JP2567151Y2 - Mixer - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mixer, and more particularly, to an overload protected mixer. 2. Description of the Related Art Mixers are used for frequency conversion and phase detection of spectrum analyzers, selective level meters, PLLs, radio receivers and the like. FIG. 2 shows a general form of a general double-balanced mixer. The mixer shown is a diode bridge 10 consisting of four diodes 11 to 14 connected in series to form a loop.
Is used. The pair of transformers 17 and 111 are connected to the diode bridge 1 so that input signals from a pair of input signal sources such as the local oscillator 15 and the high-frequency source 19 can be supplied.
Connected to 0. The nodes B and D of the diode bridge are connected to both ends of the secondary winding 38 of the transformer 17, and the center tap of the secondary winding 38 is grounded. Similarly, nodes A and C of diode bridge 10
Are connected to both ends of the secondary winding 312 of the transformer 111. An output signal is generated at the center tap of the secondary winding 312 of the transformer 111. This signal is shown in FIG. 2 as the output signal IF between the center tap and ground. When this mixer is used for frequency conversion,
The amplitude of the input signal from the local oscillator 15 is generally set to be much larger than the amplitude of the high-frequency input signal. In this case, the voltage drop across the diodes 11 to 14 is
7 depends on the voltage supplied to the diode loop. Since the center tap of the secondary winding of the transformer 17 is grounded, the local oscillator 15 drives at a voltage obtained by subtracting the voltage at the node D from the node B. Therefore, when node B is at a high voltage, diodes 11 and 12 are reverse biased, and diodes 13 and 14 are forward biased. Accordingly, the path from the node A to the center tap of the secondary winding of the transformer 17 via the diodes 11 and 12 has a high impedance, and the center C of the secondary winding of the transformer 17 via the diodes 13 and 14 from the node C. -The path to the tap has low impedance. Therefore, diodes 11 to 14
Operate as switches, each having a high impedance or low impedance state, in response to a local oscillator signal. The diodes 13 and 14 are connected to a local oscillator 15
The characteristics of these diodes are made uniform with respect to the quasi-directional current due to the input signal of FIG. If the impedances are equal, the node C is at the ground potential when there is no high-frequency input signal. When the high-frequency input signal is small compared to the local oscillator input signal, the voltage at the node C keeps substantially the ground potential. However, when the high-frequency input signal is small, a large voltage amplitude is generated at the node A due to the diodes 11 and 12 having high impedance due to reverse bias. Therefore, the transformer 111
The lower end of the secondary winding is maintained at ground potential, while the upper end of the winding is floating (i.e., nodes B and D and this secondary winding because diodes 11 and 12 are in a high impedance state due to reverse bias). It is effectively cut off from the upper end of the line). Therefore, the output signal I
F is equal to the signal generated in the lower half winding by the high frequency input signal. When the local oscillator input signal swings to the negative side,
The diodes 11 and 12 are forward biased, and the diodes 13 and 14 are reverse biased. Since the diodes 11 and 12 have the same characteristics, they have the same forward bias impedance in the operating state. For the same reason as described above, the upper side of the secondary winding of the transformer 111 is effectively held at the ground potential, and the lower side is in a floating state. As a result, the polarity of the output signal IF is inverted each time the polarity of the local oscillator input signal is inverted. Therefore, the output signal IF is proportional to the product of the high-frequency signal and the square wave, which is an alternating voltage having the same amplitude and opposite polarity. Therefore, the output signal includes a frequency component equal to the frequency of the high-frequency input signal multiplied by an integer times the frequency of the local oscillator input signal and added or subtracted. In fact, in the case of a mixer, the output signal contains a frequency component that is slightly different from the above description, and is equal to the result of adding or subtracting the integer multiple of the frequency of the high frequency signal from the integer multiple of the input signal from the local oscillator. It is. [0006] The local oscillator input signal is generally selected to be large enough to allow current to flow near the point of maximum forward bias current such that the diode will not be damaged. This is because the impedance at the time of forward bias is reduced so that the nodes of the two forward-biased diodes are almost at the ground potential. (Since the high-frequency input signal is small, it is appropriate to evaluate the effect of this signal.) The important impedance is a value obtained by differentiating the voltage between both ends of the diode with the current flowing through the diode, that is, dV / dI, and is inversely proportional to the current I. However, setting the operating conditions in this way has the disadvantage that the diode is particularly susceptible to damage when there is an input signal exceeding the rated operating input signal to the diode. [0007] In many applications, the mixer is located at an input port of the device, and a local oscillator input signal and / or a high frequency input signal is provided thereto by a user. Therefore, in such a case, it is necessary to protect the mixer when the input signal exceeds the rated range. The main current method of providing protection against excessive power that can damage the diode is to inform the user of the maximum allowable power of the diode and adhere to it (obviously in an unreliable way). However, there is a case in which a circuit for impedance matching or attenuation called an amplifier or a pad (pads) having a built-in overload protection function is provided in front of the mixer. This overload protection may be in the form of a voltage clamp that limits the maximum input voltage to a certain predetermined value. As another protection method, there is a circuit shown in FIG. In the figure, a diode bridge 20 has a pair of diodes on each side (ie, a diode 21,
213, diodes 24, 214, diodes 23, 2
15 and diodes 24, 216).
