Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
1999-04-24
2001-04-10
Wamsley, Patrick (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C375S222000
Reexamination Certificate
active
06215430
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related in general to digital signal processing, and more particularly to an improved method and system for processing a digital signal for subsequent analog transmission.
BACKGROUND OF THE INVENTION
Digital-to-analog (D/A) conversion is the process of converting digital codes into analog signals. Analog-to-digital (A/D) conversion is the complimentary process of converting a continuous range of analog signals into digital codes. Such conversion processes are necessary to interface real-world systems, which typically monitor continuously varying analog signals, with digital systems that process, store, interpret, and manipulate the analog values.
With the increased sophistication of cellular telephones, hand-held camcorders, portable computers, and set-top cable TV boxes, the requirements or performance criteria of D/A and A/D circuits has increased. These and other similar applications generally have low power and long battery life requirements. They may also have high speed and high resolution requirements.
One example of an application for a high performance digital-to-analog converter (DAC) is converting a digital signal, representing a desired modulated output signal in a digital transmitter, to an analog signal with a relatively high intermediate frequency. The relatively high intermediate frequency is desirable so that filtering subsequent mixer images is easier after the analog signal is mixed to the final radio frequency.
In the past it has been difficult to obtain a useful, high intermediate frequency signal from a low cost DAC because of the sinx/x filtering characteristic typical of a sample-and-hold action of the DAC that reduces the amplitude of signals with higher intermediate signals.
FIG. 1
illustrates a prior art application of a DAC used to convert a modulated digital intermediate frequency signal into an analog signal suitable for transmission, such as radio frequency transmission. As shown in
FIG. 1
, digital signal source
20
provides a modulated digital intermediate frequency signal that represents data to be transmitted. Such data may represent voice, video, or a data file that may be software, or some sort of user data such as a document. The modulated digital intermediate frequency signal is typically a serial stream of digital bits that comprise symbols that have been processed for transmission over a channel. Such processing may include interleaving and error coding to improve the efficiency of transmission over the channel.
The output of digital signal source
20
is coupled to a digital-to-analog converter (DAC)
22
. DAC
22
converts digital codes into a signal having discrete analog voltages.
The output of DAC
22
is coupled to the input of lowpass filter
24
, which attenuates all but the first baseband image in the analog signal output by DAC
22
. Lowpass filter
24
may be implemented with a surface acoustic wave device or other frequency selective device, which are well known in the art.
Following the output of lowpass filter
24
, the analog signal is mixed up to an intermediate frequency (IF) by mixer
26
having an input from a local oscillator with frequency F
L01
. Note that this mixing function may be thought of as a “frequency translating function” because the frequency of a signal component may be translated, up or down, to a new frequency. In one embodiment, an IF (local oscillator frequency F
L01
) near 200 MHz is used. Mixer
26
may be implemented with an integrated circuit sold under part number JYM-20H, available from Mini-Circuits, Brooklyn, N.Y.
Mixer
26
is followed by bandpass filter
27
and second mixer
28
that mixes the intermediate frequency output of mixer
26
up to the final transmission frequency, which may be a radio frequency (RF). In one embodiment, an RF (local oscillator frequency F
L02
) near 2 GHz is used. Mixer
28
may also be implemented with part number JYM-20H, available from Mini-Circuits. Bandpass filter
27
selects, or passes, one of the mixing product signals produced by mixer
26
.
Using two mixing stages with a first stage IF at 200 MHz provides a 400 MHz frequency spacing between the mixing product signal pair at the output of mixer
28
. This rather large spacing permits the use of an economic, low order filter following mixer
28
(not shown) to select one of the signals in the mixer image pair for final amplification and transmission.
The output of mixer
28
may then be forwarded to an amplifier (not shown) for amplifying a signal to a level that may be transmitted over a channel. The channel may be a radio frequency channel, in which case the signal is transmitted wirelessly from a transmitter to a receiver. Alternatively, the channel may be in another medium, such as a coaxial cable or an optical fiber. In such alternative media, signals output by DAC
22
may still be mixed up to another frequency for the purpose of frequency division multiplexing.
Referring now to
FIG. 2
, there is depicted a graph of frequency components, and their amplitudes, that are present in the analog signal output by DAC
22
.
In graph
40
, amplitude is plotted against frequency. On the frequency axis, F
L
is the sample frequency of digital signal source
20
. A plurality of signal components, including baseband signal component
42
and aliased signal components
44
, are shown at various frequencies. Each signal component is in a separate Nyquist band. A first Nyquist band is shown at reference numeral
46
and contains baseband signal component
42
. If digital signal source
20
provides a complex digital signal, first Nyquist band
46
is twice as large, extending from zero to the sample frequency F
L
. Nyquist bands having frequencies higher than the frequency of the first Nyquist band are referred to as “super-Nyquist bands.” These super-Nyquist bands are shown at reference numerals
48
.
The amplitude of aliased signal components
44
is determined by a filtering characteristic of DAC
22
. Filtering characteristic
50
, shown in
FIG. 2
as a dotted line, has the shape of the mathematical function six/x. Such a filtering function is typical of a DAC having a sample-and-hold output signal. Thus, amplitudes of aliased signal components
44
are determined by the value of the filtering characteristic function at the particular frequency of the aliased signal component.
Although signal components
42
through
44
have been represented in graph
40
as having a single frequency, these signal components may have some finite bandwidth because the signals may have several frequency components spanning such a bandwidth.
In one embodiment of the prior art, F
L
may equal 100 MHz. At the output of DAC
22
, lowpass filter
24
selects baseband signal
42
and filters out aliased signal components
44
. Mixers
26
and
28
, together, mix baseband signal
42
up to a 2 GHz frequency, which may be 20 times the frequency of F
L
. Two mixers are typically required because at the transmission frequency a mixer image needs to be filtered from the transmitted signal and it is difficult to filter such a mixer image when its frequency is close to the frequency of the transmitted signal. By using two mixers and mixing in two stages, the transmitted signal and its mixer image are separated in frequency, which makes the mixer image filter easier to implement because it can be designed with fewer poles.
Because filters with a higher number of poles are more difficult to design and implement, an upsampler may be used prior to the DAC in order to separate the baseband signal from the aliased signal components. This allows the use of a filter with fewer poles to filter the aliased signal components from the baseband signal.
As shown in
FIG. 3
, upsampler
60
and lowpass digital filter
62
are used to process the signal output by digital signal source
20
prior to being input to DAC
66
. Upsampler
60
performs a “zero stuffing” function wherein one digital symbol is input into upsampler
60
and, for example, three digital symbols are output from upsampler
60
. Of
Laird Kevin Michael
Pinckley Danny Thomas
Smith Paul Fielding
Motorola Inc.
Terry L. Bruce
Wamsley Patrick
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