JPS6124843B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a self-oscillation mixing circuit suitable for use mainly in tuners for television receivers.

FIG. 1 shows a conventional self-oscillation mixing circuit for a tuner for a television receiver. The figure shows a grounded base type self-oscillating mixing circuit, where 1 is a mixing transistor, 2
is a high frequency input terminal, 4 is an intermediate frequency output terminal, 5 is a power supply terminal, 6 is an intermediate frequency tuning coil, 7 and 8
1 is an intermediate frequency tuning capacitor, 11 and 12 are capacitors and coils that determine the local oscillation frequency, and 13 is a feedback capacitor. When a high frequency signal is input from the high frequency input terminal 2, it is mixed and amplified by the transistor 1, and only the intermediate frequency signal is sent to the terminal by an intermediate frequency tuning circuit composed of an intermediate frequency tuning coil 6 and intermediate frequency tuning capacitors 7 and 8. Output from 4. In addition, since the local oscillation signal output necessary for mixing operation is obtained by oscillating with transistor 1, local oscillation, mixing, and amplification can be performed with one transistor 1, which has a small number of components and is inexpensive. There are advantages.

However, in terms of characteristics, if the emitter current flowing through transistor 1 is reduced in order to increase the conversion gain, the cross-modulation disturbance characteristics and intermodulation disturbance characteristics will deteriorate, and furthermore, the conversion gain itself is not sufficient. There is. Particularly in television receivers in the United States, 6-channel color beat interference occurs, and the cause of this is the poor intermodulation interference characteristics of the mixing circuit, making it difficult to put it into practical use.

FIG. 2 shows a conventionally used mixing circuit that improves the drawbacks of the mixing circuit shown in FIG. 1 in terms of characteristics. In Figure 2, the same numbers as in Figure 1 indicate the same parts, 9 is the second transistor of the mixed circuit, and 10 is the connection point between the collector of the first transistor 1 and the emitter of the second transistor 9. This is a capacitor connected between and ground. This circuit is
The emitter of the first transistor 1 is grounded, and the second
This is a so-called cascode mixed circuit in which these transistors are connected in cascade with the base of transistor 9 being grounded. Since this circuit can take a large emitter current, it has good cross-modulation interference characteristics and intermodulation interference characteristics, and also has high conversion gain.
The drawbacks of the mixed circuit shown in the figure in terms of characteristics have been improved. In particular, the intermodulation interference characteristics can be significantly improved by appropriately selecting the capacitance value of the capacitor 10. Furthermore, since the conversion gain is high, even if the gain of the high frequency amplifier circuit, which is generally connected before the mixing circuit and is used to amplify the high frequency signal, is low, a sufficient gain can be obtained for the tuner as a whole. Therefore, the level of the high-frequency signal input to the mixing circuit can be lowered, so that cross-modulation interference characteristics and intermodulation interference characteristics can be further improved. As mentioned above, the circuit shown in Figure 2 has good characteristics, but in order to obtain a local oscillation output,
A local oscillation circuit including at least one transistor is required, and the combination of the mixing circuit and the local oscillation circuit requires at least three transistors and their bias circuits, resulting in a large number of components.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above, and to provide a mixing circuit that has a simple circuit configuration, uses a small number of transistors, has a high conversion gain, and has good cross-modulation and intermodulation interference characteristics. be.