This distributes the power consumption on each side to the two diodes in the pair, and the diode bridge can handle twice the power without damage. However, assuming that this diode bridge has the same operating characteristics as the circuit shown in FIG. 2, it has the same forward current characteristics as the diode bridge shown in FIG. Therefore, the mixer of FIG. 3 requires twice the input power of the mixer shown in FIG. 2, and the risk of damage is increased only by the increase in power at the same rate as the circuit shown in FIG. Appears similarly.
Thus, in the prior art, the current flowing through the mixer is
The technical idea of protecting the mixer by controlling the eyes
Is not planned, and therefore provides insufficient protection against overload.
there were. SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned disadvantages of the prior art and to provide a mixer which eliminates the risk of destruction due to overload. SUMMARY OF THE INVENTION In a preferred embodiment of the present invention, a double balanced mixer with inherent overload protection is provided. The mixer uses a diode bridge having four sides connected at four points to form a closed loop. A first input source is connected via a first transformer to a first pair of opposing nodes in the loop. Also, the second
Are connected to other node pairs in the loop via a second transformer. In the preferred embodiment, each side of the loop includes a diode and a variable impedance element. In another embodiment of the invention, the sides in the loop include diodes and the leads from each transformer to the diode loop include a variable impedance element. The cathode of each diode is connected to the anode of another diode in the loop. In the preferred embodiment, this connection is made through a variable impedance element. The center of the secondary winding of the first transformer
Tap is maintained at a fixed voltage V _1. The output signal is generated by the difference between the center tap and the voltage V ₀ which secondary winding of the second transformer. The characteristics of the four diodes and the four variable impedance elements are aligned to have the same characteristics under recommended operating conditions. By performing such matching, both input signal frequency components in the output signal IF can be removed. This is useful because the most important component of the output signal is the sum or difference between the frequencies of both input signals. The variable impedance element can take various configurations. In any case, the variable impedance element must have low impedance in the rated operating state of the circuit, and without the variable impedance element, the diode bridge is High impedance must be present for input signals that far exceed the rated operating conditions that would be damaging. In a preferred embodiment, a junction FET (JFET) having a gate connected to a source or a drain is used as the variable impedance element. An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a circuit diagram of a double balanced mixer having an inherent overload protection means according to a preferred embodiment of the present invention. The mixer has a diode bridge 30 connected at nodes A, B, C and D such that four sides form a loop ABCD. Diode bridge 3
Each side of 0 includes one of the diodes 31 to 34, respectively. Each cathode of these diodes 31 to 34 is connected to another one of these diodes via a variable impedance element. A signal from an input signal source such as a local oscillator 35 is supplied as a first input signal. In some applications, this input signal is provided by the user and is completely under the control of the user. The local oscillator 35 is connected to one end of the primary winding 36 of the first transformer 37. The other end of the primary winding is maintained at a reference potential V _0. In this embodiment, the reference potential V ₀ which is a ground potential. Transformer 37
Both ends of the secondary winding 38 each node B, is connected to the D, also the center tap is kept at a reference potential V _1. In this embodiment, the reference potential V ₁ was a ground potential. A second signal from an input signal source such as the high-frequency signal source 39
Are supplied. This signal is typically supplied by the user, in which case there is no need to generate a high frequency signal in the mixer. The high frequency signal source 39 is connected to one end of the primary winding 310 of the second transformer 311. The other end of the primary winding is maintained at a reference potential V _2. This reference potential is the ground potential in this embodiment. One end of the secondary winding 312 of the transformer 311 is connected to the node A, and the other end is connected to the node C. The output signal IF is the secondary winding 31
