A Simple HF Up Converter
For use with the RTL SDR or Fun Cube Dongle
by MattDawson, GW0VNR
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.
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.
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.
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.
- Saronix STAA9FC-106.2500T oscillator module U1
- 39R 0.25W metal film resistor R1
- 150R 0.25W metal film resistors R2,3
- 1N5711 schottky barrier diodes D1-4
- 500mA fast polyfuse F1
- 100n ceramic capacitor C1
- 120p NP0 ceramic capacitor C2,5
- 180p NP0 ceramic capacitor C3
- 27p NP0 ceramic capacitor C4
- 5-22p variable trimmer capacitor C6,7
- T37-2 Micrometals toroids L1,2
- FT37-43 Micrometals toroids T1-3
- 5T 5mm dia air spaced coils L3,4
- Connectors to suit
- FR4 single sided copper-clad PCB
PCB gerber archive is here