{"id":8698,"date":"2020-07-11T10:59:00","date_gmt":"2020-07-11T00:59:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2020\/07\/11\/q-31-receiver-filter-fix-with-the-nanovna\/"},"modified":"2025-07-22T01:09:24","modified_gmt":"2025-07-21T15:09:24","slug":"q-31-receiver-filter-fix-with-the-nanovna","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2020\/07\/11\/q-31-receiver-filter-fix-with-the-nanovna\/","title":{"rendered":"Q-31 Receiver Filter Fix with the NanoVNA"},"content":{"rendered":"
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The picture above shows my problem. As predicted by the Murata data sheet and as warned by R.A. Penfold,<\/a> that nice 455 kc ceramic filter has a significant response at around 640 kc. This caused me to hear Brother Stair twice as often as I should have. Clearly a spur exorcism was called for. <\/b><\/div>\n

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Click on the picture for a better image. As noted last time, my first idea was to build a series 640 kc trap LC circuit and put it ahead of the ceramic filter. But I had trouble getting the desired high Q. So I then thought about putting a wider 455 kc filter ahead of my 12 kc filter. I would, of course, need one that did NOT have the 640 kc spur. I found a 455B filter in my junk box. I used the NanoVNA to look at its response. No spur at 640 kc! Yea! <\/b><\/div>\n

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Next I put the two filters together, 455B first, then the 12 kc filter. Success! You can see on the NanoVNA that there is no spur at 640 kc. <\/b><\/div>\n
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With close to the desired termination impedances, the passband at 455 kc looked pretty good. I just put 1500 ohm resistors in series at the input and output of the dual filter combo. <\/b><\/div>\n

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It worked. <\/b>Spur exorcised! <\/b>I no longer hear each SW broadcast stations at two spots on my dial. <\/b><\/div>\n

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The picture above shows my problem. As predicted by the Murata data sheet and as warned by R.A. Penfold, that nice 455 kc ceramic filter has a significant response at around 640 kc. This caused me to hear Brother Stair twice as often as I should have. Clearly a spur exorcism was called for. Click … Continue reading “Q-31 Receiver Filter Fix with the NanoVNA”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":8699,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[93,16,191,270,72],"tags":[],"class_list":["post-8698","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-ceramic-resonators","category-filters","category-nanovna","category-quarantine","category-short-wave-listening"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/8698","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\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/comments?post=8698"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/8698\/revisions"}],"predecessor-version":[{"id":8703,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/8698\/revisions\/8703"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/8699"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=8698"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=8698"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=8698"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}