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 you can 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 you can listen to them.

Circuit Working:

Parts list:

VLF referring to wireless and electrical frequencies transmitted in the range of 3kHz to 30kHz, encompasses interesting signals including rare audible ones below 15kHz such, as the slow dit dah wave.

These low frequencies can pick up signals like electrical storms and powerful lightning before the onset of a thunderstorm, manifesting as crunching, whistling or crackling noises.

For frequencies above 15kHz, the circuit can detect signals like Loran, Military, foreign broadcasts, CW signals and beacon signals between 160kHz and 190kHz.

The circuit diagram presented allows the construction of a circuit covering VLF from less than 10kHz to above 260kHz with the option to modify components for operation from 200 kHz to 500 kHz.

To achieve this, one can exchange input filter components remove and substitute capacitors and modify the inductor configuration.

Care must be taken during construction with components mounted on a 2 x 3 inch perfboard or circuit board for compactness.

It is emphasized to keep components and leads short ensuring isolation of the crystal oscillator circuit from the front end circuitry.

For optimal performance, the circuit is recommended to be enclosed in a metal case powered by a battery due to its low current draw.

Connection to a shortwave receivers antenna should be via a shielded or coaxial cable with a strong ground connection to both the converter and the receiver.

A fairly long wire antenna is suggested and the receivers mode switch should be set to the CW position.

Testing involves generating a loud tone through the receivers BFO, heterodyning with the 3.5 MHz signal from the converters crystal oscillator.

The dial values are then utilized to scan the converted frequency.

Specific instructions are provided for constructing inductor L3 and adjusting the circuit for different frequency bands.

Formula and Calculations:

During the design phase, some circuit parameters are calculated using certain formula for oscillator frequency:

Oscillator Frequency: The oscillator frequency calculation depends on the intended output frequency range if the converter mixes the VLF signal with a different frequency using a local oscillator LO.

For instance, the oscillator frequency fLO may be computed as follows if you wish to convert a 100 kHz signal to a frequency range of 4 MHz to 5 MHz:

f_LO = f_out – f_in

where,

• It shows the necessary local oscillator frequency f _LO in a heterodyne VLF converter circuit in order to get the appropriate output frequency f_out depending on the frequency of the incoming VLF input signal f_in.

Explanation of the formula and how VLF converters use it:

Heterodyne Conversion: Frequency converters frequently employ this approach.

It creates a new frequency at the output f_out by combining the incoming VLF signal f_in with a signal from a local oscillator f_LO.

f_LO (Local Oscillator Frequency): The frequency of the signal produced by the local oscillator inside the converter circuit is known as fL_O, or local oscillator frequency.

It is essential in establishing the output frequency.

f_in (VLF Input Signal Frequency): The frequency of the very low frequency signal you wish to convert to a greater range is indicated by f_in.

f_out (Output Frequency): This is the intended frequency range for the converted output signal.

How formula functions:

In essence, the formula assists you in determining the necessary local oscillator frequency by taking into account the frequency of the entering VLF signal and the desired output range.

Sum and difference frequencies are produced by non linear components such as transistors or diodes by combining the f_in and f_LO signals inside the converter circuit.

Usually, the difference between the frequency of the entering VLF signal fi_n and the frequency of the local oscillator f_LO is used to determine the required output frequency f_out.

For Example:

Suppose you have a VLF signal that is at 100 kHz f_in and you wish to convert it to a higher frequency range of 4 MHz to 5 MHz f_out.
The local oscillator frequency f_LO would be as follows using the formula:

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

How to Build:

Building the VLF converter circuit involves assembling the components and following the construction tips provided.

Component Placement:

• Lay out the components on the perfboard or circuit board according to the circuit diagram.
• Pay attention to keeping leads short and make sure the crystal oscillator circuit is isolated from the front end circuitry.

Soldering:

• Solder the components onto the board.
• Double check for any solder bridges or cold joints.
• Use a socket for the crystal oscillator if you plan to swap it for another frequency.

Enclosure:

• Place the circuit in a metal case to shield it from external interference.
• Ensure that the case is properly grounded.
• Drill holes for connectors, switches and any adjustments needed.

Power Supply:

• Connect a battery to power the circuit.
• Ensure the battery can supply the required voltage and current.

Connection to Shortwave Receiver:

• Connect the converters RF output to the shortwave receivers antenna using a shielded or coaxial cable.
• Establish a strong ground connection for both the converter and the receiver.

Antenna Setup:

• Connect a fairly long wire antenna to the converter.
• This antenna enhances the reception capabilities of the circuit.

Mode Setting:

• Set the receivers mode switch to the CW (continuous wave) position.

Testing:

• Power up the circuit and the shortwave receiver.
• Use the receivers BFO (beat frequency oscillator) to generate a loud tone which should heterodyne with the 3.5MHz signal from the converters crystal oscillator.
• Test the circuit by scanning through the converted frequencies using the dial values as indicated.

Adjustments:

• Fine tune the circuit if necessary, considering the range selection and frequency adjustments mentioned in the instructions.

Caution:

Remember to follow safety precautions double check connections, and refer to the provided circuit diagram and instructions throughout the construction process.

If you encounter any issues use a multimeter or oscilloscope for troubleshooting.

Conclusion:

The construction and tuning of the circuit involve careful placement and soldering of components, consideration of proper grounding connection to a shortwave receiver and adjustment of frequency ranges.

The VLF converter circuit allows enthusiasts and hobbyists to explore and listen to signals in the VLF spectrum that are not easily detected by standard radio receivers.

References:

Very low frequency