Sunday, 15 January 2012


Receive VLF is not very simple. Today the pc sound card help us to this aim but this is not the only way. TechLib reports some projects to approach the problem easily also without using pc!
The projects are collected to TechLib page. Follow only one simple sound card receiver.

Here is an amplifier that includes a high-pass filter to help reduce the fundamental line frequency component:

The high-pass frequency is set by the 500k and 1meg resistors, change them keeping the same ratio to change the response, higher values giving a lower frequency response. The values shown will reduce 60 Hz by about a factor of 6 but don't hesitate to use lower values for more attenuation. Another way to reduce the hum if you are willing to sacrifice some low frequency response is to change the output capacitor from 10 uF to 0.047 uF  (adds another roll-off at about 350 Hz). The overall gain is set by the 10k and 180 ohm; increase the 10k to 22k if hum permits. The selected capacitor at the antenna is chosen to give a total of about 2000 pF including the cable capacitance but this value is not critical if the hum is low enough and the gain is high enough. The power supply is an ordinary bench supply with the negative terminal grounded to a good earth ground. Surprisingly, no isolation was needed between the amp and computer once the earth ground was connected!

The more I play with this, the less important the low frequencies seem to be. My version of this amp replaces the 10 uF output cap with the suggested 0.047 uF.   The plot below shows how low the line hum is before filtering. The system can now handle really big pulses without running out of headroom, as you can see.

Note the little on-off squiggles near the end of the trace. Those are from a transmitter in Hawaii!

Try your amp with just the good old line power ground first but don't be discouraged by trouble. My amplifier happens to be right next to a ground wire that connects directly to an underground copper water pipe. Connecting the shield of the coax to the earth ground at the amplifier's input completely eliminated grounding problems. I can't emphasize the benefits of a good ground at the amplifier input enough! If you don't have a cold water pipe handy try running a ground wire out a window and attach it to a ground rod or an outside water faucet near the water main. Such a good ground is handy for all sorts of radio projects. In this case we are providing a ground for audio frequencies so the main consideration is the resistance of the wire and quality of the ground; bends, turns and length are not important.

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