[AMRadio] No active devices: An electro-mechanical RX for VLF

Donald Chester k4kyv at charter.net
Sat Jul 13 19:42:34 EDT 2013

It is basically a direct conversion receiver. It has a 2 pole passive
preselector with RF bandwidth of about 800Hz, which also impedance matches
the antenna to the mixer. The mixer uses saturating ferrite cores, in a way
not unlike the SAQ keying circuit. The BFO signal saturates the cores at
both the positive and negative peaks of its waveform, and so the core
windings present an inductance that varies at twice the BFO frequency, in
series with the signal path.

The BFO signal is produced by a small high frequency alternator. This is
actually a 200 step/revolution stepper motor, salvaged from a scrap hard
disk drive. When driven as a generator, it produces a reasonable sine wave -
each winding produces a signal in phase quadrature with the other - with 100
cycles for each revolution. So as an HF alternator, it produces quite high
frequencies when rotating at moderate speed. Driven at 4800 rpm by a DC
servo motor, it generates a couple of watts at 8kHz, which effectively gives
a 16kHz BFO signal, due to the frequency doubling action of the mixer. This
"oscillator" is connected to the rest of the circuit with long cables,
because of the mechanical noise it produces! 


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