AM Radio Receiver With Additional IF Stage

Published:2012/10/22 21:55:00 Author:muriel | Keyword: AM Radio Receiver, Additional IF Stage | From:SeekIC

Pictured above is the same circuit with an additional IF stage added forgreater sensitivity. Overall gain can be adjusted with the 1K resistorsin the emitter leg of the 2N3904 transistors. The circuit board was assembledusing multiturn 10K pots in place of the 1K resistors and then adjusted forbest performance. The pots are the 2 little blue items just to the left ofthe tuning cap. I think I ended up with about 750 ohms. The emitter bypasscaps are not needed since there is plenty of gain available without them.The caps (two yellow items near the pots) are still in the board but notconnected. I didn't know if they were needed or not, so I put them in thereanyway and later disconnected them. Removing the bypass caps also increasesthe input impedance so that both IF stages can use the black IF coils whichhave higher secondary impedances (and thus more voltage) than the yellow orwhite coils. You might be able to replace the yellow coil with a black onefor greater signal transfer since the input to the first transistor is muchhigher without the bypass cap, but I didn't try it. You may notice one of theblack coils is actually white in the picture but it was rewound for a highersecondary impedance. Actually, it was removed from a junk radio purchased fora dollar and didn't have any secondary, so I added a 27 turn secondary whichis close to what the black coils use. Overall, the performance is very goodexcept for the AGC circuit, which has limited range and may not be ableto compensate for very strong stations which may overload the circuit.The AGC voltage is derived from the IF amplitude at the cathode of thedetector diode (output of T4). As the IF amplitude increases, the DCvoltage at the gate of the JFET will move negative, below ground.The audio signal is present on both the gate and source terminals of theJFET, but the audio DC offset voltage will change as the IF amplitudechanges. This DC voltage (about 2 volts) is fed back through a 15K resistorand the two IF coil secondaries to control the transistor bias points.The audio signal is filtered out by the 47uF cap leaving a stable DCvoltage at the base of the transistors. As the base voltage drops, theemitter voltages also drop resulting in less operating current and lowergain for two IF stages. But the range is limited to maybe only 6-12dBwhich isn't enough to compensate for very strong signals. One solutionto the problem is a manual gain control consisting of a switch anda few turns of wire around the antenna coil which can be seen in thepicture (3 turns of solid insulated white wire on left side of loopstick).Closing the switch loads the antenna coil and reduces the signal level.



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