JPS6243561B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a balanced modulator for use in a mixer stage in the front end of a signal receiver, the purpose of which is to provide a balanced modulator for use in a mixer stage in the front end of a signal receiver. It is an object of the present invention to provide a double-balanced balanced modulator that generates little modulation distortion and intermodulation distortion and has low noise characteristics for small input signal levels.

A generally well-known double balanced modulation circuit is shown in FIG. In Figure 1, the transistor
Q ₁ , Q ₂ , transistor Q ₃ , Q ₄ , transistor
Q ₅ and Q ₆ are emitter-coupled differential pair transistors and constitute the basic part of the double-balanced modulation circuit.
Also, transistors Q ₇ and Q ₈ are transistors Q ₅ ,
It is a constant current transistor for Q ₆ , and its current value is determined from the current value of the current mirror diode D ₁ . The resistors R ₁ , R ₂ , and R ₃ are used to further stabilize the operation of the current mirror. Input terminals 1 and 2 are normally differential input terminals for local oscillation signals, 3 and 4 are differential input terminals for received signals, and 5 is an external resistor for determining the bias current of differential pair transistors Q ₅ and Q ₆ . and a power supply connection terminal, 6 is a ground terminal, 7 and 8 are differential output terminals of this balanced modulation circuit, and 9 and 10 are external resistance connection terminals for adjusting the frequency conversion conductance of this balanced modulation circuit. . The main characteristics of the balanced modulation circuit in FIG. ₅ and _Q6 , and the level of the local oscillation signal applied to terminals 1 and 2. That is,
Frequency conversion conductance gm is terminal 9, 10
If the resistance value of the external resistor connected between is R _E ,
It is approximately given by the following equation.

gm=k1/R _E ......(1) However, k is a constant that depends on the local injection signal level applied to the differential input terminals 1 and 2 of the local oscillation signal, but this injection signal level is sufficiently large. value, it converges to a constant value and becomes 4/π. In addition, the noise figure is the above local injection signal level, the external resistance connected between terminals 9 and 10, and the terminals 3 and 4.
Although it is affected by the signal source impedance of the received signal connected to the differential pair transistor Q ₅ ,
It is greatly affected by the bias current of _Q6 , and it is known that the noise figure usually deteriorates when a large bias current is used. Furthermore, the maximum allowable signal input level to this balanced modulation circuit is determined by the bias current of the differential pair transistors Q ₅ and Q ₆ and the terminals 9 and 1.
The signal input level that can be applied to the differential terminals 3 and 4 when the differential pair transistors Q ₅ and Q ₆ can operate without entering the nonlinear region of saturation or cut-off, determined by the external resistor connected between ,
In other words, the amplitude value of the maximum allowable signal input level V _snax
is given by the following equation.

V _snax = R _E · I _B ......(2) However, R _E is the same as in equation (1). I _B is the bias current value of the differential pair transistors Q ₅ and Q ₆ , and here, the transistor Q _Five ,
Assume that a bias current I _B equal to Q ₆ is flowing.

The main characteristics when the double-balanced modulation circuit is used as a front-end mixer of a signal receiver have been explained above.

By the way, the required performance for a mixer in the front end section of a signal receiver is to have a large frequency variable conductance and a low noise figure in order to improve the sensitivity of the receiver, and to minimize intermodulation distortion and intermodulation distortion as much as possible. Although the maximum allowable signal input is large, judging from the various characteristics of the balanced modulation circuit described above, it is clear that it is difficult to satisfy all of the above-mentioned desired performances. In other words, if we try to increase the frequency conversion conductance, R _E
It is necessary to reduce the value, but at this time, the maximum allowable signal input level deteriorates, as is clear from equation (2). Therefore, in order to increase the maximum allowable signal input level without reducing the frequency conversion conductance, it is desirable to increase the bias current shown in equation (2), but in this case, the noise figure will generally deteriorate. I had a problem with it.

