JP3839123B2 - Tunable frequency variable filter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a tuning frequency variable filter that is used in a transmitter / receiver of frequency hopping communication and is configured to be able to switch a tuning frequency at a high speed corresponding to the hopping frequency, and more particularly to an improvement of the switching circuit.
[0002]
[Prior art]
As a tuning frequency variable filter of a high frequency band of, for example, 20 to 70 MHz used for a transceiver of a frequency hopping communication device, a tuning capacitor is switched by a switching element such as a pin diode (PIN diode) to change the tuning frequency. Filters are in practical use.
[0003]
In a conventional tuning frequency variable filter, for example, a plurality of series circuits of additional capacitors and pin diodes having the same or different capacitance values are connected in parallel to a main tuning circuit constituting a parallel resonant circuit, and a bias current is used. The tuning frequency is changed by changing the additional capacitance value by turning on / off the pin diode of each parallel arm.
[0004]
FIG. 3 is a diagram showing a partial configuration example of a conventional tuning frequency variable filter, which is a one-stage portion having n switching circuits. The number of switching circuits (n) is often eight, and the filter has many cascade connection configurations of two to three stages.
In the figure, Z ₀ is a main tuning circuit equivalently composed of a parallel resonance circuit of a main inductance and a main capacitor. For example, a λ / 4 cavity resonator or a helical resonator is used.
C _{1 to} C _n are additional capacitors for making the tuning frequency variable. When the pin diodes D _{11 to} D _n1 are on, they are added in parallel to the main tuning circuit Z ₀ , and the tuning frequency changes.
[0005]
L _{11 to} L _n1 are choke coils, and when a bias voltage V _B is applied, direct current flows through the pin diodes D _{11 to} D _n1 .
C _{12 to} C _n2 are high frequency bypass capacitors.
V _B is a positive or negative bias voltage, and is a control voltage for turning on / off the pin diodes D _{11 to} D _n1 .
[0006]
Figure 4 is an explanatory view of a conventional switching circuit, (A) is a circuit illustration of the switching circuit 1 Tsunotoki, (B) the voltage of a point when the pin diode D ₁₁ is turned on it is a characteristic illustration of V _a.
[0007]
The operation principle of this circuit will be described.
FIG. 5 shows an equivalent circuit of a pin diode. (A) is a reverse bias, the switch is in an OFF state, and (B) is an ON state in which a forward current flows. It can be seen that the pin diode is not completely turned on / off unlike the ideal switch. For example, when the pin diode is off, the equivalent capacitance C _p = 1.5 pF and the equivalent resistance r _p = 1000 kΩ, and when it is on, the equivalent resistance r _s = 0.2 Ω.
[0008]
In FIG. 4, a bias voltage V _B is applied to the pin diode D ₁₁ through a choke coil L ₁₁ having an alternating high impedance. When the bias voltage is positive, a forward current flows, and the pin diode D ₁₁ has a low resistance r _s . When the bias voltage is negative, D ₁₁ indicates a high resistance r _p . By turning on / off the pin diode of the switch element D ₁₁ by positive / negative of the bias voltage V _B , one end of the capacitor C ₁ is grounded or opened.
[0009]
When a forward bias current is applied by applying a positive bias voltage to the pin diode, the resistance value of the pin diode decreases and one end of the additional capacitor C ₁ is grounded. The additional capacitor C ₁ is a parallel resonant circuit that is a main tuning circuit. It functions as an additional capacitor of Z ₀ and the tuning frequency is lowered.
[0010]
Further, when a reverse bias is applied to turn off the pin diode, the resistance value of the pin diode increases, and one end of the additional capacitor C ₁ is opened, so that it does not function as an additional capacitor and the tuning frequency is that of the parallel resonant circuit Z ₀ . Resonant frequency.
By the operation as described above, the tuning frequency can be changed by switching the capacitor (capacitance value).
[0011]
[Problems to be solved by the invention]
However, the conventional tuning frequency variable problem of the filter when the pin diode by the application of the bias voltage V _B is turned off, excessive current flows to the capacitor C _1, the charge is accumulated in the capacitor C _1. For this reason, the following problems arise.
1) In addition to -V _B applied to the capacitor C ₁ , a voltage corresponding to the AC power input to the filter is generated in the resonance circuit Z ₀ , and the point a of the capacitor C ₁ as shown in FIG. 4B. A voltage in which direct current and alternating current are superimposed is applied to. Therefore, since charges are accumulated in C ₁ when the pin diode is turned off, a capacitor with a high withstand voltage is necessary and expensive.
2) when the pin diode D ₁₁ is off, -V _B is charged to the combined capacitance of the bypass capacitor C ₁₂ tuning frequency varying capacitor C _1. Therefore, the time constant when the pin diode D ₁₁ operates the switch increases in proportion to the combined capacitance, as a result, there is a problem that the switching speed becomes slow takes switching time.
3) Generally, when a high-voltage DC bias is applied to a capacitor, a physical change due to a potential difference occurs, and the capacitance value of the capacitor changes slightly. When an AC signal is input to a circuit that causes this change, intermodulation distortion is generated. In particular, when the input high-frequency signal is a digital modulation system, the occupied band is expanded, which is a serious problem.
[0012]
Furthermore, in the example of this circuit in which the potential is applied to one side, even if the same capacitor is used, the amount of occurrence is significant. In the case of the conventional circuit, since a bias voltage is applied to the capacitor C ₁ and a high frequency signal is superimposed on the capacitor C ₁ , there is a problem that the capacitance value of the capacitor C ₁ changes and distortion is likely to occur.
[0013]
An object of the present invention is to solve the problems of the prior art, fast switching speed, low breakdown voltage of the additional capacitor C _1, a tuning frequency variable filter comprising a capacitor switching circuit having no effect on the intermodulation distortion It is to provide.
[0014]
[Means for Solving the Problems]
