This Very Low Frequency (VLF) Converter Circuit talks about one circuit.
It can catch very low radio waves VLF from air and make them into sound for hearing.
Normal radios do not get these VLF waves but they only work with higher waves.
But this circuit is different it can find low waves and change them to sound so people can listen.
Circuit Working:
Parts list:
| Category | Value | Quantity |
|---|---|---|
| Resistors | 1.5Meg 1/4 watt | 1 |
| 330Ω 1/4 watt | 1 | |
| 220k 1/4 watt | 1 | |
| 1k 1/4 watt | 3 | |
| Capacitors | Ceramic 0.05pF | 1 |
| Ceramic 15pF | 1 | |
| Ceramic 0.1μF | 3 | |
| Ceramic 680pF | 2 | |
| Electrolytic 470μF 16V | 1 | |
| Semiconductors | Transistor 2N3904 | 2 |
| Transistor MPF102 | 1 | |
| Crystal 3.5 MHz | 1 | |
| Inductors Standard 1.5mH (L1, L2, L3) | 3 | |
| Inductors Standard 2.5mH (L1, L2, L3) | 3 | |
| Battery 9V | 1 | |
| Switch On/Off | 1 |
VLF mean very low frequency, it has radio signal between 3 kHz and 30 kHz.
These signals have interesting sounds like “dit dah” wave which can be heard below 15 kHz.
Before thunderstorm come this low frequency can catch things like electric storm and big lightning.
It may sound like crack, whistle or crunch noise.
This circuit can also catch other signals like Loran, military signal, world broadcast, CW and beacon.
These come in higher range from 160 kHz to 190 kHz.
We can also change circuit parts to make it work from 200 kHz to 500 kHz.
The circuits plan can pick up VLF from under 10 kHz to more than 260 kHz.
This is done by changing coil which is the inductor to take out and put new capacitor and also change some filter parts in the front.
Parts need to be placed carefully on small board size of 2 by 3 inch so it stay small and easy to build.
To stop crystal oscillator from mixing with front part all parts and wires should be short.
Because circuit uses little power it is good to put in metal box and use battery for better work.
To connect antenna to shortwave radio use shielded wire or coax cable with good ground.
Use long wire for antenna and put receiver in CW mode.
When testing a strong tone come from receiver BFO it mixes with 3.5 MHz signal from the circuit crystal.
Then we can turn the dial to scan the changed frequency.
There are full steps for making coil L3 and how to change circuit to catch different signal bands.
Formula and Calculations:
Using one formula for oscillator frequency, there are some circuit things that are calculated when making this circuit.
Oscillator Frequency:
If circuit use one local oscillator (LO) to mix VLF signal with another signal we can calculate its frequency like this:
To change one 100 kHz signal into output between 4 MHz to 5 MHz oscillator frequency (fLO) is:
fLO = fout – fin
where:
- fin is the input signal for VLF signal
- fout is the output frequency we want
- fLO is the oscillator frequency we need to make
This formula help choose right LO frequency to mix with VLF and get good output.
Heterodyne Conversion:
This is common method in converter circuits.
It mixes with local oscillator signal (fLO) with VLF input (fin) and make new signal at different frequency.
What each part means:
Local Oscillator (fLO):
This is frequency from the oscillator part inside the converter which helps shift the signal.
VLF Input Signal (fin):
This is very low frequency signal we want to change.
Output Frequency (fout):
This is final frequency we want after mixing for example in MHz range.
How formula works:
By looking at what VLF signal we get and what output we want we use the formula:
fLO = fout – fin
Inside circuit parts like transistor or diode mix both signals and make two new frequencies for sum and difference.
We pick the one we want.
Example:
If we have 100 kHz VLF signal and we want output from 4 MHz to 5 MHz we do like this:
fLO = fout – fin
So,
fLO = 4 MHz – 100 kHz = 3.9 MHz (to)
fLO = 5 MHz – 100 kHz = 4.9 MHz
So oscillator must work between 3.9 MHz and 4.9 MHz to get output in that range.
How to Build:
To build a Very Low Frequency (VLF) Converter Circuit following steps are required to be followed for connections and assembling:
Putting Components:
- Use the circuit diagram and put all parts on the board (circuit board or perfboard).
- Keep crystal oscillator part far from the front end part and wires should be short.
Soldering:
- Solder all parts on board.
- Check again for bad solder or short connections which are called the solder bridges.
- If want to use other crystal oscillator use a socket to change it easy.
Metal Box (Enclosure):
- Put whole circuit inside the metal box to keep it safe from outside signals.
- Ensure box is strong and has good ground.
- Drill holes for switch, wire and any change are needed.
Powering the Circuit:
- Use battery to give power.
- Check battery gives correct voltage and enough current for circuit.
Connecting to Radio (Shortwave Receiver):
- Use coax cable or shielded wire to connect converter output to radio antenna input.
- Make good ground between converter and radio.
Setting Antenna:
- Put long wire as antenna and connect to converter.
- Long wire helps circuit to get more signals.
Choosing Mode:
- On shortwave radio select CW mode for continuous wave.
Testing:
- Turn ON the radio and the circuit.
- Use radios BFO for beat frequency oscillator to make loud tone.
- This tone mixes with 3.5 MHz from converters crystal.
- Use tuning dial to scan and check converted signals.
Modifying Circuit:
- If needed change parts for other frequencies.
- Follow instruction for how to change range and components.
Be Careful:
- Always stay safe.
- Check all wires and parts before turning ON.
- Use circuit diagram and follow all steps.
- If problem happens use multimeter or oscilloscope to check.
Conclusion:
Putting parts carefully, soldering right, grounding good, connecting to radio and setting frequency all are important for this circuit.
With this Very Low Frequency (VLF) Converter Circuit new learners and hobby people can listen to very low signals that normal radios cannot hear.