Very Low Frequency (VLF) Converter Circuit

This article explains a circuit that can pick up very low radio waves VLF from the air and turn them into sounds to hear.

Regular radios often miss these VLF waves because they focus on higher frequencies.

This circuit is different it can find these lower waves and convert them so one can listen to them.

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 which stands for wireless and electrical frequencies broadcast between 3 kHz and 30 kHz includes exciting signal, such as the slower dit dah wave which is an unusual audible signal that occurs below 15 kHz.

Before a thunderstorm forms these low frequencies may detect signs such as electrical storms and strong lightning which might sound like crunching, whistling or crackling.

The circuit is capable of detecting signals such as Loran, military, international broadcasts, CW and beacon signals between 160kHz and 190kHz for frequencies higher than 15kHz.

With the possibility to adjust components for operation from 200 kHz to 500 kHz the circuit schematic presented above allows the creation of a circuit that includes VLF from less than 10 kHz to over 260 kHz.

This is achieved by changing the inductor set up removing and replacing capacitors and changing input filter components.

Components must be carefully placed on a 2 by 3 inch circuit board or perfboard for compactness during building.

To ensure that the crystal oscillator circuit remains separate from the front end circuits it is important to keep parts and leads short.

Due to its low current the circuit is advised to be housed in a metal box and run on a battery for best results.

A shielded or coaxial cable with a solid ground connection between the converter and the receiver is the best way of connecting to the antenna of a shortwave receiver.

It is advised to use a fairly long wire antenna and to set the receivers mode switch to the CW position.

A strong tone is generated through the receivers BFO during testing and it is heterodyned with the 3.5 MHz signal from the converters crystal oscillator.

The transformed frequency is then scanned using the dial settings.

For building inductor L3 and modifying the circuit for various frequency bands detailed instructions are given.

Formula and Calculations:

Using a certain oscillator frequency formula some circuit features are calculated during the design phase:

The oscillators frequency:

If a local oscillator (LO) is used in the converter to mix the VLF signal with another frequency the oscillator frequency calculation depends on the required output frequency range.

To convert a 100 kHz signal to a frequency range of 4 MHz to 5 MHz for example the oscillator frequency fLO is calculated as follows:

f_LO = f_out – f_in

where,

• It shows the necessary local oscillator frequency (LO) in a heterodyne VLF converter circuit to produce the correct output frequency based on the frequency of the incoming VLF input signal fin.

Explanation of the formula and how VLF converters use it:

Heterodyne Conversion:

This technique is frequently used in frequency converters.

A new frequency is produced at the output point when one mixes a signal from a local oscillator (fLO) with the incoming VLF signal fin.

fLO or local oscillator frequency:

The frequency of the signal produced by the local oscillator inside the converter circuit is known as the local oscillator frequency or fLO.

It is required to figure out the output frequency.

fin (frequency of the VLF input signal):

Fin indicates the frequency of the extremely low frequency signal that one want to convert to a higher range.

Output Frequency or fout:

This is the range of frequencies that the converted output signal is supposed to have.

How the formula works:

By considering the frequency of the entering VLF signal and a desired result range the formula basically helps to calculate the required local oscillator frequency.

Nonlinear components like transistors or diodes combine the fin and fLO signals inside the converter circuit to create sum and difference frequencies.

The necessary output frequency fout is often found by comparing the frequencies of the local oscillator (fLO) and the entering VLF signal fin.

For Example:

Assume that a VLF signal at 100 kHz that needs to be converted to a higher frequency range of 4 MHz to 5 MHz.

The local oscillator frequency (fLO) is found using the following formula:

f_LO = f_out – fi_n = (between) 4 MHz – 100 kHz = (between) 3.9 MHz and 4.9 MHz

How to Build:

To build a Very Low Frequency (VLF) Converter Circuit following steps are required to be followed for connections and assembling:

Positioning of Components:

• Using the circuit schematic as a guide and arrange the parts on the circuit board or perfboard.
• Make sure the crystal oscillator circuit is separated from the front end circuit and take care to keep the lines short.

Soldering:

• The parts should be soldered onto the board.
• Verify again for any cold joints or solder bridges.
• If want to switch out the crystal oscillator for a different frequency use a socket.

The enclosure:

• To protect the circuit from external influences place it in a metal casing.
• Make sure the case has a solid foundation.
• Be sure to drill holes for switches, connections and any necessary modifications.

Source of Power:

• To supply electricity to the circuit connect it to a battery.
• Check that the battery can provide the necessary current and voltage.

Connecting up with a shortwave receiver:

• Use a coaxial or shielded cable to connect the converters radio frequency output to the shortwave receivers antenna.
• Both the converter and the receiver should have a solid ground connection.

Tuning the Antenna:

• Connect an antenna with a fairly length wire to the converter.
• This antenna improves the circuits ability to receive signals.

Selecting the Mode:

• Select the continuous wave (CW) setting on the receivers mode switch.

Testing:

• Turn on the shortwave radio and the circuit.
• To create a loud tone that should heterodyne with the 3.5MHz signal from the converters crystal oscillator use the receivers beat frequency oscillator (BFO).
• To test the circuit use the dial values as mentioned to scan through the converted frequencies.

Modifications:

• If required alter the circuit by taking note of the frequency and range selection changes specified in the instructions.

Be careful:

• Do not forget to take safety precautions.
• Throughout the building process make sure all connections are correct and use the circuit diagram and necessary instructions.
• Use an oscilloscope or multimeter for troubleshooting if the circuit into any problems.ng.

Conclusion:

Careful component placement and soldering, careful consideration of the correct grounding, connection to a shortwave receiver and frequency range adjustment are all part of the circuits design and tuning.

With the help of the VLF converter circuit beginners and hobbyists can investigate and hear VLF signals that are difficult for standard radio receivers to pick up.

References:

Very low frequency