{"id":3619,"date":"2021-07-14T12:14:00","date_gmt":"2021-07-14T02:14:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2021\/07\/14\/mythbuster-video-4-first-signals-75-meter-bandpass-filter-yaesu-vfo-output\/"},"modified":"2025-07-22T18:04:39","modified_gmt":"2025-07-22T08:04:39","slug":"mythbuster-video-4-first-signals-75-meter-bandpass-filter-yaesu-vfo-output","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2021\/07\/14\/mythbuster-video-4-first-signals-75-meter-bandpass-filter-yaesu-vfo-output\/","title":{"rendered":"Mythbuster Video #4 &#8212; First Signals, 75 meter Bandpass Filter, Yaesu VFO output"},"content":{"rendered":"<p><iframe loading=\"lazy\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture\" allowfullscreen=\"\" frameborder=\"0\" height=\"247\" src=\"https:\/\/www.youtube.com\/embed\/Nq34RAmvcNI?rel=0\" title=\"YouTube video player\" width=\"440\"><\/iframe><\/p>\n<div><\/div>\n<div><b>This receiver required almost no coaxing or tweaking, probably because I had been so careful about testing and measuring each of the stages. <\/b><\/div>\n<div><b><br \/><\/b><\/div>\n<div><b>I have been pleasantly surprised at how well the receiver works without an RF amplifier ahead of the first mixer. But I need to know how much AF gain I have in order to understand how\/why the entire receiver works so well. I think I have about 35 db of gain (combined) through the two TIAs and the crystal filter. That would mean that all of the remaining gain is provided by the AF amplifiers (with some loss in the product detector). <\/b><b>I haven&#8217;t really measured the gain of the AF preamp\/LM386 combo, and I had some trouble measuring the input impedance of the pre-amp with the NanoVNA. <\/b><\/div>\n<div><b><br \/><\/b><\/div>\n<div><b>The 75 meter LC filter to the left of the VFO is actually a bandpass filter, not the lowpass filter. And what I call &#8220;the mixer&#8221; to the right of the VFO is really the Product Detector\/BFO.<\/b><\/div>\n<div><b><br \/><\/b><\/div>\n<div><b>For the 75 meter bandpass filter, I used the ELSIE program. <\/b><\/div>\n<div><\/div>\n<div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><a href=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Mythbuster75BPfilterschematic.png\" style=\"margin-left: 1em; margin-right: 1em;\"><img loading=\"lazy\" decoding=\"async\" border=\"0\" data-original-height=\"732\" data-original-width=\"1013\" height=\"289\" src=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Mythbuster75BPfilterschematic.png\" width=\"400\" \/><\/a><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b>75 meter Bandpass Filter designed in Elsie. 10 turns on a T50-2 toroid yield .46uH.<\/b><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><a href=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Mythbuster75BPfilterpassband.png\" style=\"margin-left: 1em; margin-right: 1em;\"><img decoding=\"async\" border=\"0\" data-original-height=\"777\" data-original-width=\"1033\" src=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/Mythbuster75BPfilterpassband.png\" width=\"320\" \/><\/a><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b>Here&#8217;s the plot from Elsie on the 75 meter BP filter. <\/b><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><a href=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/MythbusterVFOoutput.jpg\" style=\"margin-left: 1em; margin-right: 1em;\"><img loading=\"lazy\" decoding=\"async\" border=\"0\" data-original-height=\"2048\" data-original-width=\"1536\" height=\"400\" src=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2021\/07\/MythbusterVFOoutput.jpg\" width=\"300\" \/><\/a><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b>Alan W2AEW asked for a picture of the VFO output. <\/b><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b>On this shot I had the probe between the<\/b><b>VFO and the <\/b><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b>outboard booster amp that I built to bring it to 7dbm. <\/b><\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>This receiver required almost no coaxing or tweaking, probably because I had been so careful about testing and measuring each of the stages. I have been pleasantly surprised at how well the receiver works without an RF amplifier ahead of the first mixer. But I need to know how much AF gain I have in &hellip; <a href=\"https:\/\/www.homebrewradio.us\/blog\/2021\/07\/14\/mythbuster-video-4-first-signals-75-meter-bandpass-filter-yaesu-vfo-output\/\" class=\"more-link\">Continue reading<span class=\"screen-reader-text\"> &#8220;Mythbuster Video #4 &#8212; First Signals, 75 meter Bandpass Filter, Yaesu VFO output&#8221;<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":3620,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[16,132,57,53],"tags":[],"class_list":["post-3619","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-filters","category-mythbuster","category-superhet-receivers","category-vfo"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/3619","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=3619"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/3619\/revisions"}],"predecessor-version":[{"id":3623,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/3619\/revisions\/3623"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/3620"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=3619"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=3619"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=3619"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}