Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2001-10-11
2003-04-15
Jeanglaude, Jean Bruner (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C455S209000, C375S297000, C375S344000
Reexamination Certificate
active
06549153
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the field of digital-to-analog conversion, and particularly to digital-to-analog conversion in radio transmitters.
2. Background Information
Digital-to-Analog Converters (DACs) are often found in signal generation applications such as radio transmitters. See for example U.S. Pat. No. 6,032,028 and U.S. Pat. No. 5,010,585.
As shown in
FIGS. 1A-1D
, the signal at the analog output of a DAC contains not only the desired signal, but also alias signals (or spectral copies) which have to be removed in a reconstruction filter, as shown for example in
FIGS. 1A-1D
.
As shown in
FIG. 1A
, a digital input signal is provided to an input
102
of the DAC
104
. An output
106
from the DAC
104
provides an analog signal to an input of a reconstruction filter
108
, and the reconstruction filter
108
outputs a reconstructed analog signal on its output
110
.
FIG. 1B
shows a magnitude spectrum of the digital signal provided to the input
102
of the DAC
104
.
The spectral copies in the output from the DAC
104
, fall off in amplitude as (sin(x))/x due to the zero-order hold (ZOH) of the DAC. This can be seen in
FIG. 1C
, which shows a magnitude spectrum of the signal output from the DAC
104
. The sinc response
142
behavior of the magnitude spectrum of the signal, which is due to the ZOH of the DAC, can be seen in FIG.
1
C.
FIG. 1C
also shows an ideal frequency response
140
of the reconstruction filter
108
.
FIG. 1D
shows a magnitude spectrum of the signal output in the transmit frequency spectrum from the reconstruction filter
110
, when the reconstruction filter
110
operates in accordance with the ideal frequency response
142
.
For performance reasons, the DAC outputs a desired signal at a fairly low frequency. Consequently, the reconstruction filter
108
is often implemented using discrete LC technology. If high filter performance is required, the filter will often be bulky and require trimming.
In a typical radio transmitter known in the prior art, the output signal from the DAC is frequency upconverted in a mixer after passing through the reconstruction filter. An example of this is shown in
FIG. 2
, where an output of the reconstruction filter
108
is provided to a mixer
212
. The mixer
212
mixes the output from the reconstruction filter
108
with a signal from a local oscillator
214
. The frequency of the signal from the local oscillator
214
is represented as f
LO
. The output of the mixer
212
is provided to an IF filter
216
.
In typical frequency mixers, the input signal is not only frequency-translated with the local oscillator frequency f
LO
, but is also frequency-translated with its odd harmonics. Any of the spectral copies that survive the reconstruction filter
110
, or that are present in circuit configurations where the signal from the DAC
104
is provided directly to the mixer
212
(i.e., no reconstruction filtering is performed) can cause unacceptable distortion in or close to the signal band of interest. The distortion occurs because the spectral copies mix with the fundamental or harmonic frequencies of the local oscillator
214
.
FIGS. 3A
,
3
B show an example of this distortion.
FIG. 3A
shows a magnitude spectrum of a signal provided from the DAC
104
directly to the mixer
212
(where no reconstruction filtering is performed).
FIG. 3B
shows a magnitude spectrum of an output signal from the mixer
212
, which results when the signal of FIG.
3
A and the signal from the local oscillator
214
are input to the mixer
212
. For the sake of illustration, the harmonics of the local oscillator
214
are assumed to have the same magnitude level as the fundamental frequency f
LO
of the local oscillator
214
.
If the mixer
212
is well balanced, then only odd-order harmonics are of practical importance. These odd-order harmonics fall off in amplitude approximately following a Fourier series expansion of a square wave. The spectrum of the output of the mixer is simply found by displacing the double-sided spectrum of the output signal from the DAC, by all possible integer multiples (positive and negative) of the local oscillator frequency f
LO
.
For these reasons, the reconstruction filter
108
needs to be placed before the mixer
212
. The filter requirements will become dependent on the frequency plan of the transmitter. In narrowband transmitters, its is possible through careful frequency planning to prevent frequency-converted spectral copies from appearing inside the frequency band of interest. For transmitters having bands wider than narrowband transmitters, effective frequency plans are very difficult to achieve due to the larger bandwidth of the signal.
Moreover, high performance DACs usually have current source output, and suffer from decreasing linearity as the voltage swing over the load of the DAC increases. It is hence beneficial to keep the load impedance as low as possible, in order to help the DAC perform in as linear a fashion as possible. On the other hand, the linearity of some types of mixers is enhanced or benefitted by driving the mixer with a current source (i.e., high impedance source). However, it is not possible to provide both a low impedance load for the DAC, and a high impedance source for the mixer, without signal attenuation which in turn degrades the attainable signal-to-noise ratio (SNR). In summary, the following problems exist in prior art systems: 1) a bulky and often expensive reconstruction filter is required in front of the mixer, and 2) the reconstruction filter cannot present both an optimal load for the DAC, and an optimal source for the mixer, without causing signal loss. Accordingly, a need exists for a system to overcome these problems.
SUMMARY OF THE INVENTION
In accordance with exemplary embodiments of the invention, a circuit is presented that eliminates the need for a bulky filter between the DAC and the mixer, and provides both a very low load impedance to the DAC, and a high source impedance to the mixer. In accordance with an exemplary embodiment of the invention, the DAC is connected directly to the mixer, and an IF filter after the frequency upconversion (i.e., located downstream of the mixer) is used as a reconstruction filter. In addition to eliminating the bulky prior art reconstruction filter between the DAC and the mixer, a direct connection between the DAC and the mixer provides the DAC with a low-impedance load, and provides the mixer with a high-impedance source because the DAC and the mixer are effectively current-matched. In accordance with an embodiment of the invention, the update rate of the DAC, f
DAC
, is related to the frequency of the local oscillator by the following relation: f
LO
=(n/2)(f
DAC
), where n is any positive integer. In an exemplary embodiment of the invention, n=1 so that f
LO
=(f
DAC
)/2.
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Eriksson Patrik
Petersson Peter
Jeanglaude Jean Bruner
Telefonaktiebolaget LM Ericsson (publ)
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