Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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Radio amateurs are popular with simple designs of radio receivers designed to receive transmissions from local radio stations operating in the long and medium wave bands. The original design of such a radio receiver was proposed by the Japanese radio amateur Kazihiro Sunamura (JF10ZL). Brief information about this receiver is posted on its home page on the Internet at . Please note: the number 10 instead of the letter O in his email address is not a typo! Its URL looks like this.
The design uses the well-known idea of building low IF receivers, which makes it possible to solve the problem of signal filtering using simple RC filters. The local oscillator frequency is also determined by RC elements, and there is no input oscillatory circuit at all. For the reception of closely spaced one or two broadcast radio stations, this solution will be acceptable in most cases.
Schematic diagram of the receiver (without audio frequency amplifier) is shown in the figure.
The signal from the short wire antenna WA1 is fed to a mixing stage based on an operational amplifier DA1 and a diode VD1. It is a so-called "precision diode" (see Shilo V. L. "Linear integrated circuits". - M: Radio and communication, 1979. - Approx. ed.). The introduction of a semiconductor diode into the feedback circuit of an operational amplifier makes it possible to implement a node with an almost ideal current-voltage characteristic for detecting or converting a signal.
The input impedance of the "precision diode" is relatively high, which ensures good efficiency of the input circuit. The signal from the antenna is fed to the non-inverting input of the op-amp. The voltage from the local oscillator is also supplied here. It is made on DA4 and generates rectangular pulses, the repetition frequency of which can be changed by a variable resistor R4 in the range from 590 to 720 kHz. With the intermediate frequency value of 40 kHz chosen by the author, this provides reception in the band from 550 to 680 kHz. At higher frequencies, with the used operational amplifiers, the reception quality deteriorated noticeably.
The next stage is a low-pass filter on the op-amp DA2 with a cutoff frequency of about 40 kHz, and an IF amplifier is assembled on the op-amp DA3.
The detection of the AM signal is carried out by another "precision diode" on the DA5 op-amp and the VD2 diode, the signal from which passes through the low-pass filter to the DA6 op-amp, which selects audio frequencies. Variable resistor R22 - volume control.
The receiver is fed from a bipolar source with voltages of +7 and -7 V.
When repeating the receiver, it should be borne in mind that relatively high speed is required only from the operational amplifiers DA1 and DA4. Diodes VD1 and VD2 are germanium (this is important) low-signal.
The antenna is a piece of wire about 1 m long. With such an antenna, the author confidently received two local radio stations. This is not surprising, since the total gain in the path was about 80 dB.
Author: Kazihiro Sunamura
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