Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1999-04-16
2003-05-06
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S328000, C375S351000
Reexamination Certificate
active
06560301
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally related to methods and apparatuses for frequency translation and frequency selectivity.
2. Related Art
FIG. 1
is a block diagram of an example conventional receiver
112
.
FIG. 2
is a flowchart representing the operation of the receiver
112
.
In step
206
, a band-select filter
102
filters an RF (radio frequency) spectrum
114
. An example RF spectrum
114
is shown in FIG.
4
A. The RF spectrum
114
includes signal components at frequencies f
1
, f
2
, f
3
, and f
4
. Assume, for purposes of example, that the receiver
112
is configured to receive signals at frequency f
3
.
Typically, the band-select filter
102
is a wide-band filter. The characteristics of the band-select filter
102
are generally illustrated in FIG.
4
B. The band-select filter
102
has a center frequency f
c
, and a band-select bandwidth
402
. In the example shown in
FIG. 1
, where the receiver
112
is receiving an RF spectrum
114
, the center frequency f
c
of the band-select filter
102
is within the RF range. For example, the center frequency f
c
may be 900 MHZ. Depending on the application, the band-select bandwidth
402
may be as much as 50 MHz, or greater. In the example where the center frequency f
c
is 900 MHZ and the band-select bandwidth
402
is 50 MHZ, the passband (i.e., the band of frequencies that pass through a filter with little loss, relative to frequencies outside of the band) of the band-select filter
102
is 875 Mhz to 925 MHz. According to these specifications, the quality factor of the band-select filter
102
, or Q, is equal to 18 (as described further below, Q is equal to the center frequency divided by the bandwidth, or 900 MHz÷50 Mhz in this example). This Q factor is typical for a band-pass filter operating at RF. In fact, generally, high Q factors at high frequencies are difficult to realize using conventional filter techniques, and have at best limited tuning capabilities.
The band-select filter
102
in step
206
operates to filter out signals outside its passband. For example purposes, assume that f
1
and f
4
are outside the passband of the band-select filter
102
, and f
2
and f
3
are inside the passband of the band-select filter
102
(this is the case in the example of FIGS.
4
A and
4
B). Accordingly, in this example, the band-select filter
102
operates to filter out the signal components at frequencies f
1
and f
4
. The band-select filter
102
passes the signal components at frequencies f
2
and f
3
. The result of the operation of the band-select filter
102
is shown in FIG.
4
C.
In steps
208
and
210
, the signal output by the band-select filter
102
(herein called the band-select filtered spectrum
408
for reference purposes) is processed by a low-noise amplifier (LNA)
104
and a mixer
106
. The LNA
104
operates to amplify the band-select filtered spectrum
408
, and the mixer
106
operates to down-convert the band-select filtered spectrum
408
in a well known manner.
Both the LNA
104
and the mixer
106
have limited dynamic ranges over which their operation is linear. Outside of these ranges, the LNA
104
and the mixer
106
exhibit non-linear operation. The broader the band select filter
102
(i.e., the wider the pass band), the more energy is able to reach the LNA
104
and the mixer
106
. Consequently, the broader the band select filter
102
, the greater the chance that the respective dynamic ranges of the LNA
104
and the mixer
106
will be exceeded. For purposes of example, assume that the signal component
420
at frequency f
3
combined with the undesired signal component
421
at frequency f
2
exceed the linear ranges of the LNA
104
and the mixer
106
(this is a common practical example).
When operating on a signal that is outside their linear ranges (i.e., when operating in a non-linear manner), the LNA
104
and/or the mixer
106
generate spurious signal components. In the given example, when operating on the signal components
420
and
421
, the LNA
104
and/or the mixer
106
generate spurious signal components
404
. See FIG.
4
D. Some of these spurious components
404
may coincide and interfere with signals at desired frequencies. For example, as noted above, the receiver
112
is tuned to receive signals at frequency f
3
(in the example of
FIGS. 4A-4G
, frequency f
7
corresponds to f
3
after downconversion; similarly, frequency f
6
corresponds to f
2
after downconversion).
In the process of operating on the signal components
420
and
421
, the LNA
104
and/or the mixer
106
generate a spurious signal component
404
C at frequency f
7
. This spurious component
404
C coincides with the desired signal component
420
at frequency f
7
. This spurious component
404
C interferes with the desired signal component
420
.
In step
212
, a channel-select filter
108
filters the signal generated by the LNA
104
and the mixer
106
(this signal is herein called the processed spectrum
410
for reference purposes). The characteristics of the channel-select filter
108
are generally shown in FIG.
4
E. The channel-select filter
108
has a center frequency at frequency f
7
and a channel-select bandwidth
406
. The center frequency f
7
of the channel select filter
108
is at a lower frequency than the center frequency of the band select filter
102
. For example, the center frequency f
7
of the channel select filter
108
may be 10 MHZ. Depending on the application, the channel-select bandwidth
406
may be, for example, 50 KHz. According to these specifications, the quality factor of the channel-select filter
108
, or Q, is equal to 200 (as indicated above, and described further below, Q is equal to the center frequency divided by the bandwidth, or 10 MHz÷50 KHz in this example). This Q factor is typical for a narrowband band-pass filter operating at IF (intermediate frequency). As this example illustrates, it is possible to realize higher Q factors at lower frequencies using conventional filter techniques.
As shown in
FIG. 4F
, the effect of the channel-select filter
108
in step
212
is to filter-out the signal component at frequency f
6
and spurious components
404
A,
404
B, and
404
D, but to pass any signals at frequency f
7
. Both the desired signal component
420
and the spurious component
404
C exist at frequency f
7
, and are within the passband of the channel-select filter
108
. Thus, both the desired signal component
420
and the spurious component
404
C are passed by the channel-select filter
108
.
In step
214
, an amplifier
110
amplifies the signal output from the channel-select filter
108
(this signal is called the channel select filtered signal
412
for reference purposes). The channel select filtered signal
412
includes both the desired signal component
420
and the spurious component
404
C. Consequently, the amplifier
110
amplifies both the desired signal component
420
and the spurious component
404
C.
In other words, once the spurious component
404
C is generated, it follows the desired signal component
420
in all downstream processing.
As noted above, the spurious signal component
404
C may make it difficult if not impossible to properly receive the desired signal component
420
. Accordingly, because the receiver
112
utilized a wide-band, band-select filter
102
prior to amplification and frequency translation by non-linear components (i.e., by the LNA
104
and the mixer
106
, respectively), the receiver
112
suffers from potentially degraded performance. The potential for signal interference as described above limits the receiver
112
's applicability.
SUMMARY OF THE INVENTION
The present invention is directed to methods and apparatuses for frequency selectivity and frequency translation. The invention is also directed to applications for such methods and apparatuses.
Briefly stated, the invention operates to filter an input signal, and to down-convert the filtered input signal. According to embodiments of the present invent
Bultman Michael J.
Cook Robert W.
Looke Richard C.
Moses, Jr. Charley D.
Sorrells David F.
ParkerVision, Inc.
Pham Chi
Phu Phuong
Sterne Kessler Goldstein & Fox P.L.L.C.
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