Telecommunications – Receiver or analog modulated signal frequency converter – Frequency modifying or conversion
Reexamination Certificate
2000-04-20
2004-06-22
Appiah, Charles (Department: 2686)
Telecommunications
Receiver or analog modulated signal frequency converter
Frequency modifying or conversion
C455S296000, C455S309000, C327S310000, C327S334000, C333S032000
Reexamination Certificate
active
06754480
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a baseband analog circuit suitable as a receiver. More specifically, the present invention relates to a baseband analog circuit suitable as a receiver in, for example, a CDMA (code division multiple access) type mobile phone.
2. Description of the Related Art
A conventional baseband analog circuit
21
will be described with reference to FIG.
2
. The baseband analog circuit
21
includes a demodulator
22
for demodulating an input signal into a baseband analog signal, and a low-pass filter
23
having the demodulated signal input thereto. The demodulator
22
is implemented with, for example, bipolar transistors and the output impedance thereof is high. The low-pass filter
23
is also implemented with bipolar transistors.
A transmission signal of frequency 800 MHz is received by an antenna
24
and is subsequently output to a frequency converter
25
. At the frequency converter
25
, the transmission signal is frequency-converted into an intermediate frequency signal having a frequency of approximately 85 MHz and is output to the demodulator
22
. At the demodulator
22
, the intermediate frequency signal is demodulated into a baseband analog signal and is output to the low-pass filter
23
. Since the baseband analog signal demodulated at the demodulator
22
has a band up to approximately 600 kHz, the cutoff frequency of the low-pass filter
23
is approximately 600 kHz. The baseband analog signal passed through the low-pass filter
23
is output to an A/D converter (analog-to-digital converter)
26
, where it is converted into a digital signal.
However, a problem exists in that the low-pass filter
23
produces noise, which decreases the C/N (carrier to noise ratio) because the input impedance of the low-pass filter
23
and the output impedance of the demodulator
22
are large. Another problem is the decrease in the N/F (noise figure) of the low-pass filter
23
because the output impedance of the demodulator
22
is larger than the equivalent noise resistance of the low-pass filter
23
.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to solve the foregoing problems by providing a low-pass filter with improved noise characteristics.
To this end, according to a first aspect of the invention, a baseband analog circuit is provided. The baseband analog circuit includes a demodulator to demodulate a received signal and to output a baseband analog signal, a low-pass filter to filter the demodulated baseband analog signal, and an impedance converter, provided between the demodulator and the low-pass filter, having a lower output impedance than the output impedance of the demodulator.
The impedance converter may comprise an emitter-follower circuit. The output impedance of the impedance converter, and thus impedance seen by the input terminal of the low-pass filter, is decreased from the conventional receiver. Accordingly, the noise occurring at the low-pass filter is reduced thereby improving the C/N. The output impedance of the emitter-follower circuit may be not greater than the equivalent noise resistance of the low-pass filter. The low-pass filter may comprise a complementary metal-oxide semiconductor (CMOS) circuitry.
In this manner, an inexpensive baseband analog circuit can be provided.
REFERENCES:
patent: 5369470 (1994-11-01), Hansen
patent: 5623514 (1997-04-01), Arai
patent: 5815035 (1998-09-01), Yamagishi et al.
patent: 6134452 (2000-10-01), Hufford et al.
patent: 6-291560 (1994-10-01), None
Alps Electric Co. ,Ltd.
Appiah Charles
Brinks Hofer Gilson & Lione
Iqbal Khawar
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