JPH077366A - Temperature correcting circuit of narrow band pass filter by crystal filter - Google Patents

Abstract

PURPOSE:To maintain an output level, a band width characteristic and passing band width, which follow a temperature variation so as to be constant by controlling a tuning frequency of the crystal filter by reading out correction data from a temperature correction table derived in advance in accordance with an ambient temperature. CONSTITUTION:Set values of the corresponding D/A converters 13, 23 extending over all working temperatures at every crystal filter and written in a temperature correction table 20. A deviation of a tuning frequency of each crystal filter 14, 24, which follows a variation of an ambient temperature is corrected by a control voltage applied to variable capacity diodes 12, 22. This control voltage is supplied from the D/A converters 13, 23, and a correction value corresponding to the ambient temperature is read out of the table 20 and set. A correction temperature is inputted from a temperature sensor 18 and used as a temperature parameter at the time of referring to the temperature correction table 20 by a control part 19. Fine adjustment of the tuning frequency of the filter 14 is executed by varying a series capacity value of the variable capacity diode 12 connected to the filter 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crystal filter circuit of a multistage structure used for a narrow bandpass filter used at an intermediate frequency, in which a change in pass loss level, a change in pass band width, and a change in ripple. However, the present invention relates to a circuit for performing temperature correction so as not to occur with a change in temperature.

[0002]

2. Description of the Related Art A conventional circuit configuration will be described with reference to FIG. 5 which is a block diagram of a conventional narrow band bandpass filter circuit. Generally, as for the bandwidth selectivity characteristic of the crystal filter circuit used for the bandpass filter, in order to obtain the Gaussian curve characteristic, two or more crystal filters are combined to obtain a predetermined pass bandwidth characteristic and ripple.

That is, by combining a plurality of single-pole filters in stages and appropriately setting the tuning frequencies of the respective crystal filters (see FIG. 4 (a)), a comprehensive characteristic curve of the bandpass filter selectivity ( 4) (see 60 in FIG. 4A).

In FIG. 5, a filter block 8 having a three-stage structure is provided.
1, 82 and 83 are shown. Here, there are amplifiers 21, 31, 41 for amplifying the attenuation loss by the crystal filter of each stage and receiving a signal with high impedance so as not to affect the filter block.
Then, there are trimmer capacitors 55, 56, 57 for finely adjusting the tuning frequencies of the crystal filters 14, 24, 34, and after the crystal filters, for example, a tuning L is used as a load.
By providing C circuits 15, 16, 25, 26, 35, 36,
Tune to the frequency of the bandpass filter. These constitute a bandpass filter.

Each crystal filter has a unique frequency,
Since there is a variation in the center frequency, each stage is adjusted by the trimmer capacitor, and the respective tuning frequencies are shifted and set as shown in 61, 62, 63 of FIG. 4 to obtain the required bandpass characteristic curve 60. .

However, the problem here is that the crystal filter itself has a temperature characteristic of a cubic curve, as shown in the diagram of the characteristic example of the center frequency of the crystal filter in FIG. 3, and
Since the respective frequency deviation amounts are different, various characteristics of the bandpass shift with temperature.

In the temperature characteristic diagram of the center frequency of FIG.
The pass characteristics in the normal state, which are combined at the points, are shown in 6 of FIG.
The bandpass characteristic of 0 is shown in the figure. On the other hand, when the temperature changes and the tuning frequency shifts, for example, at positions 51, 52, and 53 in FIG. 3, the bandpass characteristic of 70 in FIG. 4B changes to the original band. The path characteristics change.

As a result, there are a problem that the output level changes as shown by 77 in FIG. 4 and a problem that the pass band width changes as shown by 66 to 76 in FIG. This 3d
As the b pass bandwidth, for example, a bandwidth value of 30 Hz or 200 Hz is required. The output level is, for example,
Within an allowable range of about ± 1 db is required.

Therefore, conventionally, the temperature stability is maintained by individually measuring the temperature characteristics and selecting the crystal filter parts having uniform characteristics, or by using the expensive crystal filter parts having good temperature characteristics. It was However, with this method, it was difficult to maintain a predetermined bandpass characteristic with high accuracy over the entire temperature range.

[Problems to be Solved by the Invention]

Conventionally, there is a point that the output level after passing through the narrow band pass filter is changed, or the band width and the pass band width are changed due to the temperature change.