It is generated as the second center tap and the difference between the voltage of the reference potential V _1. Each side of the diode bridge 30 has a J
Variable impedance elements such as FETs 313 to 316 are provided. Each of the JFETs 313-316 is associated with each side of the diode bridge 30 and provides inherent overload protection. Since the gate of each of the JFETs 313 to 316 is directly connected to the source, when the drain current _ID is small, the impedance between the source and the drain is low. Also, as the drain current _ID increases, the source-drain impedance increases. JFET
This characteristic can be seen from the curve for V _GS = 0 in FIG. In selecting each of these JFETs, as long as the mixer is not overloaded and the diode associated with each JFET is operating at forward bias, the source-drain impedance of the JFET will be The impedance is set to be equal to or less than the impedance of the associated diode. Each of these JFETs also has a sudden increase in its differential impedance (its JFE) as the diode current approaches the maximum diode current that the diode can carry without damage.
It must be preferably several orders of magnitude greater than the differential impedance of the diode associated with T), and is thus selected to limit the current flowing through the associated diode. In fact, JFETs 313-316 transition from a low differential impedance state to a high impedance state when the current level is near, but lower (eg, 90% of the maximum current level) of the associated diode. As can be understood from FIG. 4, JFET has _ID = 0.
Near the first quasi-linear region (low differential impedance state). The differential impedance (dV / dI) in the first quasi-linear region has a value several orders of magnitude lower than the differential impedance in the quasi-linear region (high differential impedance state) that appears again when the drain current _ID is increased. In the selection of the JFETs 313 to 316 shown in the circuit of FIG. 1, when the diodes 31 to 34 are not overloaded, the forward bias current flowing through the diodes 31 to 34 falls within a quasi-linear region near _ID = 0. To do.
The maximum current of the diodes 31 to 34 is JFET31.
Try to fall within another quasi-linear region of 3-316. The characteristics of the JFETs 313 to 316 are also uniformed, so that when the diodes 33 and 34 on both sides of the node C are forward-biased, the potential of the node C basically becomes V ₁ , and the potential of the node a when the both sides of the diodes 31 and 32 are forward biased basically to such becomes the reference potential V _1. The drain current curve as a function of gate-source voltage has the same characteristics for both MOSFETs and JFETs. Therefore, as another simple choice of the variable impedance element, a MOSFET having a gate and a source connected can be used. If an FET in which a gate and a source (that is, majority carriers are supplied from here) are used is used, characteristics as shown in FIG. 4 can be obtained. Therefore, either n-channel or p-channel FETs
But can be used. Since the characteristic curve when the potential difference between the gate and the source is not zero is similar to that when the potential difference is zero, a more complicated variable impedance generator in which the voltage between the gate and the source is a function of the drain current is used. Can be used. When such a variable impedance generator is used, it can be designed so that the transition from the low impedance state to the high impedance state becomes steeper, but there is a disadvantage that the circuit becomes more complicated. FIG. 5 is a view showing another embodiment of the present invention. In this embodiment, the protection of the diodes 51-54 forming the diode bridge 50 from overload is:
Four leads 517- from transformers 57 and 511
520 each having a variable impedance element 513-5.
16 are inserted one by one. These variable impedance elements 513 to 516 are selected based on the same criteria as in the embodiment shown in FIG. That is, these variable impedance elements must have the same characteristics, and have an impedance substantially equal to or lower than the forward biased impedance of the diodes 51 to 54 when not in an overload state, Also, as the current approaches the maximum value of the diodes 51 to 54, the impedance of these variable impedance elements must have a much larger value than the diodes 51 to 54. However, the variable impedance elements 513-
516 is a variable impedance element 1313-3 in FIG.
