Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
2000-05-30
2003-07-29
Cong Le, Thanh (Department: 2684)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S324000, C455S318000, C455S304000
Reexamination Certificate
active
06600911
ABSTRACT:
FIELD OF TECHNOLOGY
The present invention relates to a receiver in a radio frequency communications system, and relates more specifically to an even harmonic direct-conversion receiver using an even harmonic quadrature mixer.
DESCRIPTION OF RELATED ART
Direct conversion reception is one method used to achieve size and weight reductions in a receiver for a radio frequency communications system. Direct conversion reception refers to a reception method in which a received radio frequency signal is frequency converted directly to a baseband signal and then demodulated without an intermediate frequency conversion step. The even harmonic direct-conversion receiver using an even harmonic quadrature mixer has been used to improve deterioration in reception sensitivity.
FIG. 12
is a block diagram of an exemplary even harmonic direct-conversion receiver according to the related art.
Shown in
FIG. 12
are an antenna
1
; a low noise amplifier
2
for amplifying a signal received by the antenna, that is, a radio frequency signal; a bandpass filter
3
; and an even harmonic quadrature mixer
4
comprising two unit even harmonic mixers. A local oscillation signal output from local oscillator
5
and the received signal (of frequency f
rf
) passed through bandpass filter
3
are input to the even harmonic quadrature mixer
4
. It is to be noted that the frequency (f
p
) of the local oscillation signal output from the local oscillator
5
is approximately one-half the frequency of the received signal, and the even harmonic quadrature mixer outputs a baseband signal (of frequency |f
rf
−2
fp
|) by mixing the received signal and the local oscillation signal. Also shown in
FIG. 12
are a low pass filter
6
; a baseband signal amplifying circuit
7
comprising a plurality of baseband signal amplifiers for amplifying the baseband signal; and a signal processing circuit
11
. The signal processing circuit
11
comprises an A/D converter
8
, digital filter
9
, and a digital operating circuit
10
.
Operation of this even harmonic direct-conversion receiver is described next.
Referring to
FIG. 12
, a signal received at antenna
1
is amplified by low noise amplifier
2
, and noise components at frequencies outside the received frequency band are strongly suppressed by the bandpass filter
3
. The received signal passed by the bandpass filter
3
is then mixed with the output signal from the local oscillator
5
by the even harmonic quadrature mixer
4
(see
FIG. 14
) comprising two unit even harmonic mixers. The even harmonic quadrature mixer
4
then outputs I channel and Q channel baseband signals. The frequency of the oscillation wave output by the local oscillator
5
is approximately half the frequency of the signal received by the antenna
1
.
The I and Q channel baseband signals output from the even harmonic quadrature mixer
4
are filtered by the low pass filter
6
to remove extraneous signals other than baseband signals. The baseband signals filtered by the low pass filter
6
are amplified to a level appropriate for input to the A/D converter
8
by the baseband signal amplifying circuit
7
, which comprises a plurality of baseband amplifiers. The amplified baseband signals are converted to digital signals by the A/D converter
8
, passed through the digital filter
9
, and demodulated data is then obtained by the digital operating circuit
10
.
It is possible to suppress deterioration in reception sensitivity due to secondary intermodulation distortion, autodetection of the local oscillation wave, and other mixing of even harmonics by using an even harmonic quadrature mixer in an even harmonic direct-conversion receiver as described above. In addition to enabling receiver downsizing by use of a direct-conversion design, it is therefore also possible to improve reception sensitivity by using an even harmonic quadrature mixer.
It should be noted that the local oscillator
5
outputs to the even harmonic quadrature mixer
4
a local oscillation wave at approximately half the received signal frequency. At the same time, a radiation wave of substantially the same frequency as the local oscillation wave is also emitted from the local oscillator
5
. This is referred to below as the radiation wave. When this radiation wave is input from the antenna
1
or input terminal of the low noise amplifier
2
, the radiation wave from the local oscillator
5
is input to the low noise amplifier
2
in addition to the received signal.
Due to the nonlinearity of the amplifier, the low noise amplifier
2
thus outputs the received signal, the radiation wave, and a harmonic component of each. The second harmonic of the radiation wave is substantially the same frequency as the received signal. It therefore cannot be removed by the bandpass filter
3
disposed downstream, and is thus input to the even harmonic quadrature mixer
4
. As a result, a dc offset voltage caused by the radiation wave is contained in the baseband signals output from the even harmonic quadrature mixer
4
.
While the dc offset voltage contained in the baseband signals output from the even harmonic quadrature mixer
4
are relatively small, the dc offset voltage input to the A/D converter
8
becomes extremely high due to the extremely high gain of the baseband signal amplifying circuit
7
. This then becomes a factor in data recognition errors when the data is subsequently evaluated by the digital operating circuit
10
, and the bit error rate deteriorates.
FIG. 13
shows another even harmonic direct-conversion receiver using an even harmonic quadrature mixer according to a second example of the related art. It should be noted that like parts are identified by like reference numeral in FIG.
12
and
FIG. 13
, and further description thereof is thus omitted below.
Shown in
FIG. 13
are a first low noise amplifier
2
a
, and a second low noise amplifier
2
b
disposed between the first low noise amplifier
2
a
and bandpass filter
3
. A signal received from the antenna
1
is amplified by the first low noise amplifier
2
a
and second low noise amplifier
2
b
. The amplified signal is then input to the even harmonic quadrature mixer
4
, and then demodulated to the original data by the downstream signal processing circuit
11
.
A receiver thus comprised to amplify a signal received from the antenna using a plurality of low noise amplifiers for input to the even harmonic quadrature mixer is used, for example, in mobile communications base station equipment, and is used to amplify extremely weak received waves to an input level appropriate to a downstream even harmonic quadrature mixer while also maintaining a sufficient NF.
It is also possible for a radiation wave to be input from antenna
1
or the input terminals of first low noise amplifier
2
a
or second low noise amplifier
2
b
with a configuration as shown in FIG.
13
. When a radiation wave is input from second low noise amplifier
2
b
, a drop in reception sensitivity can be expected similarly to that in the first prior art example described above. If a radiation wave is input before the first low noise amplifier
2
a
, a second harmonic component of the local oscillation wave (radiation wave) will be contained in the components of the received signal amplified by the first low noise amplifier
2
a
. As a result, the dc offset voltage will increase as the signal is subsequently amplified by the second low noise amplifier
2
b
and baseband signal amplifying circuit
7
. This then becomes a factor in data recognition errors when the data is subsequently evaluated by the downstream signal processing circuit
11
, and the bit error rate deteriorates.
It should be noted that to achieve a desired NF through amplification of a weak received signal to a desired signal level by a plurality of low noise amplifiers, the gain of the first low noise amplifier
2
a
is typically not very high and the second low noise amplifier
2
b
is set to a high gain level. If a particularly strong radiation wave is input to the first low noise amplifier
2
a
in this case, t
Ikematsu Hiroshi
Itoh Kenji
Morishige Hideki
Cong Le Thanh
Lei T. Richard
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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