In the present invention, a first transistor whose emitter is grounded and a second transistor whose base is grounded are connected in cascade to constitute a mixing circuit with good intermodulation disturbance characteristics, intermodulation disturbance characteristics, and exchange gain. A local oscillation circuit is also constructed using the first and second transistors.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 3 is a circuit diagram of essential parts of a self-oscillating mixing circuit showing one embodiment of the present invention, and the same parts as in FIGS. 1 and 2 are designated by the same numbers. 11 is a resonant capacitor for determining the local oscillation frequency; 12
13 is a feedback capacitor for local oscillation operation, and 14 and 15 are capacitors for optimizing the amount of feedback and oscillation conditions. In FIG. 3, transistors 1 and 9 are connected to operate as local oscillation transistors as well as to operate as mixed transistors. First, local oscillation operation will be explained. The local oscillation frequency is approximately determined by the resonant frequency of the resonant circuit constituted by the resonant capacitor 11 and the resonant coil 12. The signal at the local oscillation frequency resonates with this resonant circuit, and a high voltage is generated across the resonant circuit. This signal is fed back to the emitter of the first transistor 1 by the feedback capacitor 13, this signal is amplified by the first transistor 1 and the second transistor 9, and again by the resonant capacitor 11 and the resonant coil 12. The voltage is applied to the constructed resonant circuit, which continues to oscillate, and operates as a local oscillation circuit. At this time, high frequency input terminal 2
When a high frequency signal is input to the first transistor, it is mixed with a local oscillation signal by the first transistor, and the resulting intermediate frequency signal is amplified by the first and second transistors, and the second transistor mixes the high frequency signal with the local oscillation signal. An intermediate frequency signal can be obtained from the intermediate frequency output terminal 4 from the collector 9 through an intermediate frequency tuning circuit consisting of an intermediate frequency tuning coil 6 and an intermediate frequency tuning capacitor 8. More specifically, the mixing operation of the first embodiment of the present invention shown in FIG. It is considered to be equivalent to a transistor mixed circuit in which a local oscillation signal is injected into the emitter of transistor 1. Therefore, the capacitance of the capacitor 14 connected between the emitter of the first transistor 1 and the ground is connected to the capacitance of the feedback capacitor 13 and the capacitor 1 without being grounded with respect to the local oscillation signal frequency, that is, as a short-circuit impedance.
The oscillation signal voltage is obtained at the emitter of the first transistor 1 by the voltage division of the capacitor 5, and the impedance is low to some extent, that is, suitable for the high frequency signal frequency injected into the base of the first transistor 1. The ground impedance is selected so that a high frequency signal voltage is sufficiently applied between the base and emitter of the first transistor 1. Furthermore, these capacitors 13,
The capacitance values of 14 and 15 are set so that positive feedback is applied to the cascode connection circuit consisting of the first transistor 1 and the second transistor 9 to satisfy the oscillation conditions, and the local oscillation signal frequency is 128MHz,
When the high frequency signal frequency is about 83 MHz, the capacitance value of the feedback capacitor 13 is 3 pF, the capacitance value of the capacitor 15 is 270 pF, and the capacitance value of the capacitor 14 is 3 pF to perform self-oscillation mixing operation. When the local oscillation frequency and high frequency signal frequency change, the capacitance values of these capacitors 13, 14, and 15 may be set appropriately. Note that the capacitor 14 is a capacitor for appropriately setting oscillation conditions, and may not be necessary depending on the operating frequency of the self-oscillation mixing circuit, the circuit connected to the previous stage, etc.

The conversion gain of the mixed circuit shown in Figure 3 is sufficiently high, but if you wish to obtain an even higher conversion gain, a trap circuit tuned to the intermediate frequency may be connected between the base and emitter of the first transistor 1 or between the base and ground. All you have to do is connect to . It has long been known to add a trap circuit as a means of improving the conversion gain of a mixing circuit, but in the self-oscillating mixing circuit shown in FIG. 3, the capacitance value of the grounded capacitor 15 at the emitter of the first transistor 1 is as described above Since it is necessary to have a certain level of impedance with respect to the oscillation signal frequency, it is impossible to make the impedance large enough to have a low impedance with respect to the intermediate frequency. Therefore, when adding a trap circuit, the first It is more effective to connect between the base and emitter of the transistor. In FIG. 4, 16 is an intermediate frequency trap coil, and 17 is a trap capacitor, which form an intermediate frequency trap. The effect of adding this trap is that in addition to improving the conversion gain mentioned earlier, it also prevents the intermediate frequency interference signal input from the high frequency input terminal 2 from being input to the mixing circuit, improving the intermediate frequency interference characteristics. You can also aim for

FIG. 5 shows the conversion gain characteristics with respect to the emitter current of the transistor 1 of the embodiment of the present invention shown in FIGS. 3 and 4. In FIG. It shows the conversion gain characteristics when the local oscillation signal frequency is 138MHz, the high frequency signal frequency is 83MHz, and the intermediate frequency is 45MHz. The solid line A in FIG. 5 is the conversion gain characteristic of the embodiment of the present invention shown in FIG. 3, and the maximum conversion gain is obtained when the emitter current is approximately 5.5 mA. Fifth
A dashed-dotted line B in the figure shows the conversion gain characteristic of the self-oscillation mixing circuit with one transistor shown in the conventional example shown in FIG. In the case of the self-oscillating mixing circuit of the present invention, the conversion gain is higher than 7 dB, and this is thought to be due to the use of two transistors as in the conventional cascode mixing circuit shown in FIG. Furthermore, while the emitter current showing the maximum conversion gain of the self-oscillating mixing circuit shown in FIG. 1 is 1 to 2 mA, the emitter current showing the maximum conversion gain characteristic of the self-oscillating mixing circuit of the present invention is 5 to 6 mA,
The mixing operation is not operated at a low emitter current with poor nonlinear distortion characteristics, but is operated at an emitter current more than three times that of the self-oscillating mixing circuit shown in Figure 1, and as a result, the cross-modulation disturbance characteristics are improved. Thus, a self-oscillating mixing circuit with good intermodulation interference can be obtained. Furthermore, the dotted line C in FIG. 5 shows the conversion gain characteristic of the embodiment of the present invention shown in FIG. 4, and the conversion gain is improved compared to the embodiment shown in FIG. It is thought that the current of the intermediate frequency component flowing through the base-emitter junction of No. 1 increases, and the conversion gain improves.

