2017年7月9日日曜日

Hot-melt glue for RTL-SDR direct sampling

I have a generic Chinese RTL-SDR dongle which, as you probably know, is a very inexpensive USB device that, when connected to a PC with the appropriate software, can be repurposed as a generic software-defined radio capable of receiving from 24 MHz to more than 1 GHz. More on these devices can be read at, for example, http://www.rtl-sdr.com/.

For me, however, MW and HF (0-30 MHz) are more interesting, and there are 2 approaches to receiving MW and HF on an RTL-SDR dongle:

  1. Up-conversion
  2. Direct sampling, where an antenna is connected directly to the internal chip, bypassing the tuner. Reference: http://www.rtl-sdr.com/rtl-sdr-direct-sampling-mode/.
This post concerns direct sampling. The summary is that I used hot-melt glue instead of soldering to attach a wire to pin 4. The following photos show the details.

I opened up my dongle and noticed it had two small copper islands which appeared to be designed for direct sampling. This makes it much easier to implement the direct sampling modification, as the necessary antenna wire can be connected to the relatively large copper island, instead of the tiny pin on the chip. Nevertheless, even though the copper island is "relatively large" compared to the pin size or the size of the nearby SMT capacitors, the actual size of the copper island is still quite small for hand soldering.



 

Though my board, as shown above, had copper islands, I actually expected to see a hole in the circuit board through which a wire could be pushed. The webpage at http://www.radioforeveryone.com/p/the-new-smart-manufacturer-link-can-do.html shows several examples of RTL-SDR board layouts with a pre-drilled hole in the same location. The idea is then simply to push a small wire through the hole, removing the need for soldering. Here is a sample photograph (from the above web page) of a board with a pre-drilled hole for the direct sampling modification.



My mistake: trying to drill my own hole.


Since I expected a hole where I only had a copper island, I thought perhaps I could drill a hole myself. I used a sharp hand tool and a magnifying glass to exactly place the tip of the tool in the center of the island.


I stopped drilling when a small hole had been created in the center of the island. Further drilling would have (due to the wide tool diameter) completely scraped away the copper island, so I did not attempt to drill all the way through the board.

To affix the wire to the board, first I placed a drop of hot-melt glue on top of the crystal, to hold the wire in place some distance from the desired connection point.



Then, with the wire held firmly in place by the hot melt glue, I bent the wire to lie exactly in the hole (or rather, the depression) that I had just drilled.



Unfortunately, even in this seemingly-perfectly-positioned condition, testing continuity with a multimeter revealed that the wire had no electrical contact with the copper island! It seems that my drilling had the effect of removing so much copper from the exact center of the island, that when a wire was pushed into the exact center of the newly-created hole, it was unable to make any electrical contact to the edges of the copper island lining the hole.

Salvaging my work


I bent the wire to make contact with the copper island's edges instead of the center of the hole. Testing with a multimeter confirmed contact between the copper island and the wire.

Then, being careful not to disturb the precariously-poised wire, I injected two dabs of hot-melt glue to encase the connection and hopefully hold it in place mechanically.


Finally, I added one more dab of hot-melt glue at the far side of the wire, and routed the wire outside through the RTL-SDR case.


Not working yet...


I ran my SDR software (gqrx on Linux) but unfortunately connecting an antenna to the newly-connected wire showed no change, indicating the wire was not actually connected anymore to the copper island.

Applying pressure


Through experimentation I found that if I pressed down hard on the hardened dab of hot-melt glue (that encased the connection point), the electrical connection would be made. Therefore, I can only conclude that either during handling the glue broke slightly loose, and/or during drying perhaps the glue changed shape slightly, shifting or lifting the wire away from the copper island.

To fix this, I placed a small piece of paper, folded about 4 times, on top of the hardened dab of glue, and closed the plastic case on top of the folded paper. The pressure of the closed case on the folded paper was enough to push the hardened dab of glue downwards to allow the encased wire to make electrical contact with the pad.

Reception results


FM broadcast signals are present in many places in the HF spectrum. This makes the quality of reception unacceptable (as the FM signals drown out the weak HF signals of interest), and a low-pass filter (or band-pass filter) will be mandatory for acceptable reception.

In the evening, with a 2-meter wire antenna, I could receive local AM stations clearly.

At 7 MHz, again there were severe problems with FM broadcast interference. However, I could weakly pick up one Chinese shortwave station, which is normally extremely loud when received on other shortwave receivers. No ham stations at 7 MHz were audible.

Next steps

  1. Use both copper pads and a balun for a differential input into the chip.
  2. Add lowpass filter to remove FM interference. Shielding the device may also be necessary.
  3. Add a separate low noise amplifier to boost the antenna signal. The software I use (gqrx) seems no longer to be able to control the device's internal LNA when direct sampling mode is chosen.

Addendum


The hot melt glue, if applied sparingly, can fairly easily be peeled off in case of a failed attempt, making this method perhaps easier than directly attempting to solder to the tiny pads.