Crystal filter with switchable bandwidth

In the quartz filter, described by V. Zalnerauskas ("Radio", 1982, No. 6, p. 23), bandwidth change, connecting reed relays in parallel to the quartz resonator capacitors. But in cases where the size of the device and power consumption are critical, as switching devices can be used p-i-n diodes or FETs.

In Fig. 1 shows a diagram of shestidesiatnika filter with switchable bandwidth on quartz resonators at a frequency of 10.4 MHz.

Fig.1

Its frequency response is depicted in Fig. 2. When applied to the control input voltage -5 to -12 V field-effect transistors are closed and the channel resistance is large.

Fig.2

In this case, the filter bandwidth is 2.7 kHz. If apply voltage +5...+12 V, field-effect transistors opens and the capacitors C6, C8, C10, C13 is connected in parallel to the quartz resonators. Bandwidth will be narrowed to 0.7 kHz. The capacitance of these capacitors are chosen experimentally. Unlike the above-mentioned filter in this capacitors are not connected to all resonators. This is because when you switch the capacitors in parallel to the quartz decreases the steepness of the low-frequency slope. At the same time narrow the band manages by connecting capacitors in parallel, three or four of the six capacitors. The filter consumes little power, has a small footprint. It can be assembled on a printed circuit Board and installed in the finished instrument. Drawing of the PCB is shown in Fig. 3.

In the device, you can use transistors CPA - CPD, CPA, CPB. For smooth adjustment of the bandwidth of the capacitance of the capacitors connected in parallel, it is necessary to change smoothly. This can be implemented by applying a multi KPE or varactors. However, due to the spread of the parameters of resonators to obtain a satisfactory frequency response of the filter for different bandwidths, very difficult. Therefore, a more appropriate, apparently, is the filter, the frequency band which is changed abruptly by connecting in parallel capacitors resonators, which are selected taking into account the parameters of each resonator.

Under the scheme, similar to the one mentioned in the article, you can do cosmically filter, it is necessary only to consider that to exclude nonlinear effects in field-effect transistors, the signal level should not exceed 1 V.

Author: I. Nechaev, Kursk; Publication: N. Bolshakov, rf.atnn.ru