As explained above, compared to the conventional example shown in FIG. 1, the self-oscillating mixing circuit of this embodiment can be operated with an increased emitter current and has a high frequency conversion gain. The input level of the high-frequency signal input to the device can be reduced. Therefore, intermodulation interference and cross-modulation interference can be improved. Furthermore, the first transistor 1 and the second transistor 9 perform not only frequency conversion and intermediate frequency amplification operations but also oscillation operations, and the number of components is reduced compared to the conventional cascade mixing circuit shown in FIG. Therefore, it has the advantage that a low-cost circuit can be constructed.

An application example in which the self-oscillating mixing circuit according to the present invention is used in a VHF tuner of a television receiver is shown in FIG. 6, and the operation of the circuit according to the present invention will be explained in detail.
23 and 24 are capacitors connected to the emitter of the first transistor 1, and 25 is a capacitor connected to the emitter of the first transistor 1, which is switched between low band (low frequency band) and high band (high frequency band). , a switching diode for making the amount of feedback appropriate for each frequency band, 26 a DC blocking capacitor, 27 a coil for improving intermodulation interference characteristics, 28 a damping resistor for the coil 27, 29 and 30 a second The auxiliary feedback capacitor 31 connected between the input and output of the second transistor 9 and the first transistor 1 is a tracking correction capacitor that also serves as DC blocking, 32 is a tuning diode for changing the local oscillation frequency, and 33 is a high voltage A high band tuning coil for oscillating the band, 34 a low band tuning coil for oscillating the low band, 35 a switching diode 36 for switching the oscillating frequency band, a power supply terminal to which a power supply voltage is applied when receiving the low band; 37 is a power supply terminal that applies power supply voltage during high band reception;
38 is a tuning voltage terminal.

When receiving a low band signal, a voltage is applied to the power supply terminal 36 and no voltage is applied to the power supply terminal 37. Therefore, the switching diode 35 is cut off, and the tuning coils 33 and 34 are connected in series, and a parallel resonant circuit is formed by this, the tuning diode 32, and the tracking correction capacitor 31, and a local resonance circuit corresponding to the low band is formed. Resonates with the oscillation frequency. In addition, the switching diode 25
is in a conductive state, so a parallel connection of capacitors 23 and 24 is connected to the emitter of the first transistor 1, and at the same time, the amount of feedback by the feedback capacitor 13 is set to a value suitable for the low band oscillation frequency, and at the same time, the first By increasing the grounded emitter capacitance of the transistor 1, the grounded emitter impedance at a low band high frequency signal frequency is lowered, thereby preventing a reduction in conversion gain.

On the other hand, when receiving high band signals,
A voltage is applied to the power supply terminal 37 and no voltage is applied to the power supply terminal 36. Therefore, the switching diode 35 becomes conductive, the tuning coil 34 is equivalently short-circuited, and the parallel resonant circuit formed by the tuning coil 33, the tuning diode 32, and the tracking correction capacitor 31 generates a high band signal. It resonates at the local oscillation frequency corresponding to the Also,
Since the switching diode 25 is cut off, the capacitor 23 is the only capacitor connected to the emitter of the first transistor 1, and the amount of feedback by the feedback capacitor 13 is set to a value suitable for the high band oscillation frequency. At this time, the grounded emitter capacitance value of the first transistor 1 decreases compared to when receiving the low band signal, but since the high frequency signal frequency is high, the grounded impedance of the first transistor 1 with respect to the input high frequency signal frequency does not change much. Conversion gain hardly decreases.

By the above-described operation, a local oscillation output corresponding to the low band and high band is obtained, and when a high frequency signal is inputted from the high frequency input terminal 2 at this time, a predetermined intermediate frequency signal is obtained from the intermediate frequency output terminal 4.

