This invention relates generally to analog to digital converters, and relates in particular to delta-sigma analog to digital converters.
Analog to digital converters are employed for converting analog signals to digital signals in a wide variety of applications, including instrumentation and communication such as modem and wireless communication. Conventional analog to digital converters generally include Nyquist rate converters and over-sampled converters. Nyquist rate converters typically are designed to provide a certain output rate and to provide a single conversion operation per output sample. Over-sampled converters typically over-sample a given analog input signal. One type of over-sampled converter is a delta-sigma converter, such as disclosed in U.S. Pat. No. 6,414,615.
Many communications systems, however, are designed to be able to handle communication operations in two or more operational modes (such as GSM, WCDMA and TD-SCDMA etc. for wireless communication system). It is typically desirable that the same hardware be able to be used for each of the desired operational modes. Analog to digital converters, however, are typically either designed to support one mode of operation only, requiring duplication of the analog to digital function in the hardware, or include duplicative hardware. For example, while TD-SCDMA and WCDMA are both air interfaces for third generation (3G) terminals, TD-SCDMA downlink requires 2.56 Msps (2.56 million samples per second) analog to digital conversion with relatively high signal to noise ratio, while WCDMA requires 7.68 Msps analog to digital conversion with relatively low signal to noise ratio.
The difference between the two standards relates, in part, to the bandwidth of the signal and the performance of the analog to digital converter. WCDMA (wideband code division multiple access) uses a signal that has an occupied bandwidth of 5 MHz RF or 2.5 MHz baseband. TD-SCDMA (time division-synchronous code division multiple access) has an occupied bandwidth of 1.6 MHz as RF signal or 800 kHz used as a baseband signal. To support WCDMA, therefore, one needs a converter with a higher speed. For TD-SCDMA one needs 8 to 10 bits performance. The tradeoff, therefore, is that WCDMA requires a faster converter with fewer bits, while TD-SCDMA permits a slower converter with more bits. The problem, therefore, is how to have a converter that can be configured in two ways and be reasonably efficient for both jobs. Although one solution is to design a fast converter that gives 8 to 10 bits performance, but this would be inefficient and more costly and require more silicon area.
Another conventional dual-mode modulator is disclosed by T. Burger and Q. Huang in A 13.5-mW 185-Msample/s ΔΣ Modulator for UMTS/GSM Dual-Standard IF Reception, IEEE Journal of Solid-State Circuits, vol. 36, No. 12 (December 2001). A system disclosed therein provides that a resonator is disabled by disabling a feedback path from a third integrator to a second integrator. All integrators and feed-forward paths, however, remain active, and applicants have found that when a high frequency resonator feedback path is disabled and all integrators and feed-forward paths remain active, a delta-sigma modulator may not be sufficiently stable.
There is a need, therefore, for an analog to digital converter system that is efficient and economical to manufacture, yet supports two modes of operation.
A dual-mode delta-sigma analog to digital converter system and method are disclosed that use a feed-forward modulator and include a low frequency resonator circuit and a high frequency resonator circuit and include a feed-forward path from the final integrator in the high-frequency resonator circuit to a summer. The digital converter system includes a selection unit for permitting the high frequency resonator circuit and the low frequency resonator circuit to be employed in a first mode of operation. The system also permits the high frequency resonator circuit and the feed-forward path from the final integrator in the high-frequency resonator circuit to the summer to be disabled in a second mode of operation.
The following description may be further understood with reference to the accompanying drawings in which:
The drawings are shown for illustrative purposes only.
The invention provides a single delta-sigma analog to digital architecture that may be configured to efficiently operate as a fourth-order analog to digital converter for WCDMA applications and may be configured to efficiently operate as a third-order analog to digital converter for TD-SCDMA applications.
As shown in
During WCDMA operation, the system employs a frequency division duplex (FDD) method (at 3.84 MegaChips per second) whereby uplink and downlink transmission use two separate radio frequencies. In the FDD mode, each uplink and downlink uses the different frequency band. A pair of frequency bands that have a specified separation are assigned for the system. During TD-SCDMA operation, the system employs a time division duplex (TDD) method (at 1.28 MegaChips per second) whereby uplink and downlink transmissions are carried over the same radio frequency by using synchronized time intervals. In the TDD method, time slots in a physical channel are divided into transmission and reception portions. Information on uplink and downlink, therefore, are transmitted alternately.
As shown in
When the switches 77, 79 and 93 are connected as shown in
The noise transfer function is shown at 100 in
The switching at switches 77, 79 and 93 may be implemented as switched capacitor inputs. To disable these, a signal may be multiplexed with a zero signal. The unused integrator may also be shut off to save power.
Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the invention.
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/627,578 filed Nov. 12, 2004.
Number | Name | Date | Kind |
---|---|---|---|
5230098 | Seki | Jul 1993 | A |
6005896 | Maruyama | Dec 1999 | A |
6087969 | Stockstad et al. | Jul 2000 | A |
6362762 | Jensen et al. | Mar 2002 | B1 |
6414615 | Cheng | Jul 2002 | B1 |
6556159 | Fei et al. | Apr 2003 | B1 |
6765517 | Ali | Jul 2004 | B1 |
6822592 | Gandolfi et al. | Nov 2004 | B1 |
6839012 | Kawamura | Jan 2005 | B1 |
6911928 | Orsier et al. | Jun 2005 | B1 |
6943715 | Radja et al. | Sep 2005 | B1 |
6977546 | Stapleton | Dec 2005 | B1 |
20040036640 | Kawamura | Feb 2004 | A1 |
Number | Date | Country |
---|---|---|
0954107 | Nov 1999 | EP |
Number | Date | Country | |
---|---|---|---|
20060119491 A1 | Jun 2006 | US |
Number | Date | Country | |
---|---|---|---|
60627578 | Nov 2004 | US |