Low cost SDR update (2): Encapsulation

The next task was to put the receiver in a box. As I wanted to make the receiver powered by 12V I needed room for a DC to DC converter and possibly some external filtering (to get rid of 198kHz droitwich for instance). For the same reason I wanted a screened metal box

Searching the mouser diecast box lists I could not find on  just big enough but i did find one that would do, so i ordered it. I did have to 3D print some 3mm threaded spacers of appropriate lengths as the 2 boards were at different heights above the bottom of the box.

Initially I used an LT1084 Low volt drop regulator to regulate the incoming 12V down to 5V directly to supply the FPGA board and hence the Receiver board, but it got hot, not good for reliability. So I considered using just a chinese DC/DC converter to drop the 12V supply directly down to 5V. However having had these boards fail in the past and put the full input voltage at the output I devised an alternative. I used the Chinese LM2596  DC/DC converter set to produce 8V output then used an LT1084 Low volt drop regulator to produce the 5V. This way if the DC/DC failed and produced full output the LDO regulator would protect the board. This was the method eventually used.

Low cost SDR (1)

Occasionally interesting projects do get shown on the BATC QO100 net which runs on Thursday evenings.

One such occasion in November 2024 was when GI4DOH mentioned he was using a cheap A to D board in front of a commercial FPGA board to do cw skimming and rtty skimming instead of the expensive Red Pittaya board

I had been using my ANAN 10 to do my skimming which is overkill . It is limited to 4 receivers. His system could do 16 so time to Investigate!

Full information was available at https://pavel-demin.github.io/qmtech-xc7z020-notes/ who was the designer of the  A to D board. All the necessary files were available to get the boards assembled in China. Luckily I found someone who was getting a batch made so I ordered one from him. The cost (shipped) was £58

The FPGA board QMTECH ZYNQ7000 XC7Z020 Development Board was available from Aliexpress  for around £80. It came with a 5 Volt Power supply

The only other items needed were a 2x25 pin header and socket to mate the boards,  a heatsink to keep the FPGA from overheating and an edge mount SMA socket. I found the 2x25 pin connectors recomended in the links to be too tight a fit, needing a screwdriver to seperate them (risking damaging the board) so I ended up using some single row  headers and sockets, cut to length and superglued together

The A/D PCB has a reminder to remove R14 and R15 on the FPGA board before mating the two boards and to connect 3.3V wires between the 2 boards (the positive lead on the RX board goes furthest away from the edge of the board), It didnt take long to assemble.

 Software was easy. Format an SDHC card then copy the contents of qmtech-adc-20240614.zip to it. I also copy the start.sh from sdr_receiver_hpsdr_77_76 directory to the root directory so that on power up the receiver can be accessed by HPSDR

After power up I found the IP address assigned by the router and pointed HPSDR running on my PC . It connected and I could see signals

Overnight I compared the 600m WSPR decodes on the new receiver with the 600m WSPR decodes on the ANAN-10. They were within 1dB of each other, the new receiver has good sensitivity!

Now to make the receiver protected from accidents from falling items!