Therefore, the problem to be solved by the present invention is to provide a means for performing independent temperature correction for each of a plurality of crystal filters so that a constant filter characteristic curve can be obtained at any operating temperature. To control. This aims to stabilize the output level, the bandwidth characteristic, and the pass bandwidth.

[0012]

In order to solve the above-mentioned problems, the present invention provides at least two sets of filter blocks having the structures A to E below. A: Crystal filter 14 B: Load element C: Amplifier 21 that amplifies the attenuated level D: Variable capacitance diode 12 that corrects the tuning frequency of the crystal filter E: Apply a control voltage to the variable capacitance diode, D
A converter 13

Here, the load element is an element that constitutes a load impedance, and is, for example, a tuning LC load circuit including a tuning coil 15 and a tuning capacitor 16 or a load circuit including a ceramic filter element. Also, in the above, the crystal filter and the varactor diode are AC-connected in series to form the tuning frequency of the filter. And
A control unit 19 for setting and controlling a predetermined value in the DA converter, a correction table 20 for storing correction values read from the control unit, and a temperature sensor 1 for detecting an ambient temperature.
8 and are provided.

As a result, when the ambient temperature changes by more than the temperature correction unit, the correction data for each crystal filter corresponding to the temperature is read from the temperature correction table and D
This is a means for setting the A converter and changing the control voltage of the variable capacitance diode to correct it to a predetermined tuning frequency.

By taking the above-mentioned means for a plurality of filter blocks, it is possible to maintain a stable narrow-band bandpass filter characteristic against temperature changes.

As the connecting means, the variable capacitor is connected to the input of the crystal filter, and the output of the crystal filter is connected to the input of the tuning coil, the tuning capacitor and the amplifier. The temperature signal from the temperature sensor is input to the control unit. The temperature correction table is connected to the control unit. From the control unit to the D
It is connected to the input of the A converter and the control voltage is applied to the variable capacitance diode from the output of the DA converter.

Then, the control section controls the execution of all of them and serves as temperature correction circuit means of the narrow band pass filter.

Further, in the above structure, as shown in FIG. 2, there is a constituent means in which a bypass switch 17 is added to each filter block, which is connected in parallel with the crystal filter.

With this configuration, means for use when obtaining the correction data of the temperature correction table and bypassing the crystal filter other than the one to be obtained to obtain the characteristic of a single crystal filter is realized.

[0020]

With respect to the tuning frequency, a variable capacitance diode is connected in series with the crystal filter to form the tuning frequency of the filter. And by changing this series capacitance value,
With a fine adjustment variable function. By giving an arbitrary control voltage to the variable-capacitance diode to the DA converter, a function of changing the series capacitance is realized.

The temperature correction table has a function of storing DA converter setting data for each filter block and for each temperature unit. The temperature correction data detects a signal from the temperature sensor, and the control unit reads the correction data from the temperature correction table.

The tuning coil and the tuning capacitor function as a load impedance without lowering the Q of the crystal filter. The amplifier is also a high-impedance input circuit and acts so as not to reduce the Q of the crystal filter. The bandpass filter configured as described above has a function of always maintaining stable filter characteristics against changes in ambient temperature. As a result, the band pass filter characteristic does not change as in the conventional case, and the performance is improved.

(Embodiment 1) An embodiment of the present invention will be described with reference to the block diagram of the crystal filter circuit shown in FIG.

An outline of the implementation of this embodiment will be described first. FIG. 1 shows a configuration example in which the filter block has two stages. The control voltage applied to the varactor diodes 12 and 22 corrects the deviation of the tuning frequency of each crystal filter 14 and 24 due to the change of the ambient temperature. The control voltage is supplied from the DA converters 13 and 23, and the correction value corresponding to the ambient temperature is read from the temperature correction table 20 and set.

Here, the contents of the temperature correction table are as follows:
For each crystal filter, the corresponding DA converter setting value is obtained in advance over the entire operating temperature and is written in the temperature correction table. Also, the corrected temperature is
It is input from the temperature sensor 18 and used as a temperature parameter when the control unit 19 refers to the correction value table.

The crystal filter 14 has a natural frequency, but by connecting the variable capacitance diode 12 in series,
The tuning frequency can be finely adjusted by changing the series capacitance value. That is, the crystal filter and the variable capacitance diode are AC-connected in series to determine the tuning frequency of the filter. The variable capacitance diode is AC-coupled, and a control voltage is applied from the DA converter 13 in terms of DC.

