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A Simple HF Up Converter

For use with the RTL SDR or Fun Cube Dongle

by MattDawson, GW0VNR

The Problem

The RTL2832, often coupled with an Elonics E4000 silicon tuner, is able to output 8 bit IQ data directly. This makes it ideal for software defined radio applications. Unfortunately, the E4000 and the tuner used in the popular AMSAT Fun Cube Dongle only goes down to 64MHz or thereabouts. This simple project extends the range downwards using a simple mixer technique and allows reception of the 0-30MHz range. As well as making these parts a useful HF receiver, it also allows analysis of things like crystal and bandpass filters when used in conjunction with a noise bridge.

Schematic

attachment:UC_SCH.png

The theory of operation is quite simple. The incoming signal is filtered by a low pass filter with a 30MHz cutoff and is mixed in a double balanced diode ring mixer with a 106.25MHz local oscillator provided by U1, powered from the USB bus. The resultant signal is the sum of the original signal and the LO, putting it well within the tuner's range. There is a trap to keep the LO out of the open front end of the tuner, which would otherwise affect the rudimentary AGC these devices employ.

U1 is a surface mount device but it is rather large and is extremely simple to solder having only four pins. Of course, this means we need a real PCB.

High resolution schematic of this circuit is here.

PCB

attachment:HRB_PCB.png

The PCB is single sided FR4. It can be made using any of the common techniques, including toner transfer. The file should be printed at 300DPI using a decent graphics manipulation package such as The GIMP.

Overlay

attachment:UC_PLACE.png

The oscillator module is mounted on the other side of the board. It is useful to construct a small tin shield to completely cover this module to stop the LO getting into downstream stages.

Parts List

PCB gerber archive is here

HFUpConverter (last edited 2012-12-03 16:01:02 by MattDawson)