Next, the effect of the coil 27 shown in FIG. 6 will be described. As is conventionally known, intermodulation interference characteristics can be improved by connecting a capacitor between the connection point between the first transistor 1 and the second transistor 9 and the ground. However, since the self-oscillation mixing circuit according to the present invention performs local oscillation operation, adding a capacitor makes it difficult to oscillate particularly at high frequencies. Therefore, in the present invention, intermodulation interference characteristics are improved by adding a coil in place of the conventional capacitor. The effect of adding the coil 27 is shown in FIG. Figure 7 shows the intermodulation interference characteristics when receiving the 6th channel of the United States when the capacitance value of the capacitor 26 is 2200 pF and the inductance value of the coil 27 is changed from about 10 nH to 70 nH. An interference ratio of -55dB was obtained. It is clear that the intermodulation interference characteristics can be improved by setting the inductance value of the coil 27 to a certain fixed value. This means that the intermodulation distortion when receiving US 6 channels is the local oscillation signal frequency and 6 channels.
It is estimated that this is caused by a difference signal between the frequency component of the sum of the video and audio carrier waves of the channel, and in the self-oscillating mixing circuit of the present invention, not only transistor 1 but also transistor 9 is included.
This is caused by second-order distortion, and by adding an inductance, a high-frequency current bypass is formed, which changes the transfer characteristics of high-frequency signals in the self-oscillating mixing circuit, which is a cascade connection of transistors 1 and 9, and improves the second-order distortion characteristics. It seems that things are improving.

In the self-oscillating mixing circuit of the present invention, the inductance 2
At high oscillation frequencies, the impedance of transistors 7 becomes high and has almost no effect on the oscillation operation, and at low oscillation frequencies, the amplification degree of the cascaded circuit of transistors 1 and 9 at the oscillation signal frequency decreases due to inductance, but transistors 1 and 9 Since the gain of the amplifier element increases as the frequency decreases, it is possible to compensate for this decrease, obtain the necessary amplification degree even at low frequencies, and perform stable oscillation operation. As mentioned above, when the method for improving intermodulation interference using a capacitor in the separately excited cascode mixing circuit shown in the conventional example shown in FIG. This, combined with the fact that the amplification of the transistor decreases as the frequency increases, makes it difficult to operate stably at high oscillation frequencies. However, the method using an inductor according to the application example of the present invention stabilizes the oscillation operation over a wide range of frequencies and stabilizes the oscillation operation over a wide range of frequencies. This has the effect of improving modulation interference characteristics. As a result, it has become possible to put it into practical use as a tuner for receiving American television signals, which was difficult to do with a self-oscillation mixing circuit using a single transistor, and it also eliminates the need for a separate local oscillation circuit compared to the conventional cascode mixing circuit. This has the effect of reducing the number of parts and providing a low-cost circuit. Note that resistance 2
8 is for averaging the impedance of the coil 27 over a wide frequency range, and is not necessarily required.

Further, the capacitor 29 has the effect of improving the stability of the local oscillation frequency against fluctuations in the power supply. As mentioned above, during high band reception, the switching diode 35 becomes conductive, and the tuning coils 33 and 3
4 is grounded by a switching diode 35 and a bypass capacitor, but is actually connected to the emitter of the second transistor 9 by a feedback capacitor 29 due to the lead inductance of the switching diode and bypass capacitor. will be returned. Therefore, the feedback capacitor 29 is effective in both the low band and the high band, and this feedback can reduce the change in the output impedance of the second transistor 9 when the power supply fluctuates.
The local oscillation frequency can be stabilized. capacitor 2
FIG. 8 shows the effect of 9. The figure shows the power supply voltage ±
It shows the local oscillation frequency change when the local oscillation frequency changes by 10%, and when a capacitor 29 with a capacitance of 2pF is added, as shown in curve B, the frequency change is approximately It can be improved by half. The capacitor 29 is considered to operate as a negative feedback for the oscillation operation, and as the power supply voltage increases and the oscillation amplitude increases, the output impedance of the collector of the transistor 9 changes, causing fluctuations in the oscillation frequency.
9, a part of the oscillation voltage is negatively fed back to prevent the oscillation amplitude from changing excessively, and when the power supply voltage decreases and the oscillation amplitude becomes smaller, the amount of negative feedback decreases, and the drop in the oscillation amplitude is similarly kept small. Overall, it is considered that changes in oscillation amplitude due to power supply voltage fluctuations can be reduced, changes in output impedance of the collector of transistor 9 can be reduced, and oscillation frequency fluctuations can be suppressed to a small level.