The temperature sensor 18 is provided in the vicinity of the crystal filter, the ambient temperature is detected and digitally converted, and the setting data for the DA converter 13 is input to the controller 19. The control unit 19 reads the temperature correction data stored in the temperature correction table 20 for each resolution of 1 ° C. from the temperature correction table 20, and writes and sets the temperature correction data in the DA converter. The correction data of the temperature correction table is previously obtained for each of the crystal filters over the entire temperature range.

As a result, the DC voltage output from the DA converter is applied to the variable capacitance diode,
A predetermined tuning frequency can be set by changing the capacity.
This variable range has a variable range that covers the variation of the natural frequency of the crystal unit and the variable range of the tuning frequency. For example, in the case of 3.58MHz crystal, ±
A variable amount of about 500 Hz is realized.

(Embodiment 2) Another embodiment of the present invention will be described with reference to the block diagram of FIG. The difference from the first embodiment is that bypass switches 17 and 27 are added. The purpose of this bypass switch is to use it when obtaining correction data.

The first use of the bypass switch is to obtain the correction data of the temperature correction table. That is, the switch of the crystal filter for which correction data is to be obtained is opened and the other switches are closed, and a known frequency and level are applied to the input 10 to measure the output signal level of the output 29. In this state, while continuously changing the output voltage of the DA converter, the center frequency, bandwidth characteristic and output level of a single crystal filter are obtained.

This is carried out for each crystal filter, the correction data is calculated based on this, the data is actually set in the DA converter, the predetermined filter characteristic is confirmed, and then written in the temperature correction table. This is done in a similar manner over the entire temperature range.

The second use of the bypass switch is a method of performing calibration at any time by the same method as described above. This can eliminate influences over time of parts, mechanical changes, etc., and is used when it is necessary to always maintain highly accurate bandpass filter characteristics. In this case, the function of applying a known frequency and level to the input 10 from the outside and the function of measuring the output signal level of the output 29 are built-in, and the function is performed at any time by the control unit.

[0032]

Since the present invention is configured as described above, it has the following effects. Conventionally, a change in output level due to a temperature change has an allowable error of about ± 3 dB, but is improved to about ± 0.5 dB. Also, changes in the -3db pass bandwidth can be significantly reduced. As a result, the performance as a narrow band pass filter can be remarkably stabilized over the entire temperature range.

Further, it is possible to obtain a predetermined bandpass characteristic with an inexpensive crystal filter without using an expensive crystal filter component having good temperature characteristics by selecting and using a crystal filter component having uniform characteristics. it can.

[0034]

[Brief description of drawings]

FIG. 1 is a configuration block diagram of a first embodiment of a crystal filter circuit according to the present invention.

FIG. 2 is a configuration block diagram of a second embodiment of a crystal filter circuit in which a bypass switch is added to the present invention.

FIG. 3 is a diagram showing an example of temperature characteristics of a center frequency of a crystal filter.

FIG. 4 is a diagram of an example of a filter selectivity characteristic.

FIG. 5 is a block diagram of a conventional narrow band bandpass filter circuit.

[Explanation of symbols]

 11, 21, 31 Amplifier 12, 22 Variable capacitance diode 13, 23 DA converter 14, 24 Crystal filter 15, 25 Tuning coil 16, 26 Tuning capacitor 17, 27 Bypass switch 18 Temperature sensor 19 Control section 20 Temperature correction table

Claims (2)

[Claims] 1. A narrowband bandpass filter device composed of a plurality of stages of crystal filter blocks, comprising: at least two sets of filter blocks consisting of the following A to E; A: crystal filter (14) B: load element C : Amplifier (21) that amplifies the attenuated level D: Variable capacitance diode (12) that corrects the tuning frequency of the crystal filter E: Apply a control voltage to the variable capacitance diode, D
A converter (13) Control unit (19) for setting and controlling a predetermined value in the DA converter, correction table (20) for storing correction values read from the control unit, and temperature sensor for detecting ambient temperature (18), the temperature signal of the temperature sensor is received by the control unit, temperature correction data is extracted from the temperature correction table based on the temperature signal, and the temperature correction data is set in the DA converter. A temperature correction circuit for a narrow band bandpass filter using a crystal filter, characterized in that the analog voltage of (1) is applied to the variable capacitance diode and the above is provided. 2. A temperature correction circuit for a narrow bandpass filter using a crystal filter according to claim 1, wherein a bypass switch (17) is added in parallel with the crystal filter.