A LO signal made from a standard 10 MHz crystal wired in a Pierce oscillator configuration can also be used. A fancy 10 MHz TCXO unit is not required for this circuit, though it's recommended. The 10 MHz LO signal is about +6 dBm at this point. The output from the TCXO is passed through an optional 10 MHz crystal filter to further reduce the phase noise or spurs produced by the TCXO. The local oscillator port of the SRA-8 mixer is fed via a 10 MHz Temperature-Compensated Crystal Oscillator (TCXO) unit from a surplus Qualcomm OmniTRACS unit. The Mini-Circuits SRA-8 mixer was chosen because of its exceptional low frequency reponse (down to 500 Hz) on its RF port and its high port-to-port isolation. Filtering the RF input to the mixer is highly recommended in order to keep as much AM radio interference out of the mixer as possible. The low-pass filter utilizes high-Q silver mica capacitors and T-50-3 powdered iron toroid inductors for optimal response. The GBPPR VLF Converter consists of a Mini-Circuits SRA-8 mixer feed via a 5-pole Butterworth 500 kHz low-pass filter on its RF port. The overall concepts can be used for your own design, if necessary. The circuit shown here was designed for high-performance RF test lab or TSCM operations, so some of the parts may be expensive or difficult to track down. There are numerous VLF converter schematics on the Internet, many of which are much simpler in design. At this slightly higher frequency, it's easier to construct narrowband filters or utilize a receiver with a finer tuning frequency "step." For example, the 15.75 kHz horizontal synchronization signal from a standard NTSC video camera would be converted to 10.01575 MHz. This front-end converter uses a stable 10 MHz Local Oscillator (LO) to "upconvert" the 1 - 500 kHz band to 10.001 - 10.500 MHz.Ī standard HF receiver (or spectrum analyzer, computer soundcard, etc.) can then be used to further break down and examine the target RF signal(s).
This is a Very-Low Frequency (VLF) converter to increase the frequency response of a radio receiver operating below 500 kHz.
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