Encyclopedia of radio electronics and electrical engineering / radio reception
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The manufacture of winding products such as inductors and transformers often causes difficulties for radio amateurs. In simple and relatively low-frequency receivers, coils can be dispensed with by replacing them with active RC filters or gyrators (active electronic devices that mimic the properties of inductance).
The figure shows a part of the AM signal receiver circuit, in which tuning to a received radio station and selection of input signals are carried out using a gyrator ("CQ ham radio", 1995, January). This receiver in the author's version is designed to receive signals from radio stations of low-frequency frequency standards operating in the frequency band of 17 ... 34 kHz. In Russia, a similar time and frequency standard operates at a frequency of 66,6 kHz.
(click to enlarge)
A gyrator that simulates an inductor is formed by operational amplifiers DA1 and DA2, as well as resistors R1-R5 and capacitor C3. The value of the inductance of this "coil" can be determined from the formula L=R1(R2+R4)R5C3/R3. If we choose R1=R3, then this formula is simplified to L=R5C3(R2+R4). For the values of the elements shown in the figure, the tuning range of the equivalent inductance by the variable resistor R4 will be approximately 11 ... 44 mH. In this case, the parallel input "oscillatory circuit", which includes capacitor C2, will be tuned in the frequency band 17 ... 34 kHz.
From this "circuit" the signal goes to the radio frequency amplifier on the operational amplifier DA3. Its gain can be adjusted by a variable resistor R6 within 0...40 dB.
Cascade on the operational amplifier DA4 - the so-called "precision rectifier" (detector). Diodes VD1 and VD2 are included in it in a negative feedback circuit, which significantly linearizes the transfer characteristic of the detector at low signals.
Since the author's version of the receiver was intended to receive special signals containing second pulses, the RC filter at the output of the detector (R12C5) has a cutoff frequency of only 16 Hz.
The receiver uses a quad operational amplifier 4136, which greatly simplified the design. The parameters of the OU included in it are close to the parameters of K140UD8A. Diodes VD1 and VD2 - 1N38. They can be replaced with any high-frequency germanium diodes, starting with diodes of the D9 series.
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Comments on the article:
Sergei
I assembled the RF part of this circuit, it works. Of the minuses - a rather low quality factor, the gain changes during tuning. I used tl074 bipolar 2x6 power supply.
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