The present invention aims to eliminate the above-mentioned drawbacks when using a double-balanced modulation circuit as a mixer in the receiver front-end section, and when the signal input level to the receiver is small, the above-mentioned frequency The parameters shown in equations (1) and (2) are set to maintain good conditions with emphasis on conversion conductance and noise figure, and when the signal input level to the receiver is large, Parameters are set to increase the maximum allowable signal input level shown by the equation In order to suppress intermodulation distortion as much as possible, we propose reducing the local injection level and reducing the frequency conversion conductance.

The present invention will be described below with reference to drawings of embodiments.

FIG. 2 is a block diagram showing an embodiment of a receiver including the balanced modulator of the present invention, and FIG. 3 is a block diagram showing a configuration centered on a double balanced modulation circuit having various functions of the present invention. FIG. 2 is a circuit diagram showing an example of a balanced modulator according to the present invention. In FIG. 2, a received signal is applied to a signal input terminal of a balanced modulator 12 via an antenna 11. (In order to simplify the explanation, the high frequency amplifier circuit section, frequency selection circuit section, etc. are omitted.) An oscillation signal from a local oscillation circuit 13 is further applied to this balanced modulator 12 through a local oscillation injection terminal. . Moreover, this balanced modulator 12
Further, the output signal obtained from the intermediate frequency level detector 16 is added to the output signal, and by utilizing the fact that the output signal obtained from the intermediate frequency level detector 16 increases when the level of the received signal is high, the received signal level is When is large, as described above, the bias current of the double balanced modulation circuit included in the balanced modulator 12 is increased and the local injection level to the double balanced modulation circuit is decreased. The above balanced modulator 1
The output of 2 is applied to an intermediate frequency amplifier 15 through an intermediate frequency filter 14, and the output of the intermediate frequency amplifier 15 is applied to the above-mentioned intermediate frequency level detector 16 and demodulator 17. The output of the demodulator 17 is applied through a terminal 18 to a subsequent audio amplification section (not shown), and finally an audio output is obtained using a speaker or the like.

In each block shown in FIG.
An example of this will be described in detail with reference to FIG. In addition, an intermediate frequency level detector 16 that can be configured with a local oscillation circuit 13, an intermediate frequency amplification circuit 15, an envelope detection circuit, etc., and an AM or
Since the signal demodulator 17 for FM and the like can be realized using well-known circuit technology, further detailed description will be omitted.

Next, to explain the connection relationship between each terminal of the balanced modulator 12 shown in FIG. 3 and each block in FIG. 2 in correspondence with FIG. 2, the signal input connected to the antenna in FIG. The terminal corresponds to the terminal 20 shown in FIG. 3, and the terminal into which the local oscillation signal is injected corresponds to the terminal 19 in FIG.
The signal output terminal whose frequency has been converted to 4 corresponds to the terminal 21 shown in FIG. The terminal 23 shown in FIG. 3 is connected to the intermediate frequency level detector 1 shown in FIG.
This is a terminal connected to the output terminal of No.6. Also, the third
Terminals 22 and 24 shown in the figure are a DC power supply connection terminal and a ground terminal, respectively.