There is one important difference as compared to 16. That is, the variable impedance elements 513 to 516 must be bidirectional. In the circuit of FIG. 1, for example, diodes 31 and 32 basically block a current flowing in a direction from node B to node D. Therefore, the variable impedance elements 313 and 314 do not need to have bidirectionality, but need only have desired characteristics in a single direction, that is, only in a direction from the node D to the node B. Unlike this, the variable impedance element 513
In the case of ５516, the element must be a bidirectional element. This is because it is necessary for a current of both polarities to flow, and whether or not the variable impedance elements 513 to 516 exhibit the current limiting function is determined not by the polarity of the current but by the absolute value of the current. . Suitable examples of such variable impedance elements 513 to 516 include, for example, L.M.
J. 114 of "Field Effect Transistor" by Sevin
The bidirectional constant current source described on page can be used. The present invention can be widely applied to other than the double balanced type mixer shown in FIG. Generally, a mixer uses at least one nonlinear element. The nonlinear element generates an output signal in response to a pair of input signals. The Fourier expansion component of this output signal includes the sum or difference of the product obtained by multiplying the frequency of the Fourier expansion component of both input signals by an integer. Various mixers have been designed to suppress specific Fourier expansion components of the output signal (eg, to be able to suppress an integer multiple of the input signal). In many mixers, the non-linear element is an impedance element connected in series with the input source, the impedance of which is a function of the current flowing through the element. In order to operate as a mixer, this impedance must cause a voltage drop in a non-linear function to the current flowing in the operating region of the mixer. Also, in order to provide protection against overload, the differential impedance of this non-linear element must increase sharply in adjacent restricted areas on both sides of the operating area. Since the differential impedance of the diode increases rapidly in a reverse bias condition, one of the adjacent regions will have some inherent overload protection characteristics. However, when a deep forward bias is applied, the differential impedance does not increase so much. Therefore, in the other adjacent region, a series connection of a variable impedance element (for example, a JFET or a MOSFET, or a more complicated circuit) that transitions from a low impedance to a high impedance when the current level exceeds the operating state. Must be protected against excessive current. As described above, according to the present invention,
With a simple configuration, a mixer with sufficient protection against overload can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a mixer circuit according to an embodiment of the present invention. FIG. 2 is a diagram showing a circuit of a conventional mixer. FIG. 3 is a diagram showing a circuit of a conventional mixer. FIG. 4 is a diagram showing drain-source voltage versus drain current characteristics of a JFET. FIG. 5 is a diagram showing a circuit of a mixer according to another embodiment of the present invention. [Description of Signs] 10, 20, 30, 50: Diode Bridges 11 to 14, 21 to 24, 213 to 216, 31 to 3
4, 51-54: Diodes 15, 35: Local oscillators 19, 39: High frequency signal sources 17, 111, 37, 311, 57, 511: Transformers 36, 310: Primary windings 38, 312: Secondary windings 313- 316: JFET 513 to 516: the variable impedance element 517 to 520: lead wire A, B, C, D: node IF: output signal _{V 0, V 1, V 2} : reference potential

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Hans au Mortermans               Edmon, Washington, United States               'S Fate Ace Place We               Strike 13509                (56) References JP-A-51-54778 (JP, A)                 JP-A-54-84469 (JP, A)                 JP-A-52-75962 (JP, A)                 JP-A-53-60510 (JP, A)

Claims (1)

(57) [Rules for requesting registration of utility model] A first input port to which a first input signal is applied;
In a mixer having a second input port to which a second input signal to be mixed with the first input signal is applied and an output port, the mixer has four vertices, and an edge between the vertices Each includes a diode bridge having a diode, and a non-linear impedance is inserted into each side of the diode bridge in series with a diode included in the side, and each of the non-linear impedances is
When the dance current does not exceed the specified value, the die
Take a value equal to or less than the impedance of the
When the ambient current exceeds the predetermined value, the die
Take a value sufficiently larger than the differential impedance of the
Mixer, characterized in that that. 2. The mixer according to claim 1, wherein each of the non-linear impedances is an FET having a gate connected to a source or a drain.