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Add-on for the WSPR-TX_LP1 giving it four switched lowpass filters

Updated: May 23, 2019

The Mezzanine LP4 is a new add-on that expands the WSPR-TX_LP1 transmitter to four bands or more (US users can use more bands with careful selection of the filters, read the FAQ in the build instruction document)

Hi All!

This one took a bit longer to complete than my other products partly because some difficulty getting good overtone suppression for some of the filters, but in the end I manged to track down the problem and solve it.

I meant to release this before Christmas but because my measurement showed that two of the filters groups could not give 50dBC overtone suppression, I had to trouble shoot and eventually redesign the add-on card with a four layer PCB.

The card is a pretty easy design and is basically a few relays that switch in or out low pas filters as commanded by the Arduino. The four low pass filters are built in to two groups of two. After I built my initial prototype and measured it everything looked good and I proceeded making a first batch but when they arrived and I did some more careful measurements it showed that the top two filters A & B did not meet my design goal of 50dB suppression or more on all overtones! :-(

On some bands above 10MHz I only got 46 or 47 dB suppression. The strange thing was that if I built the same filter in spot C or D it worked fine! As you can see the design is nearly symmetrical so it was surprising to have different performance between different parts of the board. I came to the conclusion it had to be unwanted coupling to the WSPR-TX_LP1 board from some of the traces that were on the bottom part of the ad-on card facing down. So I redesigned board to be 4- layered PCB even though I wasn't 100% sure the problem was cased by unwanted coupling to the the lower board. But time was in short supply and I never designed a four layer board before so that seemed like a useful pursuit :-) To my relive my assumption was correct and the redesign fixed the problem! With the new design there no problem at all getting at least 50dB suppression of overtones and in some bands the overtones are even down 60 or 65dBC. The board sells for 19$ and four bandpass filters for it is another 10$ so it is a really inexpensive way to be QRV on four or more bands, especially if you already have a WSPR-TX-LP1 board.

The combination of the WSPR-TX_LP1 and this Mezzanine LP4 will give you the same features as the Desktop WSPR transmitter but to a lower price if you are prepared to wind a few toroids and solder in. You also need to upgrade to the latest firmware in your WSPR-TX_LP1 so some Arduino skills are needed of course.

The configuration software for the WSPR transmitters is now updated and understands how to use this ad-on card. It will indicate what low pass filters is fitted in its GUI.

Note the three green small squares next to the 30m, 20m and 17m bands. This indicate that low pass filters for these bands are fitted to the Mezzanine LP4 board (You do not have to have four filters, in this case the Mezzanine LP4 board only had three filters)

The Transmitter will still allow you to transmit on any band regardless if you have a filter for it or not.

It will try to pick the best filter for each band.

Check out the documentation page for more information . There is a comprehensive build document that goes over every aspect on how to build and integrate it to the WSPR-TX_LP1. There is also an in depth FAQ in the build document if you want to know more. The Mezanine LP4 and low pass filter kits for all Ham bands from 137kHz to 70MHz are available in the shop.


//Harry - SM7PNV

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