Next, the operation of the balanced modulator 12 shown in FIG. 3 will be explained. First, transistors Q ₁₇ , Q ₁₈ ,
Q ₁₉ , Q ₂₀ , Q ₁₅ , Q ₁₆ and resistor R ₁₄ constitute a double-balanced modulation circuit that corresponds to and operates in the same manner as the double-balanced modulation circuit shown in FIG. transistor
Q ₁₃ and Q ₁₄ are constant current sources for transistors Q ₁₅ and Q ₁₆ , and the output obtained from transistor Q ₂₁ , resistors R ₁₅ and R ₁₆ , and intermediate frequency level detector 16 applied to terminal 23 by the current mirror principle. Defined and controlled by signals. Therefore, as is clear from equation (2), the maximum allowable signal input level of this double-balanced modulation circuit is controlled by the intermediate frequency level, that is, the input signal level. In addition,
Needless to say, the resistor _R14 is for determining the frequency conversion conductance of this double-balanced modulation circuit. Next, the local oscillation signal applied to the terminal 19 is first passed through an inverted L-type attenuator composed of resistors R ₄ , R ₅ and a field effect transistor (hereinafter referred to as FET) Q ₉ to a transistor Q _{10 .} ，
It is added to a differential amplifier consisting of Q ₁₁ , resistors R ₆ , R ₇ , R ₈ , R ₉ , and R ₁₀ . The above FETQ ₉ was introduced as a variable impedance element whose AC impedance between the drain and source is controlled by the voltage between its gate and source, and the output signal obtained from the intermediate frequency level detector 16 applied to the terminal 23 is The impedance is controlled by the input signal level, and as the voltage applied to the terminal 23 that shows a direct current change increases in the positive direction, the impedance is reduced. The differential amplifier mainly composed of the above-mentioned transistors Q ₁₀ and Q ₁₁ is for transmitting a local oscillator signal to the above-mentioned double balanced modulation circuit,
A DC bias current is also supplied at the same time. Also, transistor Q ₁₂ , resistors R ₁₁ , R ₁₂ , R ₁₃
The emitter follower circuit is for supplying a base bias voltage to the differential amplifier mainly composed of the above-mentioned transistors Q ₁₀ and Q ₁₁ . Further, the capacitor _C1 shown in FIG. 3 is for coupling the local oscillator signal.

Here, the intermediate frequency level detector 1 shown in FIG.
Assuming that the output voltage of 6 is given the characteristic of increasing in the positive direction as the input voltage increases,
The DC-varying voltage applied to the input terminal 23 shown in FIG. 3 increases in the positive direction as the signal input voltage from the antenna 11 increases. Therefore,
In this case, as the signal input voltage from the antenna 11 increases, the bias current of the transistors Q ₁₅ and Q ₁₆ increases, and as a result, the maximum allowable signal input level increases as shown in equation ( ₂ ). As the AC impedance decreases, the local injection level to the double-balanced modulation circuit decreases, which reduces the frequency conversion conductance and increases the level of the frequency-converted intermediate frequency signal at the output terminal 21, resulting in cross-modulation distortion and mutual interference. Reduce modulation distortion.

It goes without saying that in the present invention, a bipolar transistor may be introduced in place of the FETQ ₉ , or a variable signal attenuation circuit using a complicated multiplier or the like may be introduced.

Furthermore, when the received signal level from the antenna 11 is sufficiently small, the bias current of transistors Q ₁₅ and Q ₁₆ of the double-balanced modulation circuit shown in FIG. 3 and the local oscillation signal applied to the double-balanced modulation circuit It goes without saying that it is desirable to set the injection level to an optimal level in consideration of the noise figure.

As described in detail above, according to the present invention, balanced modulation is achieved that has little generation of cross-modulation distortion and intermodulation distortion over a wide range of received signal levels, and has low noise characteristics for small received signal levels. It is possible to obtain a circuit.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a conventional dual balanced modulation circuit, Fig. 2 is a block diagram of a signal receiver configured using the balanced modulation circuit of the present invention, and Fig. 3 is a balanced modulation circuit of the present invention. FIG. 2 is a circuit diagram showing one embodiment of the present invention. 12...Balanced modulator, 13...Local oscillation circuit,
15...Intermediate frequency amplifier circuit, 16...Intermediate frequency level detector, 17...Demodulator, _Q10 to _Q21 ...Transistor, _R4 to _R16 ...Resistor.

Claims (1)

[Claims] 1. A double-balanced transistor balanced modulation circuit using a plurality of pairs of emitter-coupled differential pair transistors, means for increasing the bias current of the balanced modulation circuit in accordance with an increase in the input level of a signal, and an input level of a signal. The balanced modulation circuit is characterized by comprising means for reducing the level of the local oscillation signal applied to the balanced modulation circuit in response to an increase in the level of the local oscillation signal, thereby reducing distortion in the balanced modulation circuit over a wide range of signal input levels. Balanced modulator.