![]() ![]() If connections on the capacitor are reversed, then moving your hand near the capacitor will cause unwanted stability and oscillation.įinally here are some voltagee checks from my breadboard prototype.This should help in determining a working circuit:-Īll measurements made with a fresh 9volt battery and three BC109C transistors with respect to the battery negative terminal. ![]() The moving plates should be connected to the "cold" end of the tank circuit, this is the base of Q1, and the fixed plates to the "hot end" of the coil, the juction of R1 and C1. The tuning capacitor has fixed and moving plates. This will directly drive high impedance headphones or can be fed into a suitable amplifier.Īll connections should be short, a veroboard or tagstrip layout are suitable. Audio level varies on the strength of the received station but I had typically 10-40 mV. Transistor Q3 has a dual purpose it performs demodulation of the RF carrier whilst at the same time, amplifying the audio signal. This will give adjustment of sensitivity and selectivity of the receiver. R1 could also be replaced by a fixed resisor say 33k and a preset resistor of 100k. If there is a lack of sensitivity, then try increasing R1 to around 150k. If the circuit oscillates,then R1's value may be decreased try 68k. Insufficient feedback and the receiver becomes "deaf". Too much feedback and the circuit will become unstable producing a "howling sound". The 120k resistor provides regenerative feedback,between Q2 output and the tank circuit input and its value affects the overall performance of the whole circuit. This is necessary so as not to unduly load the tank circuit. Q1 and Q2 form a compund transistor pair featuring high gain and very high input impedance. I used a ferrite rod and tuning capacitor from an old radio which tuned from approximately 550 - 1600kHz. The radio circuit diagram above is that of a straightforward FM receiver. The design is simple and sensitivity and selectivity of the receiver are good.Īll general purpose transistors should work in this circuit, I used three BC109C transistors in my prototype.The tuned circuit is designed for medium wave. It is similar in principle to the ZN414 radio IC which is now no longer available. The value of the rf choke L2 is not critical, anything from 1H to 10H being suitable. The earthy end (moving vanes and spindle) is connected to the 22pF capacitor C1. One other variation in my circuit was the choice of transistor. The tunning capacitor VC1 is one of the two fm sections of a miniature fm transistor radio with built-in trimmers (VC2). Nevertheless, they can and do work very well, and this article will make use of one.This is a compact three transistor, regenerative receiver with fixed feedback. The most basic version of a simple direct conversion FM radio is covered in 3.15.1 of. They were very popular with early radio experimenters since the advent of radio in the ’30s, but because they required a bit of skill and experimentation, they fell out of fashion. ![]() Regenerative receivers work differently from the typical FM receiver circuits we just discussed. This signal is then amplified by the AF amplifier. The IF is then sent to an FM detector, which converts frequency to voltage and extracts the audio modulation. One of these is the needed intermediate frequency (IF). This is the tuning part, and the result is the creation of two new frequencies-the incoming signal with and without the local oscillator (LO) frequency. Then follows the mixer, which mixes the incoming frequency with another frequency from the local oscillator. The RF amp is a VHF amplifier tuned to the FM band and provides rejection to other frequencies and the intermediate frequency image (more on this below). The FM radio signal is picked up by the antenna, which as we discussed in a previous article on FM transmitters, is best as a quarter wavelength. ![]()
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