VLF/LF Up-Converter for SDR or Analog Receivers DC-520 KHZ

This version includes a change-over relay which, when unpowered, connects the upconverter’s antenna input directly to the IF output SMA port. A jumper is also provided to select between a 50 Ω antenna input and a high-impedance antenna input (approximately 800 Ω via a 16:1 RF transformer).

Both the 50 Ω and high-impedance antenna inputs are protected by gas plasma discharge tubes for overvoltage protection. A diplexer filter is implemented ahead of the mixer, followed by a wideband amplifier at the mixer output. The unit incorporates a high-stability 10 MHz TCXO and provides support for an external local oscillator (LO) input.

Most HF receivers exhibit limited performance at low and very low frequencies (LF/VLF), due to factors such as design priorities, cost constraints, synthesizer phase noise, and front-end linearity. The purpose of this upconverter is to allow a high-quality HF receiver to be used for reception of the 0–520 kHz spectrum at 10.000–10.520 MHz (or 0–120 kHz at 10.000–10.120 MHz for the 120 kHz model). This approach enables the user to take advantage of the superior performance and features of an HF receiver when monitoring LF/VLF signals at a higher operating frequency. For newcomers to these frequency ranges, this opens access to an entirely new area of exploration.

A very low phase-noise, high-stability 10 MHz TCXO is used as the carrier source to translate the incoming 0–520 kHz spectrum to 10.000–10.520 MHz. Frequency translation is performed using a double-balanced RF mixer, with its IF port configured as the input, following the same proven topology used in earlier HF upconverter designs.

For direct antenna connection, dedicated gas-discharge tubes are incorporated to provide overvoltage protection. Since a 50 Ω input is often suboptimal for LF/VLF reception, a second antenna input is provided using a 16:1 BALUN transformer from Mini-Circuits with guaranteed specifications. This enables a high-impedance antenna input, better suited for long-wire antenna configurations. The BALUN transformer’s frequency response nominally begins at 30 kHz rather than DC; however, testing has shown the unit remains usable at lower frequencies, with attenuation increasing rapidly as frequency decreases.

Before reaching the mixer’s IF port, the antenna signal passes through a diplexer filter consisting of a 9th-order high-pass section and a 9th-order low-pass section. The high-pass section is terminated into a 50 Ω resistive load, ensuring that all RF energy above 520 kHz (or 120 kHz), including strong MF broadcast signals and other out-of-band energy, is absorbed in the load rather than reflected between the antenna and the mixer. The low-pass section feeds the mixer for upconversion to 10 MHz. This configuration improves dynamic range and is preferred over the use of a simple low-pass filter. The diplexer crossover frequency is set at 520 kHz for the DC–520 kHz model and at 120 kHz for the DC–120 kHz model.

The upconverted signal appears at the mixer’s RF port and is applied directly to a wideband amplifier (GALI-74+) to provide proper mixer termination. This amplifier compensates for mixer and diplexer losses and provides additional gain. If this gain is not required, attenuation may be applied at the input of the user’s HF receiver.

DC power for the 10 MHz local oscillator and the wideband amplifier is supplied via a mini-USB connector. A dedicated common-mode noise rejection filter is included to suppress computer-generated noise and provide a clean DC supply. The power circuitry is intentionally robust, incorporating a 6 A fuse and heavy-gauge wiring, to accommodate installations where poor grounding may result in voltage differences between the antenna ground and the power (computer) ground, leading to increased ground currents. This power architecture has been successfully used in previous upconverter designs.

The board provides a regulated 5 V DC output for an external GPS-disciplined oscillator (e.g., GPSDO-2), allowing the internal TCXO to be disabled and an externally supplied 10 MHz signal to be applied via the SMA connector labeled “EXT LO” when superior frequency accuracy or long-term stability is required. In this configuration, jumper JP2 must be removed to disable DC power to the internal TCXO. The same SMA connector can also be used to supply 5 V DC from an external low-noise power source if powering via USB is not desired.

The choice of a 10 MHz IF output is not restrictive, as the post-mixer circuitry is wideband. If operation at 10 MHz is not suitable, the internal TCXO may be replaced with another frequency in the range of 0.5–500 MHz. The upconverted LF/VLF spectrum will then be translated to the selected frequency, as the mixer LO/RF ports operate from 0.5 to 500 MHz and the wideband amplifier (GALI-74+) covers DC to 1 GHz.

Jumper JP1 allows access to the baseband spectrum and can be used to feed a sound card or other external device. It provides a well-filtered DC–520 kHz (or DC–120 kHz) output suitable for further processing or analysis.

Components L10, C24, and C25 form part of the LO conditioning network. Inductor L10 in series with capacitor C24 creates a series-tuned network that converts the 10 MHz TCXO square-wave output into a sine wave for driving the mixer. If operation with a square-wave LO is desired—allowing longer diode conduction in the mixer and potentially improved IMD performance—a high-value capacitor (e.g., 100 nF) may be installed in the C25 position to bypass the tuned network. The same approach applies when changing the TCXO to a different LO frequency: either L10 and C24 must be retuned for the new frequency, or C25 may be populated to bypass the tuned network entirely.