Filter with tuning from 200 kHz to 20 MHz. Scheme, description

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Filter with tuning from 200 kHz to 20 MHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Buffer logic elements of the CMOS structure can also be used as a tuning element for high-pass filters. Active filters using such elements are distinguished by a high quality factor (up to 100) and a large overlap in the range - up to 50 and even up to 100.

The figure shows a circuit of a narrow-band filter with adjustable Q factor. Its resonant frequency is tuned from 200 kHz to 2 MHz by changing the transmission coefficient of the elements of the 4050 microcircuit by adjusting the supply voltage within Uset = 2,5 ... 3,5 V. Moreover, each of the six elements can be used for this purpose.

Filter with tuning from 200 kHz to 20 MHz

In the linear gain mode with this regulation, the gain of the element DD1 - here it works as a voltage-to-current converter for the transistor VT1 - changes significantly. The circuit DD1,VT1,R3,R4 forms a resistance converter having an inductive impedance at the indicated frequencies. Capacitor C1 with this inductance forms a series oscillatory circuit. Its setting is changed by the supply voltage of the microcircuit. The high quality factor of the circuit is ensured by the large input resistance of the 4050 chip. In addition, it can be adjusted with a variable resistor R2. Resistor R4 acts as a load. If it is connected between the collector of the transistor and the common wire, the resistor is selected during tuning (for a collector current of 1 ... 2 mA). The stability of the device mode for direct current is quite high due to the deep OOS in terms of voltage. The current consumed in the control circuit is practically equal to the base current VT1.

The input capacitance of this buffer element is about 80 pF. When using other elements with low input capacitance (up to 15 pF), the upper limit of the tuning frequency can be extended to 20 MHz.

Instead of a 4050 microcircuit, a domestic analogue, K561PU4, can be used in the filter. Transistor - any high-frequency low power, for example, series KT315, KT3102.

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