AM receiver

  
This is a description of an experimental AM receiver for VLF. It is crystal controlled to receive 181.818 kHz (more or less) and operates as either a single conversion superhetrodyne or a direct conversion receiver. The bandwidths are expected to be 15 Hz to 3.5 kHz in superhet mode and 15 Hz to 10 kHz in the direct conversion mode. Operation is from a single +5 volt regulator and it can be powered from a 9 volt radio battery or other source of +8V or higher. The reason I built this was to verify some of my assumptions about these components would work together, having had numerous discussions of the SA-612/MK-484 and SA-612 direct conversion receiver concepts over the last year or so with my friend Jeff, besides that, I want to have a direct conversion receiver on hand for some planned experiments.
AM receiver - schematic

Depending up the state of the mode switch on the left, either pin 8 or pin 9 of the AT90S2313 goes high. When pin 8 goes high, it supplies power to the MK-484 (ZN-414 replacement - an integrated IF/AM detector). When pin 9 goes high, it stops the 2N2907 from shunting the direct conversion signal at the output of the SA6-12. In the superhetrodyne mode, the receiver consists of a ferrite loopstick antenna, a local oscillator, mixer an IF filter and an IF amplifier and AM detector, followed by an audio amplifier. The loop antenna is a resonant circuit made of a 760 microhenry coil wound on a 5.5 cm long ferrite rod covered with one thickness of 80 grams per square meter printer paper, in parallel with a 1000 pf capacitor.. The ferrite rod was picked up at a surplus store, so I had to experiment a little to find the correct number of turns. I started by winding 100 turn closely spaced in the middle of the ferrite rod, with a tap at 50 turns. I used #30 heavy polythermaleze magnet wire. I measured the inductance between the 50 turn tap and each end of the coil, and measured it a third time for the entire 100 turns, then I calculated an inductance factor for the rod for each of the windings, and then found the average inductance factor. The inductance factor, also referred to as AL (A-sub-L) in some literature is the factor that, when multiplied by the number of turns squared, is equal to the inductance. Here, I will use...



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