{"id":1327,"date":"2019-02-06T16:08:45","date_gmt":"2019-02-06T21:08:45","guid":{"rendered":"http:\/\/abrammorphew.com\/notes\/?p=1327"},"modified":"2026-03-16T13:44:24","modified_gmt":"2026-03-16T18:44:24","slug":"7mhz-crystal-oscillator-design","status":"publish","type":"post","link":"http:\/\/abrammorphew.com\/notes\/2019\/02\/06\/7mhz-crystal-oscillator-design\/","title":{"rendered":"7MHz Crystal Oscillator Design"},"content":{"rendered":"\n<p>After constructing a 40m wire dipole that works with my SDR setup, I needed to start working on a transmission system. At the heart of virtually any RF system lies a stable oscillator, and crystal oscillators are ubiquitous in many low-power (QRP) rigs simply because they are so stable. After some rough math and a lot of simulations in LTSpice, I came up with this design to give me somewhere around 7 dBm of power. <\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"1107\" height=\"828\" src=\"http:\/\/abrammorphew.com\/notes\/wp-content\/uploads\/2019\/02\/7MHz-VXO-Rev-B.png\" alt=\"\" class=\"wp-image-1328\"\/><\/figure>\n\n\n\n<p>The first stage is simple Colpitts oscillator topology with a &#8220;bent&#8221; 7.03 MHz crystal resonator. The 30 pF variable capacitor (C1) provides around 2 kHz of tuning. The output stage is a common-collector (Q4) and emitter-follower (Q3) with a negative feedback loop. I forget exactly where I saw this configuration, but I thought I would try it out and see if it worked. As you might have guessed, the output is loaded with higher order harmonics resulting in a waveform that doesn&#8217;t resemble a sine wave at all. I made sure to include a simple second-order low-pass Butterworth filter on the output to filter the output. <\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"1900\" height=\"1639\" src=\"http:\/\/abrammorphew.com\/notes\/wp-content\/uploads\/2019\/02\/VXO_ugly_build.jpg\" alt=\"\" class=\"wp-image-1329\"\/><\/figure>\n\n\n\n<p>Pictured above is the ugly constructed version of the oscillator in all it&#8217;s dead-bug style. I built it on a scrap piece of double-sided FR4 and overall it&#8217;s performance came out fairly close to what LTSpice had predicted. I got around 6 dBm of output and the second harmonic is around 29 dB down (around -23 dBm). That&#8217;s not the cleanest of signals, but it&#8217;s about right for the filter. Below is the output shown on my HP 8595E spectrum analyzer. <\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"1900\" height=\"1447\" src=\"http:\/\/abrammorphew.com\/notes\/wp-content\/uploads\/2019\/02\/VXO_fund_Pout.jpg\" alt=\"\" class=\"wp-image-1330\"\/><\/figure>\n\n\n\n<p>For the next phase, I&#8217;ll likely be adding control of the oscillator via an ATTiny chip. This will give me the ability to automate on-off keying of the device turning it into a simple CW beacon. One thing that could be improved is the current draw (~40 mA) from the emitter-follower at Q3. Basically, it&#8217;s a class A amplifier so it&#8217;s not the most efficient design in the world, but it provides enough power for driving an ADE-1 and could run off a small solar panel if I wanted to use it in the field. <\/p>\n","protected":false},"excerpt":{"rendered":"<p>After constructing a 40m wire dipole that works with my SDR setup, I needed to start working on a transmission system. At the heart of virtually any RF system lies a stable oscillator, and crystal oscillators are ubiquitous in many low-power (QRP) rigs simply because they are so stable. After some rough math and a [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":1329,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[312,278,102,288,24,227,277,90,310,311],"class_list":["post-1327","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-notes","tag-40m","tag-amateur-radio","tag-arduino","tag-attiny85","tag-circuit","tag-cw","tag-morse-code","tag-oscillator","tag-rf","tag-sdr"],"_links":{"self":[{"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/posts\/1327","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/comments?post=1327"}],"version-history":[{"count":3,"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/posts\/1327\/revisions"}],"predecessor-version":[{"id":1360,"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/posts\/1327\/revisions\/1360"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/media\/1329"}],"wp:attachment":[{"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/media?parent=1327"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/categories?post=1327"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/abrammorphew.com\/notes\/wp-json\/wp\/v2\/tags?post=1327"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}