The tuning frequency variable filter of the present invention includes a main tuning circuit including a parallel resonance circuit of a main inductance and a main capacitor, a plurality of additional capacitors connected in parallel to the main tuning circuit to change a tuning frequency, and the plurality of capacitors. An additional capacitor switching circuit configured by a plurality of series circuits each including a pin diode connected in series to each of the additional capacitors and a bias circuit for applying a bias voltage to the pin diode; and changing the polarity of the bias voltage. In the tuning frequency variable filter configured to change the tuning frequency by changing the additional capacitance by the additional capacitor connected in parallel to the main tuning circuit by turning on and off the pin diode,
In each of the series circuits of the additional capacitor switching circuit, one end of the additional capacitor is connected to the high potential side of the main tuning circuit, the other end is connected to the cathode terminal of the pin diode, and the anode terminal of the pin diode is It is connected to a bias voltage application terminal and grounded by a bypass capacitor in an alternating manner, and a choke coil is connected between a connection point between the additional capacitor and the pin diode and the ground.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, by connecting one end of the choke coil of the conventional switching circuit to the ground and applying the bias voltage V _B directly to the pin diode D ₁₁ , the switching speed is fast and the withstand voltage of the capacitor may be low. A tuning frequency variable filter including a capacitor switching circuit having little influence on distortion can be realized.
[0016]
FIG. 1 is a block diagram showing an embodiment of the present invention, which is an embodiment using a plurality of switching circuits forming the main part of the present invention. In this figure, the one-stage part of the filter is shown, but actually, a plurality of stages are connected in cascade.
In the figure, Z ₀ is a main tuning circuit constituting a parallel resonance circuit of a main inductance element and a main capacitor. C _{1 to} C _n are additional capacitors for tuning frequency adjustment. D _{11 to} D _n1 are pin diodes that turn on / off the additional capacitors C _{1 to} C _n with respect to the ground. L _{11 to} L _n1 are choke coils. C _{12 to} C _n2 are bypass capacitors. V _B is a bias voltage, which is a positive / negative control voltage for turning on / off the switch elements D _{11 to} D _n1 .
[0017]
The operation principle of changing the tuning frequency of the filter according to the present invention is that the bias voltage V _B is applied to the control voltage application terminal, and the pin diodes (D _{11 to} D _n1 ) are turned on / off to change the tuning frequency. The tuning frequency is changed by changing the additional capacitance value by turning on / off the additional capacitors (C _{1 to} C _n ). If n application terminals of the control voltage V _B are individually combined, the tuning frequency can be changed to 2 ⁿ types.
[0018]
2 is an explanatory diagram of the switching circuit of FIG. 1. FIG. 2A is a circuit example diagram when there is one switching circuit, and FIG. 2B is a diagram of the point a when the pin diode D ₁₁ is turned on. it is a characteristic illustration of the voltage V _a.
When a positive bias is applied to V _B and a predetermined forward current is passed through the pin diode D ₁₁ , the capacitor C ₁ is grounded and the tuning frequency is lowered. When a negative bias voltage is applied to V _B , a reverse bias voltage is applied to the pin diode D ₁₁ to exhibit a high resistance, and the capacitor C ₁ becomes open and the tuning frequency increases.
[0019]
Since the pin diode D ₁₁ is grounded at both ends of C ₁ regardless of whether it is on or off, DC charge is not accumulated in the capacitor C ₁ unlike the conventional circuit. As shown in FIG. 2 (B), since unnecessary transient characteristics are eliminated, the switching speed is fast, the withstand voltage of the capacitor is low, and distortion due to application of a DC voltage to the capacitor is small.
[0020]
【The invention's effect】
As described above in detail, the following effects can be obtained by implementing the present invention.
(1) The withstand voltage of the additional capacitor C ₁ for tuning frequency adjustment may be low.
(2) The switching time is shortened and the switching speed is increased.
(3) Generation of intermodulation distortion can be eliminated.
[Brief description of the drawings]
FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention.
FIG. 2 is a diagram showing an example of a switching circuit forming a main part of the present invention.
FIG. 3 is a diagram illustrating an example of a conventional filter circuit.
FIG. 4 is a diagram illustrating an example of a conventional switching circuit.
FIG. 5 is an equivalent circuit diagram of a pin diode.
[Explanation of symbols]
Z ₀ main inductance element and main capacitor main tuning circuit C _{1 to} C _n tuning frequency variable additional capacitor D _{11 to} D _n1 pin diode C _{12 to} C _n2 bypass capacitor L ₁₁ choke coil V _B bias control voltage

Claims (1)

A main tuning circuit comprising a parallel resonance circuit of a main inductance and a main capacitor, a plurality of additional capacitors connected in parallel to the main tuning circuit to change the tuning frequency, and connected in series to each of the plurality of additional capacitors An additional capacitor switching circuit configured by a plurality of series circuits including a pin diode and a bias circuit for applying a bias voltage to the pin diode is provided, and the pin diode is turned on / off by changing the polarity of the bias voltage. In the tuning frequency variable filter configured to change the tuning frequency by changing the additional capacitance by the additional capacitor connected in parallel to the main tuning circuit,
In each of the series circuits of the additional capacitor switching circuit, one end of the additional capacitor is connected to the high potential side of the main tuning circuit, the other end is connected to the cathode terminal of the pin diode, and the anode terminal of the pin diode is A tuning frequency variable filter connected to a bias voltage application terminal and AC-grounded by a bypass capacitor, and a choke coil is connected between the connection point of the additional capacitor and the pin diode and the ground .

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