REGENERATIVE_SHORTWAVE_RECEIVER

Published:2009/7/17 2:46:00 Author:Jessie | From:SeekIC

The receiver is composed of several subassemblies: an active antenna, an amplifier (with a regeneration control and band-switching circuitry), an AM detector, a power amplifier, and some form of output device (internal speaker, external speaker, or phones), plus a multivoltage power supply. The schematic diagram of the receiver is shown in the figure, a dual-gate MOSFET, Q4 (which is analogous to a pentode vacuum tube), is used as the regenerative amplifier. The dual-gate MOSFET is configured as a Colpitts oscillator, rather than an Armstrong type. The MOSFET's feedback from source to gate is provided via R18 and C33. The circuit contains several standard fixed-value inductors, each of which is connected in parallel with a small variable capacitor. Those LO pairs along with a SP12T rotary switch (S2) are used for band selection. Regenerative amplification and AM detection are performed by two separate transistors, Q4 and Q5. The AM detector (which is called an infinite-irnpedance detector) has the advantage of not loading the RF stage appreciably, Because D3 is operated with a slight forward bias, either a germanium or a silicon diode can be used in that position. The dc voltage across R24 (the volume control) should be about 0.1 V. Regeneration is controlled by varying the voltage applied to gate 2 of Q4. The circuit also contains fine and coarse regeneration controls that allow delicate adjustments. The receiver uses an active-antenna circuit, consisting of transistors Q1, Q2, and Q3. Under most conditions, a short whip antenna is adequate.



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