Next, the effect of the capacitor 30 will be described.
The capacitor 30 is mainly effective in improving cross-modulation interference characteristics. This is shown in FIG. The figure shows the 1% cross-modulation interference characteristics of the self-oscillating mixing circuit according to the present invention, and the 1% cross-modulation interference characteristics due to interference signals two channels apart when receiving a US channel are as follows:
When the capacitor 30 with a capacitance value of 2 pF is added, the degree of improvement in the low band is particularly large compared to the case where the capacitor 30 is not added, and the effect of the present invention is clear. This is considered to be due to the slight negative feedback from the output of transistor 1 to the input, which improves the nonlinearity due to third-order distortion of the mixing operation roughly performed by transistor 1.

Note that in the embodiments described above, a case has been described in which NPN transistors are used as transistors 1 and 9, but exactly the same effect can be obtained even if PNP transistors are used. Also with NPN
The same applies if PNP transistors are used in combination.
Furthermore, in the embodiment of the present invention, the first transistor 1 and the second transistor 9 are also connected in terms of DC, but each transistor is separated in terms of DC,
A similar effect can be obtained by applying biases separately. In addition, in the embodiment, the first transistor 1 and the second transistor
Although the collector currents of the transistors 9 are made almost equal, it is clear that the same effect can be obtained even if the bias circuit is set so that the collector current values of the transistors 1 and 9 are different.

As described above, according to the present invention, two transistors are used, one of which is grounded with its emitter, to address the cross-modulation and intermodulation disturbance characteristics that were the drawbacks of the conventional self-oscillation mixing circuit using one transistor. , by connecting these in cascade with the other connected to the base, it is greatly improved, and the local oscillation transistor and bias circuit required in the conventional cascode mixing circuit are also made to perform local oscillation operation using the mixing transistor. I was able to delete it. As a result, compared to the conventional self-oscillating mixing circuit, the above-mentioned interference characteristics were improved by more than 10 dB, and the conversion gain was also increased by more than 7 bB. Further, the industrial value of the present invention is extremely large, as these favorable characteristics can be achieved with a smaller number of parts than conventional cascode mixing circuits.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing a conventional self-oscillating mixing circuit.
FIG. 2 is a circuit diagram showing a conventional cascode mixing circuit, FIG. 3 is a circuit diagram showing an embodiment of a self-oscillating mixing circuit according to the present invention, and FIG. 4 is a circuit diagram showing another embodiment of a self-oscillating mixing circuit according to the present invention. FIG. 5 is a characteristic diagram showing the conversion gain of the circuits shown in FIGS. 1, 3, and 4. FIG. 6 is a circuit diagram showing the conversion gain of the circuits shown in FIGS. A circuit diagram showing an example of application, Figure 7 is a characteristic diagram showing changes in intermodulation interference characteristics depending on the coil inductance value, and Figure 8 is a characteristic diagram showing improvement in local oscillation frequency stability by an auxiliary feedback capacitor. , FIG. 9 is a characteristic diagram showing the improvement in cross-modulation disturbance characteristics by the auxiliary feedback capacitor. 1, 9...Transistor, 2...High frequency input terminal, 4...Intermediate frequency output terminal, 13...Feedback capacitor, 23, 24...Emitter grounding capacitor, 25...Switching diode, 26...DC blocking capacitor, 27...Coil, 29,30...
Auxiliary feedback capacitor.

Claims (1)

[Claims] 1. A first transistor whose emitter is appropriately grounded by a first capacitor, whose base is connected to a high-frequency input terminal into which a high-frequency signal is input, and whose emitter is connected to the collector of the first transistor. is,
a second transistor whose base is grounded at high frequency; a resonant circuit consisting of a resonant capacitor and a resonant coil, one terminal of which is connected to the collector of the second transistor and the other terminal of which is grounded; an intermediate frequency tuning circuit having an intermediate frequency output terminal connected to the collector of the transistor, and an intermediate frequency tuning circuit having one terminal connected to the collector of the second transistor and the other terminal connected to the emitter of the first transistor. A self-oscillating mixing circuit characterized by comprising a feedback capacitor. 2. A first transistor whose emitter is appropriately grounded by a first capacitor, whose base is connected to a high-frequency input terminal into which a high-frequency signal is input, and whose emitter is connected to the collector of the first transistor.
a second transistor whose base is grounded;
A resonant circuit consisting of a resonant capacitor and a resonant coil, one terminal of which is connected to the collector of the second transistor, and the other terminal of which is grounded, is connected to the collector of the second transistor, and the intermediate frequency output terminal is connected to the resonant circuit. an intermediate frequency tuning circuit; a feedback capacitor having one terminal connected to the collector of the second transistor and the other terminal connected to the emitter of the first transistor; and a feedback capacitor connected between the base and the emitter of the first transistor. What is claimed is: 1. A self-oscillating mixing circuit comprising: an intermediate frequency trap;