I assembled the first prototype of the RFID sniffer on one of the printed circuit boards. Soldering the small SMD parts requires a good soldering iron with a fine tip, a pair of tweezers and a steady hand. I first tinned all solder pads and removed excess tin with the help of solder wick. This makes soldering the parts much easier.
Then I placed the SMD parts one by one, holding them with the tweezers and heating one of the solder pads to fix it in place. Then applying a little bit of solder to each of the other pads. Again using solder wick to remove excess solder if needed.
The trimmer and potentiometer were the hardest parts so solder, due to the small contact surfaces on these tiny components.
After connecting the battery, I pressed the switch and the LED came on. I just had to adjust the level of the potentiometer to set the threshold to light the LED only when a RFID tag is near the loop antenna.
Measuring the frequency with an oscilloscope revealed that the frequency was about 16 MHz, quite a bit higher than 13.56 MHz, but it could still detect tags. I adjusted the trimmer capacitor anyway, to set the frequency to approximately 13.56 MHz.
The full range appeared to be from 12.7 to 16.2 MHz, but it hardly affects the detection of tags. I can probably get rid of the trimmer and use slightly higher values for capacitors C1 and C2 to fix the frequency to within a few percent of 13.56 MHz.
The oscillator frequency can be measured without having a oscilloscope probe making contact with the circuit. You can construct a coil from a few turns of solid copper wire and hook it up between the tip of the probe and its ground clip, and then holding it close to the loop antenna. This will pickup the emitted RF signal just like a tag would do.