{"id":8036,"date":"2017-02-07T10:21:00","date_gmt":"2017-02-06T23:21:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2017\/02\/07\/schematic-for-the-rtl-sdr-dongle-front-end\/"},"modified":"2025-07-21T23:56:02","modified_gmt":"2025-07-21T13:56:02","slug":"schematic-for-the-rtl-sdr-dongle-front-end","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2017\/02\/07\/schematic-for-the-rtl-sdr-dongle-front-end\/","title":{"rendered":"Schematic for the RTL SDR Dongle Front End"},"content":{"rendered":"
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Here it is. Very simple. I used an obsolete 40673 dual gate MOSFET. I didn’t need both gates so I just soldered them together. You could probably substitute an easily obtained MPF-102 JFET. <\/b>The capacitors and the coil in in the gate circuit form a parallel LC filter circuit that resonates in the 40 meter ham band. The 1 Megohm resistor maintains a very high impedance for the LC circuit, helping to maintain circuit Q (sharp response). It also drains off any charge that builds up on the gates. For the antenna input I just put 2 turns on the 4.5uH coil — this too helps maintain LC filter response. The 470 ohm resistor serves as the load and limits quiescent current through the MOSFET. It should pull about 19 milliamps — with a 9V battery that should give you around 26 hours of listening time between battery changes. That’s fine with me — I don’t use this thing that much. <\/b><\/div>\n

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Realize that I’m using this with an RTL-SDR dongle that has been given the familiar modification that allows it to use direct sampling in the HF bands. Some ideas here: <\/b><\/div>\n
http:\/\/www.rtl-sdr.com\/rtl-sdr-direct-sampling-mode\/<\/b><\/a><\/div>\n
and here<\/div>\n
http:\/\/soldersmoke.blogspot.com\/2015\/04\/sdr-dongle-modified-for-hf-watch-it.html<\/b><\/a><\/div>\n
and here<\/div>\n
https:\/\/dl.dropboxusercontent.com\/u\/1987387\/Even_more_on_using_the_RTL2832U_Dongle.pdf<\/b><\/a><\/div>\n
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Here it is. Very simple. I used an obsolete 40673 dual gate MOSFET. I didn’t need both gates so I just soldered them together. You could probably substitute an easily obtained MPF-102 JFET. The capacitors and the coil in in the gate circuit form a parallel LC filter circuit that resonates in the 40 meter … Continue reading “Schematic for the RTL SDR Dongle Front End”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":8037,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[16,40],"tags":[],"class_list":["post-8036","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-filters","category-sdr"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/8036","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=8036"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/8036\/revisions"}],"predecessor-version":[{"id":8040,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/8036\/revisions\/8040"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/8037"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=8036"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=8036"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=8036"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}