Up converter for receiving of shortwave stations on PC
A cheap USB DVB-T, DAB+ and FM receiver is available on the interner, which many enthuiasts use to listen to other frequencies. This receiver, based on RTL2832 or R820T2 chipset, has many Chinese clones, but all of them have similar frequency range - from 50 MHz to 2 GHz. There is no FM or AM tuner in this USB dongle, demodulation is done solely in software - either in software made by its manufacturer, or in a universal program, such as the popular SDR Sharp. It shows a nice frequency spectrum of around 1 MHz so that you can see what is "in the air" and which frequencies to focus on while tuning.
Hardware restrictions of these cheap USB dongles do not allow receiving of shortwave frequencies, which are typically a few MHz. It can be circumventer by using an up converter which shifts these low frequencies into higher frequencies that can be received by the USB dongle.
This up converter adds 100 MHz to frequencies in the 0 - 30 MHz range, so that shortwave stations can be tuned on frequencies 100 - 130 MHz. Because no crystal oscillator oscillates precisely at 100 MHz - it is a bit less or more - it is wise to find out the exact frequency by a high "peak" around 100 MHz in SDR Sharp and write this frequency in the Shift field. Then you can happily tune on the "real" low frequencies and the software computes the frequency given by up converter. I have written value -99 992 500 into the Shift field, as can be seen in the video below.
The schematics is taken from Tomasz Watorowski's page. However, there are a few important things not described there that I had to find out myself.
Inductors are air ones, everyone can make his own depending on which kind of Cu wire and diameter of pencil or a pen to wind on is at hand. I have used this Czech page to calculate inductors of required inductance. All my inductors are from laminated 0.5 mm-diameter Cu wire that can be easily obtained from a defunct TV, transformer, etc.
- 150 nH: 6 turns, 7.4 mm coil diameter, 11 mm coil length
- 270 nH: 9 turns, 5.5 mm coil diameter, 8 mm coil length
- 330 nH: 9 turns, 7.4 mm coil diameter, 12 mm coil length
- 150 nH: 12 turns, 7.4 mm coil diameter, 15 mm coil length
Input coil of 56 μH inductance is used from a dismantled TV, I didn't make it myself.
The input is marked "SMA" in the schematic, with shielding and grounding, however, the antenna should be just a long Cu wire, connected only to the input (ignore shielding and grounding).
I only needed to buy two things to build this up converter::
Unfortunately, both are only available in SMD package.
Input voltage of 5V is taken from USB.
Last important thing is to set up RF Gain to 14-21 dB in SDR Sharp, otherwise you don't get any signal. This fact is also not mentioned anywhere (as it if was self-explaining), which has left me to wonder why the up converter is not working. Of course, you need to switch to AM mode.
I designed the PCB myself, it was easy. Its dimensions are 8 cm x 8.5 cm and it is designed to fit inside Z76 box. There is only one wire connection - from Vcc to pin 8 of SA612.
Shortwave radios are best received after sunset and at night, when signal propagates best. I wish you good luck with building this up converter and happy tuning!