ZachTek WSPR TX Mini Build Instruction

Low pass filter

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1017 WSPR TX Mini Low pass filter build

Instruction.

On the left the WSPR TX Mini as it comes delivered without a LP filter mounted - on the right with a filter

The WSPR TX Mini (product number #1017) has an unpopulated space for an output low pass filter.

This build instruction will guide the user in building and soldering the low pass filter.

Background.

The low pass filter removes the overtones from the generated RF square wave and makes the transmitter

compliant to use on one amateur band.

The user will pick what band to use and will build the filter for this band.

The filter is made up of four capacitors and three inductors. The inductors are constructed using toroidal cores

and copper wire. The capacitors are surface mount and are soldered on top of the board.

The Low Pass filter is a separate product and comes in a separate bag with product number #1021.

Document version 1.1

Page | 1

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Summary of Contents for ZachTek WSPR TX Mini

Page 1 Instruction. On the left the WSPR TX Mini as it comes delivered without a LP filter mounted - on the right with a filter The WSPR TX Mini (product number #1017) has an unpopulated space for an output low pass filter.
Page 2 Remove the link wire on the WSPR TX Mini. As a preparation step, you may have to remove a link wire from the WSPR board. Sometimes a wire is soldered in to help testing it before delivery.
Page 3 Checking the contents of the LP filter bag, product number 1021. Check that the bag contains three toroidal cores, they can be black, red, yellow or other colors depending what frequency they are made for. Typically, they are yellow. The larger bag also contains two smaller bags that holds surface mount capacitors. To avoid getting the capacitors mixed up I recommend that they stay in their bags until it is time to solder them to the board.
Page 4 Checking out the placement on the circuit board. If you check the printed text on the circuit board you will find the positions for the capacitors - C4 is not printed due to an error in the manufacturing. There are two capacitance values in the filter. C1 and C4 are smaller in value than C2and C3.
Page 5 The inductors are three and the two outer ones have the same value. In other words - L1 and L3 have the same value. L2 is a larger value so you can use a similar mental memory guide for the inductor size, - The Higher value Inductor goes in the center The value of the inductors can also be found in the “SMD low pass filter table”...
Page 6 Building and mounting the LP filter. Solder the surface mount capacitors. There are several ways to solder the surface mount capacitor depending what tools you have. I will describe how to do it with a normal soldering iron and solder wire. First, apply some solder to one side of the pads that will hold the surface mount capacitor.
Page 7 Do this for both capacitors (C2 & C3). Now that innermost two capacitors are fixed and soldered to one side let’s go ahead and solder the other pad as well. You might want to turn the board around to get the best working angle. Be quick when heating and applying solder so you avoid melting the opposite side or else the cap might come lose and move about on the board.
Page 8 OK, you have soldered in the first two caps, well done! Now repeat this same procedure with the other two caps that will go on location C1 and C4. That is - first solder to one pad by reheating the pad and holding the cap with a tweezer - then turn around the board and solder the other pads.
Page 9 Doing the inner coil – L2 Check the low pass table in the appendix and see how many turns to use for L2. Wind the L2 inductor, if you are unfamiliar how to wind toroidal inductors - see the winding guide in the appendix.
Page 10 Now use a knife to scrape of the burnt isolation until the ends are shiny all around. Finally tin the ends with the soldering iron and some solder. If you have done a good job getting the enamel off it should take solder easily, if not scrape some more with the knife and pre-tin again.
Page 11 Solder L2 to the board You place it on top of the board and solder it on the underside so you have to be able to turn the board up- side-down without the toroid falling out. You can hold the toroid in place in some way or you can bend the leads on the underside, use whatever works for you.
Page 12 Cut of the excess piece of wire from the solder point with a side cutter. OK well done so far, looking good! Now continue winding the other two - L1 and L3, they have the same number of turns, again look in the low pass filter table to find out how many turns to wind.
Page 13 The result will look like this. Last step is to verify that everything works using the built in signal generator and an oscilloscope. Document version 1.1 Page | 13...
Page 14 Verifying the output Connect the board to a PC and start the configuration program. Set it in Signal generator mode and pick the correct frequency, in the screenshot below I am testing a 30m filter so I have set the signal generator to 10MHz Document version 1.1 Page | 14...
Page 15 Connect the antenna output to your oscilloscope using a SMA to BNC cable or use the standard probe on the oscilloscope as the picture below shows. Check the oscilloscope; you should see a nice sinus curve with about 3.3V Peak-to-peak value at the frequency you have set.
Page 16 If it the output is much less in voltage then first double check the frequency of the output so it is not higher than that the LP filter frequency, eg don’t set it to 14MHz if you are testing a 10MHz filter. If no signal can be seen then double-check your solder joints.
Page 17 Appendix. SMD low pass filter table: Document version 1.1 Page | 17...
Page 18 Appendix. Winding guide: There is no special trick to winding a toroidal core, simply wind the wire in any direction and any way you like. However there are some recommendation how to do, it and you need to know what counts as a turn and what does not.
Page 19: End Of Document
One Turn Two turns If you get lost in the count then you can recount the number of turns by using the following rule: look at the core from above and count the number of wires that cross the top side of the core, se picture for an example of a 17-turn toroid END OF DOCUMENT Document version 1.1...

This manual is also suitable for:

1017