The resistors are integrated, the capacitors external. The bulky external IF filter transformers have been replaced by R-C filters. After Sony įigure below shows a low-parts-count FM radio based on a TDA7021T integrated circuit by NXP Wireless. Though, the variable capacitors could be replaced by varicap tuning diodes.Ĭompact IC radio eliminates external IF filters. L-C tuning components are still required for the radio frequency (RF) input and the local oscillator. This eliminates external IF transformers and an IF ceramic filter. Sony engineers have included the intermediate frequency (IF) bandpass filter within the 8-pin IC. IC radio comparison of (a) mechanical tuning to (b) electronic varicap diode tuning.įigure below shows an even lower parts count AM radio. V tune could be produced by a potentiometer. Increasing the reverse bias V tune decreases capacitance which increases frequency. It is ecconomic to replace it with varicap tuning diodes. The meshed plates of a dual variable capacitor make for a bulky component. Some automatic gain control (AGC) is recovered and filtered to DC and fed back into pin 9.įigure below shows conventional mecahnical tuning (a) of the RF input tuner and the local oscillator with varactor diode tuning (b). A diode at pin 7 recovers audio from the IF. Most of the amplification is in the intermediate frequency (IF) amplifier between pins 12 and 7. The external 455 KHz ceramic filter between pins 15 and 12, selects the 455 KHz difference frequency. The RF signal and local oscillator frequencies mix producing the sun and difference of the two at pin 15. The 320 pF variable capacitor tunes the local oscillator 455 KHz above the RF input signal. The 370 Pf variable capacitor tunes the desired RF signal. All capacitors and inductors, along with a few resistors, are external to the IC. The circuit in Figure below is an integrated circuit AM radio containing all the active radio frequency circuitry within a single IC. Regency TR1: First mass produced transistor radio, 1954. įor more crystal radio circuits, simple one-transistor radios, and more advanced low transistor count radios, see Wenzel A silicon transistor may be used if the base-bias resistor is changed according to the table.Ĭrystal radio with one transistor audio amplifer, base-bias. A schottky diode may be substituted for the germanium diode. The original circuit used a germanium diode and transistor. The circuit in Figure below adds an audio amplifier to the crystal detector for greater headphone volume. See Volume 6 Experiments, AC Circuits, Sensitive audio detector for details. However, the low impedance earbuds commonly used with portable audio equipment may be substituted when paired with a suitable audio transformer. The 2000Ω headphones are no longer a widely available item. No-bias-resistor causes operation as a detector. Reverse battery polarity for NPN silicon devices. However, a silicon device should still work. This is probably more sensitive, due to the lower 0.2V V BE, compared with silicon. Note the transistor is a germanuim PNP device. This more powerful circuit can readily drive 2000Ω magnetic headphones. An advantage of a transistor detector is amplification in addition to detection. Since the transistor is not biased in the linear region (no base bias resistor), it only conducts for positive half cycles of RF input, detecting the audio modulation. The circuit in Figure below produces a stronger output than the crystal detector. The ceramic earphone is desirable for all but the strongest radio signals While 2000Ω magnetic headphones are shown above, a ceramic earphone, sometimes called a crystal earphone, is more sensitive. A germanium diode, which has a lower forward voltage drop provides greater sensitvity than a silicon diode. Note that no power supply is required for a crystal radio. C2 is sized to filter the radio frequencies from the RF envelope (c), passing audio frequencies (d) to the headset. The diode passes the positive half cycles of the RF, removing the negative half cycles (c). The variable capacitor C1 allows for tuning to the various signals. Its purpose is to select one out of many available radios signals. The combination of C1 and L1 comprise a resonant circuit, refered to as a tank circuit. The antenna absorbs transimtted radio signals (b) which flow to ground via the other components. (d) Demodualted audio to headphones.Īn antenna ground system, tank circuit, peak detector, and headphones are the the main components of a crystal radio. (c) Rectified RF at diode cathode, without C2 filter capacitor.
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