Telecommunications – Transmitter – Frequency conversion
Reexamination Certificate
1999-04-15
2001-08-14
Hunter, Daniel (Department: 2746)
Telecommunications
Transmitter
Frequency conversion
C455S115200, C455S126000, C330S129000
Reexamination Certificate
active
06275684
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microwave/millimeter wave transmitter device having a frequency up convertor, and more particularly to an ALC (automatic level control) circuit for stabilizing the output level of the transmitter device.
2. Description of the Related Art
The structure of a conventional transmitter device having an ALC function (Japanese Patent Application Laid-open No. Hei 4-157927) is shown in FIG.
5
. An inputted intermediate frequency signal (IF signal) is mixed with the output of a local oscillator
20
by a mixer
21
so as to be converted into a radio frequency signal (RF signal), and then amplified by a variable gain amplifier
22
and a power amplifier
23
, thus being outputted as an RF signal. The RF signal transmission output level is detected by a detector
24
, and then sent to the variable gain amplifier
22
through an automatic gain control circuit (AGC)
25
, where the gain of the variable gain amplifier
22
is controlled to make the RF signal transmission output level constant.
However, the above-described transmitter device is expensive because the variable gain amplifier
22
of an RF-band is employed. Also, because automatic gain control is conducted after frequency conversion, in the transmitter device that alters the transmission frequency by use of the local oscillator of the synthesizer system, the variable gain amplifier
22
is required to provide an excellent frequency characteristic in a wide band within the limit of all the gain variations. In addition, when the IF input is interrupted, the variable gain amplifier
22
has the maximum gain, and the leakage of the local signal from the mixer
21
is greatly amplified, resulting in such a problem that the spurious characteristic of the RF output is deteriorated.
In view of the above, a transmitter device shown in
FIG. 6
has been known as an improvement in the above-described conventional transmitter device. This transmitter device inputs an IF input signal to a variable gain amplifier
30
to conduct a predetermined amplification, and mixes it by a mixer
32
with the output of a local oscillator
31
to convert it into an RF signal. Then, the transmitter device amplifies the RF signal thus converted is amplified by a power amplifier
33
to extract a transmission output.
In the above transmitter device, the output of the power amplifier
33
is partially branched and detected by a detector
34
to detect an RF transmission output level. The automatic gain control circuit
35
controls the gain of the variable gain amplifier
30
so that the detection output of the detector
34
becomes a predetermined value, to thereby make the RF transmission output level constant.
Accordingly, the above transmitter device per se can be manufactured inexpensively as compared with the above-described conventional transmitter device. Also, because the automatic gain control is conducted before frequency conversion, the variable gain amplifier
30
can control the gain of the IF signal having a predetermined center frequency. As a result, the variable gain amplifier
30
does not particularly require an excellent frequency characteristic in a wide band. Also, there is advantageous in that when the IF signal input is interrupted, the variable gain amplifier
30
becomes the maximum gain so that there is no case in which the leakage of the local signal from the mixer
32
is greatly amplified, thereby being capable of preventing the spurious characteristic of the RF output from being deteriorated. However, in the above-described two conventional transmitter devices, the signal branched from the output of the power amplifier is detected directly by a diode.
Since the input power to detected voltage characteristic of the general diode is that the forward voltage of the diode increases at a low temperature as shown in
FIG. 4
, the detected voltage is decreased at a reduced input power. Also, there is a risk that the diode is broken when the input power is inputted at the maximum input or more which is determined by a reverse withstand voltage of the diode junction.
For that reason, a range in which a given detected voltage is stably obtained even if an environmental temperature is varied is very narrow to the degree of about 20 dB (100 times) at most.
With the application of a circuit structure in which the output power can be made constantly a predetermined value by use of an automatic level control circuit (ALC) where a part of the output power of the transmitter device is branched and detected as a power monitor, and then compared with a reference voltage to control a variable attenuator, the reference voltage is varied so that the output power of the transmitter device can be varied. However, a variable range in which the varied output power level can be made constant with respect to the environmental temperature is about 20 dB at most because of the limited characteristic of the above diode, and in the case where the output power is controlled and varied, it is very difficult to obtain the detected voltage of sufficient and stable voltage over a wide high-frequency output power range.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above drawbacks with the prior art, and therefore an object of the present invention is to provide an ALC circuit for a transmitter device in which a logarithmic amplifier is applied at a prestage of a detector, a power obtained by branching a part of transmission output through a directional coupler is down-converted to an intermediate frequency (for example, 1 GHz or less) once so as to be converted into a frequency at which the logarithmic amplifier is operable. With this structure, even if a change in power level is the degree of several decades (for example, 1000 times), a change in the output power level of the logarithmic amplifier can be made several times (for example, 3 times) as much as the original output power level, and a change of the power of several times can be detected in a range in which linearity of the detection diode is excellent (−10 to 10 dBm, 100 times in FIG.
4
). Then, even in the case where a differential amplifier reference voltage is varied to control and vary the output power, the detected voltage of the sufficient and stable voltage can be obtained over a wide high-frequency output power range.
In order to solve the above problems, according to the present invention, there is provided an ALC circuit for a transmitter device having a frequency up-convertor of microwaves and millimeter waves, a variable attenuator and a power amplifier which comprises: a first mixer that inputs an oscillation signal of a local oscillator; a second mixer that inputs a part of the oscillation signal of the local oscillator and a part of a transmission output signal amplified up to a predetermined level by the power amplifier to output a down-converted intermediate frequency signal; a logarithmic amplifier that amplifies the intermediate frequency signal outputted by the second mixer to a predetermined level; a detector that obtains a level monitor voltage proportional to the logarithm of a signal level outputted by the logarithmic amplifier; and a differential amplifier that compares the output signal of the detector with a DC reference voltage that determines the transmission output power to amplify the comparison result; wherein the output of the differential amplifier is inputted to the variable attenuator to control the transmission output signal.
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patent: 4972512 (1990-11-01), Garskamp
patent: 5043672 (1991-08-01), Youn
patent: 5182527 (1993-01-01), Nakanishi et al.
patent: 5298811 (1994-03-01), Gilbert
patent: 5383223 (1995-01-01), Inokuchi
patent: 5557640 (1996-09-01), Chadwick
patent: 5656929 (1997-08-01), Humpherys
patent: 5796309 (1998-08-01), Nguyen
patent: 6002122 (1999-12-01), Wolf
patent: 6020787 (2000-02-01), Kim et al.
patent: 6121834 (2000-09-01), Kim
patent: 61210727 (1986-09-01), None
patent: 4-157927 (1992-05-01), None
patent: 4196622 (
Akaishi Makoto
Kaneko Tomoya
Hunter Daniel
NEC Corporation
Tran Pablo N
Young & Thompson
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