My loop tuning board has been designed and built. And it even works!

The receiving loop project needs two control circuits. The first is for remote control, and that was described in my recent articles. The second is for tuning the loop to desired frequencies in the 0.54 to 10.0 MHz range. There are two functions needed in the loop tuning board.

• A relay to switch between a small loop for the higher frequencies, and a large loop for the lower ones. As shown in the circuit diagram, the default “off” positions of the DPDT relay just connect the two loops in series, for a total of 9 turns. When the relay is “on”, only the small two turn loop is selected. The relay is powered by 5V, but 3.3V is used for control with the help of a switching transistor.
• A voltage to tune the varicaps  or variable capacitance diodes. This voltage of 0 to 8 volts will adjust the capacitance in a range of approximately 450 to 20 pF. The varicaps and loops are tied together to form a resonant circuit. Power for the varicaps is provided through large resistors to prevent loading of the resonant circuit.

Loop Tuning Board PCB

Three things of note about the printed circuit board. First, I am no longer embarrassed to publish pictures of the copper side of the board. My new temperature-controlled soldering station with its fine tip helps me solder really good and clean connections.

Second, I decided to move away from DuPont crimp connectors for external connections. Instead, I am using screw-type PCB Terminal Blocks. I picked these up from Active Tech, a local Canadian supplier. In addition to being easily able to screw down the connections, the terminal block pins are spaced 0.2” apart, rather than the normal 0.1”.

Third, where possible, I used wider spacing for pins and wider copper traces. This made things neater and easier to work with.

Notice also that I am using four diodes (two pairs) for tuning. Using diodes in pairs reduces the possibility of distortion being created by the received signals when they pass through the tuned circuit. Unfortunately, putting capacitors in series reduces capacitance in half. Which is why I added a second pair of varicaps in parallel. Parallel capacitances add.

Once you build or breadboard this circuit, you can check the capacitance my connecting a meter across the loop terminals. As you apply a variable voltage to the varicaps, you can measure the result easily.

Oh yes, make sure you connect the varicaps properly. I read the data sheet wrong and accidently put them in backwards first time around. Always have some solder wick handy.