Quartz filter with switchable bandwidth. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Quartz filters
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In the quartz filter described by V. Zhalnerauskas ("Radio", 1982, No. 6, p. 23), the passband is changed by connecting capacitors parallel to the quartz resonators with reed relays. But in cases where the dimensions of the device and power consumption are critical, p-in diodes or field-effect transistors can be used as switching products.
On fig. 1 shows a six-resonator filter circuit with a switchable bandwidth on quartz resonators at a frequency of 10,4 MHz.
Ris.1
Its amplitude-frequency characteristic is shown in fig. 2. When a voltage of -5...-12 V is applied to the control input, the field-effect transistors are closed and the channel resistance is high.
Ris.2
In this case, the filter bandwidth is 2,7 kHz. If you apply a voltage of +5 ... + 12 V, the field-effect transistors will open and the capacitors C6, C8, C10, C13 will be connected in parallel to the quartz resonators. The bandwidth will narrow down to 0,7 kHz. The capacitance of these capacitors is selected experimentally. Unlike the filter mentioned above, in this filter, capacitors are not connected to all resonators. This is due to the fact that when capacitors are connected in parallel to quartz, the slope of the low-frequency slope decreases. At the same time, it is possible to narrow the band by connecting capacitors in parallel with three or four out of six capacitors. The filter practically does not consume electricity, has small dimensions. It can be assembled on a printed circuit board and installed in finished equipment. The printed circuit board drawing is shown in fig. 3.
The device can use transistors KP305A - KP305D, KP313A, KP313B. To smoothly adjust the bandwidth, the capacitance of capacitors connected in parallel must be changed smoothly. This can be realized by using multi-section KPIs or varicaps. However, due to the spread of the parameters of the resonators, it is very difficult to obtain a satisfactory frequency response of the filter for different passbands. Therefore, more appropriate, apparently, is a filter, the band of which changes abruptly due to the connection of capacitors in parallel with the resonators, which are selected taking into account the parameters of each resonator.
According to a scheme similar to that given in the article, an eight-crystal filter can also be made, it is only necessary to take into account that in order to eliminate nonlinear effects on field-effect transistors, the signal level should not exceed 1 V.
Author: I. Nechaev, Kursk; Publication: N. Bolshakov, rf.atnn.ru
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