{"id":5891,"date":"2021-11-20T12:46:00","date_gmt":"2021-11-20T01:46:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2021\/11\/20\/the-double-crystal-lattice-filter-in-the-swan-240-smoothing-it-out-with-a-nanovna\/"},"modified":"2025-07-22T18:04:08","modified_gmt":"2025-07-22T08:04:08","slug":"the-double-crystal-lattice-filter-in-the-swan-240-smoothing-it-out-with-a-nanovna","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2021\/11\/20\/the-double-crystal-lattice-filter-in-the-swan-240-smoothing-it-out-with-a-nanovna\/","title":{"rendered":"The Double Crystal Lattice Filter in the Swan 240 — Smoothing it out with a NanoVNA"},"content":{"rendered":"<\/p>\n
<\/a><\/div>\n
<\/div>\n
In SolderSmoke Podcast #234, I said that I was scrutinizing the filter from the Swan 240 that I had picked up around 1994 in the Dominican Republic. I cannibalized it out in the Azores in the early 2000s and used the parts to build — among other things — my first SSB transmitter. I never really focused much attention on the filter that I pulled out of that old rig — I was just happy that it seemed to work. But I am now older and wiser, and I have some test gear that lets me look at the passband of that filter. <\/b><\/div>\n
<\/div>\n
<\/a><\/div>\n
<\/div>\n
First, take a look at what it is supposed to look like. This is from the manual. Yikes! That passband looks far from flat. I can almost hear homebrewers around the world shrieking in horror and disgust. <\/b><\/div>\n
<\/div>\n
<\/a><\/div>\n
<\/div>\n
<\/a><\/div>\n
<\/div>\n
Above is a description of the filter, and the schematic, again from the manual. <\/b><\/div>\n
<\/div>\n
\n
<\/a><\/div>\n
<\/div>\n
Here is what my extracted and somewhat re-built filter looked like in my NanoVNA (more shrieking!). The dip in the passband is a lot worse here — it looks like 10 db vs. 3 db in the manual. This is probably because I’m not even attempting any impedance matching on the filter — it is just seeing the 50 ohms in and out of the NanoVNA. <\/b><\/div>\n
<\/div>\n
<\/a><\/div>\n
<\/div>\n
Here is my 2002 attempt to rebuild the filter and put it in my SSB transmitter, along with my more recent attempt to flatten the passband. I no longer had the adjustable coil L8, so I made my own coil based on guidance from Ben Vester W3TLN’s January 1959 QST article on “Surplus-Crystal High-Frequency Filters.” (Ben had an early influence on Pete Juliano’s tube-rig designs.) In the picture above I have 1k pots between the filter and the input and the output of the NanoVNA, as described by Nick M0NTV<\/a>. <\/b><\/div>\n
<\/div>\n
<\/a><\/div>\n
<\/div>\n
Adjusting the 1 k pots, I could smooth out the passband quite a bit. Measuring the pots and adding the 50 ohms of the NanoVNA, it looks to me like this filter is smoother with about 280 ohms at the input and output. I may build two matching networks or some transformers. Some TIAs may also be needed. <\/b><\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

In SolderSmoke Podcast #234, I said that I was scrutinizing the filter from the Swan 240 that I had picked up around 1994 in the Dominican Republic. I cannibalized it out in the Azores in the early 2000s and used the parts to build — among other things — my first SSB transmitter. I never … Continue reading “The Double Crystal Lattice Filter in the Swan 240 — Smoothing it out with a NanoVNA”<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":5892,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[164,19,16,41,191,380],"tags":[],"class_list":["post-5891","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-azores","category-dominican-republic","category-filters","category-juliano-pete","category-nanovna","category-swan-240"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/5891","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/comments?post=5891"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/5891\/revisions"}],"predecessor-version":[{"id":5898,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/5891\/revisions\/5898"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/5892"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=5891"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=5891"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=5891"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}