Since many countries are allocating the 472 KHz band for experimental use by Radio Amateurs, a growing number of them as well as listeners have become interested on VLF and LF bands.
The fact that few radios are available on the market covering properly frequencies from below 5KHz to 500KHz, convinced us to provide radio amateurs and listeners with the high-performance receiving VLF-LF Converter that we introduce here.
The VLF-LF Up-Converter extents the range of any shortwave receiver to below 5 KHz to 500 KHz. It is connected between the antenna and a shortwave radio receiver. When it is power-on the <5KHz to 500 KHz band segment is translate to 3.500MHz to 4.000MHz, Model-350 or 4.000MHz to 4.500MHz, Model 400, allowing the reception of VLF and LF bands.
For example, if a conversion frequency of 4.000MHz - 4.500MHz, Model 400, is used, and the receiver tuned on 4.135.500MHz, you are receiving on 135.500KHz, just ignoring the digit “4” on the dial.
Alternatively most SDR radio control software lets customise the dial by the user, in this regard see below PowerSDR configuration.
The VLF-LF Up-Converter enables reception of VLF mobile maritime communications, aviation and marine navigational aids such as NDB and DGPS beacons, DGPS reference stations, NAVTEX, Standard Time & Frequency stations, LF Amateur bands, European LW broadcast stations, monitoring the sounds of nature created by planet Earth (Natural Radio) such as sferics, hiss, tweaks, whistlers, Dawn Chorus, other not well known VLF radio atmospheric sounds, and many more.
It is advisable to use a variable attenuator in front of the converter in order to improve signal to noise ratio. Its use have more or less noticeable effects depending on local conditions. If you are living in the country side far away from noise sources probably you do not need one.
In urban areas in presence of strong signals and/or noise a variable attenuator avoids saturating the IF receiver. Adjusting the attenuation to a proper value weak signals buried in the noise can be seen emerging.
In addition better energy transfer is achieved between antenna and converter due the broad impedance matching effect.
This section provides good performance right up to the start of the AM broadcast band with a sharp roll-off, giving a good rejection to MW broadcast signals avoiding introducing overload or intermodulation products in the receiver.
500KHz LPF and 3.7KHz IF trap input filter section plot.
(Up Converter model 350)
A low phase noise, crystal controlled oscillator provides +7dBm signal level to the mixer
A high quality quartz crystal of 3.5MHz or 4MHz, according to the model of converter is used. A trimmer capacitor is accessible by the user if fine adjust of frequency is required in times. A linear fixed voltage IC regulates the voltage applied to the oscillator to enhance stability and avoid drift.
The oscillator is followed by a buffer stage to improve its stability. Next, an amplifier accommodates the signal to the required level. It connects to a 4 MHz, 7th order, Elliptic Low Pass Filter which cleans up any harmonics and assures that a pure signal applies to the mixer to minimize spurious responses.
The mixer stage is designed around a Mini-Circuits SRA-8+ double balanced mixer. It is rated to operate from 500Hz, giving and excellent frequency coverage. Designed for +7 dBm of LO power, it have a conversion loss of less than 5 dB at VLF LF bands, which is compensate by the post-mixer amplifier. A double balanced mixer has the advantage of high port-to-port isolation, which keeps the strong LO signal from degrading the dynamic range of the IF receiver. Every effort has been made to terminate the ports of the mixer in a proper 50 Ω impedance to maximize its performance.
The mixer IF port is most critical in terms of proper termination, for so, a diplexer circuit provides a 50 Ω resistive load to the mixer while passing the desired signal to the IF amplifier, with minimum loss.
Next, a Band Pass filter at IF frequency connects to a Low Noise Amplifier to compensate losses, delivering around 5dB of gain to the output, necessary to overcome losses from external cable tails, relays and connectors.
A carefully PCB and shielding design keep feed through signals of the HF receiving frequency down to a very low level; therefore, weak LF signals will not be interfered by strong HF signals on the same frequency.
The HF receiver used should have strong rejection of HF signals through paths other than the antenna connector. Most HF transceivers and receivers have good performance in this respect, but some SWL receivers do not and may be used with the addition of filters and shielding.
VLF LF Up-Converter (any model)
Set of BNC-BNC coaxial tails, power cable and manual on CDR included.