces charge detection sensitivity. Another limitation of the previously disclosed ACT PTF is the tap weight dependent impedance of the weighting circuit as seen by the common summing node. This results in the effect that programming of a single tap affects all other tap weights. The present invention provides a means for the realization of ACT programmable transversal filters with high dynamic range and nearly ideal programmable tap weight characteristics.
Multiple transversal filters in parallel, or filter banks, are useful signal processing devices that can be implemented with ACT devices. Prior art ACT device filter banks have employed a separate charge transport channel for each filter, resulting in performance limitations due to interchannel non-uniformities and very high power requirements due to the requirement for SAW power in each active charge transport channel. An embodiment of the present invention overcomes these limitations by realizing a filter bank with a single charge transport channel.
Prior art selectable delay lines, such as disclosed in U.S. Pat. No. 5,144,262, have employed a direct connection from the non-destructive sense tap to the tap selection switch, resulting in moderate to poor charge detection sensitivity due to parasitic capacitance of the parallel switches. The present invention provides improved charge detection sensitivity in selectable delay lines.