![]() However, in the prototypes, the oscillators did pull slightly when connected to a very long (greater than 100 meters) antenna. The signal-tonoise ratio also is improved substantially, and the mixer is made more insensitive to pulling. But, a double-balanced mixer like this is inherently good with respect to signal overloads.Ī double-balanced mixer is symmetrical to ground and completely cancels both the received signal and the oscillator voltages in its output. Adding a tuned-RF stage might increase the gain a little, and would tend to reduce broadcast band overload. The third position is for direct input, which is particularly useful when operating with a very short antenna. One is resonant at 10.7 MHz, while the other is actually the AM oscillator coil. The bandpass-filter (BPF) arrangement is quite simple and is based on some junked IFTs from an old AM/FM clock/radio. ![]() The audio stage runs at full battery voltage and can fill a room with sound using a 6-in. If your LM386 seems a bit too lively, lower it to 4.7 ♟ to reduce the gain. The only caution revolves around the 10-mF gain-setting capacitor. The LM386 is almost a standard chip for audio use. The IC has internal automatic gain control, as well as an AM detector, and works very well at the 455-kHz IF frequency. The IF stage is based around a ZN416, which is a ZN414 10-transistor radio with an internal 18-db-gain audio buffer amplifier added ( Fig. The value of the 220 pF capacitor on pin 7 also can be increased to 1000 pF, but the oscillator may refuse to oscillate on the HF bands if the value is too large (the capacitor can be switched using a wavechange switch if convenient). This will increase the bias current, but at the expense of making the noise figure slightly worse. So, if coverage of the lower end of the broadcast band is desired, add a 22k resistor from pin 7 to ground. The internal oscillator may be reluctant to operate below 1 MHz. Of course, if the external oscillator is employed, a digital frequency readout can also be utilized. The internal oscillator can be disabled and an external oscillator used. It features an on-board oscillator, capable of up to 200 MHz or so, and provides about 18-dB conversion gain. The AN602 mixer, designed for use with cellular phones with a frequency response in excess of 500 MHz, takes care of all this. It also must have a low noise figure that isn’t adversely affected by the oscillator injection power level. Power-supply requirements are 9 to 12 volts dc at 10 mA average (up to 30 mA at full volume).Ī good mixer design should be very “strong” (i.e., not easily overloaded by strong stations) and it should have significant conversion gain. ![]() The receiver is based on the AN602 double-balanced mixer IC, with the big brother of the ZN414 (the 10-transistor radio chip), the ZN416, employed as an IF amplifier, coupled with a standard LM386 audio stage ( Fig. Also, in a conventional set, the tuning capacitor needs at least two gangs, and they must be able to track each other to maintain the sensitivity across the entire reception band. In a conventional superhet design, the antenna coil, as well as the oscillator coil, would need simultaneous adjustments. It also makes the receiver very easy to experiment with by changing the coil dimensions, etc. When constructing a superhet receiver needing just one coil/tuning-capacitor combination, there’s the obvious advantage that only one simple coil needs to be made. ![]() The receiver circuitry as presented here can be built for less than $50.ĭon’t be misled-even though the set has some pretty obvious limitations as presented, it’s still capable of worldwide reception when connected to a few meters of wire as an antenna. ![]() This idea presents a simple circuit for a superhet radio receiver that can be built up in sections, with each section tested before assembly. ![]()
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