11/18/2023 0 Comments Transistor fm radio receiver circuitIt allows more neighbor stations through and makes listening to either radio stations frustrating. A lower Q tuned circuit has a wider pass band. A tuned circuit with a higher Q has a narrow pass-band that makes it better able to pick out a station of many equally strong. Its disadvantages are a lower Q (quality), nonlinearity, lower voltage rating and a more limited capacitance range. Varactors are small and inexpensive, which makes their use advantageous in many applications. It provides an electrically controllable capacitance, which can be used in many different circuits. The varactor is also known as a variable capacitance or a varicap diode. This radio, as shown on Figure 1, uses a varactor diode instead of the usual mechanical rotary device. For those of you who would like to eliminate this mechanical component, here is a modern version of the classic detector set. Traditionally, in a crystal detector radio tuned circuits, a mechanical type variable capacitor is used. Not all Philips data sheets show it, but connecting a 10K resistor from the supply to pin 1 will disable the squelch. If you don't need that feature, just remove the. Although it performs as any other muting circuit does, the TDA7000 provides an artificial noise generator so that the receiver still sounds alive while tuned off station. The muting or squelch feature is novel to say the least. It is actually compressing the frequency range of the modulated signal. Basically, the local oscillator is shifted in response to detector output so that the bandwidth of the mixer output is never more than +/- 15Kc/s. It's quite simple in that there is what Philips call a Frequency Locked Loop. A fully modulated signal would therefore sound rather distorted. Now, you might remember that the deviation of a broadcast FM signal is +/- 75Kc/s. Like the Pulse Counting Receiver, the TDA7000 relies on a low IF so that ordinary Op Amp circuitry can take care of the gain and bandpass characteristics. The difference is that there's only one tuned circuit the local oscillator. It is a proper FM superhet receiver, with the usual local oscillator, mixer, IF amplifier, limiter, and phase detector. What is unusual about TDA7000 IC is how it operates. This project is a FM Radio based on TDA7000 and LM386 integrated circuits. 9V battery fits neatly inside a 30mm x 130mm long PVC tube. Insertion of earphones plug completes supply circuit and acts as an on/off switch. LM358 dual op-amp draws less than 1 ma so the battery drain is minimal. The two 10M resistors bias the detector diode and the op-amp input near mid-rail for better detector efficiency. Detector uses a biased 1N5711 (or similar) schottky diode with lowest forward-biassed voltage drop. Capacitance runs from 28 to 7pF which by formula gives a frequency range of 77 - 155MHz. Tuning capacitor is a 30pF Philips Beehive trimmer, with a short length of plastic tube glued - as a tuning shaft. The input tuned cct 'L' is a 2 turn loop, with 30mm diameter measured at 0.15uH on my LC Meter which intercepts RF directly as opposed to an LC cct fed with external aerial. Useful for listening to the pilot transmissions. This passive airband receiver is basically an amplified "crystal radio" designed to receive nearby aircraft transmissions on 121 - 133 MHz frequency.
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