{"id":6226,"date":"2023-03-14T11:56:00","date_gmt":"2023-03-14T00:56:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2023\/03\/14\/fixing-the-tuning-problem-in-the-high-school-direct-conversion-receiver-with-video\/"},"modified":"2025-07-22T18:02:25","modified_gmt":"2025-07-22T08:02:25","slug":"fixing-the-tuning-problem-in-the-high-school-direct-conversion-receiver-with-video","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2023\/03\/14\/fixing-the-tuning-problem-in-the-high-school-direct-conversion-receiver-with-video\/","title":{"rendered":"Fixing the Tuning Problem in the High-School Direct Conversion Receiver (with video)"},"content":{"rendered":"<p><iframe loading=\"lazy\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" allowfullscreen=\"\" frameborder=\"0\" height=\"247\" src=\"https:\/\/www.youtube.com\/embed\/kYmtgT8LrSg?rel=0\" title=\"YouTube video player\" width=\"440\"><\/iframe><\/p>\n<p><b><u>Here is the problem:<span style=\"mso-spacerun: yes;\"> <\/span><\/u><\/b><\/p>\n<p class=\"MsoNormal\"><b>For the capacitive element in the LC circuit we have essentially two 660 pF caps in series.<span style=\"mso-spacerun: yes;\"> <\/span>This results in a total capacitance of 330 pf.<span style=\"mso-spacerun: yes;\"> <\/span>I measured 362 pF.<span style=\"mso-spacerun: yes;\"> <\/span><o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b>To get a resonant frequency of 7.0 MHz with 362 pF we need 1.428 uH. <o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b>To get 1.428 uH on the PTO coil form we need about 21 turns of wire.<span style=\"mso-spacerun: yes;\"> <\/span><o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b>21 turns on our coil form yields 1.440 uH and resonates with 362 pf at 6.9708 MHz<o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b>That\u2019s pretty close to what we need, <u>but<\/u> the problem arises when we screw in the brass tuning screw. <span style=\"mso-spacerun: yes;\"> <\/span>This reduces the inductance and raises the frequency. Putting the screw all the way in reduces the inductance to 1.138 uH resulting in a resonant frequency of 7.8414 MHz.<span style=\"mso-spacerun: yes;\"> <\/span>So with a coil this large (that we must use if we want to tune down to 7.0 MHz) we end up with a tuning range that is far too large.<span style=\"mso-spacerun: yes;\"> <\/span>We only need 7.0 to 7.3.<span style=\"mso-spacerun: yes;\"> <\/span>In effect, this means that we end up using only a small portion of the tuning range:<span style=\"mso-spacerun: yes;\"> <\/span>We can turn the screw approximately 34 times, but only 6 turns keep us within the range of 7 to 7.3 MHz (the 40 meter band).<span style=\"mso-spacerun: yes;\"> <\/span>There is about 50 kHz per turn of the dial.<span style=\"mso-spacerun: yes;\"> <\/span>This makes tuning difficult.<span style=\"mso-spacerun: yes;\"> <\/span>It becomes more difficult to separate stations and tune them in.<span style=\"mso-spacerun: yes;\"> <\/span>It would be better if we could tune across the band using more turns of the dial.<span style=\"mso-spacerun: yes;\"> <\/span>At least 15 turns of the dial would be nice:<span style=\"mso-spacerun: yes;\"> <\/span>That would mean about 20 kHz per turn.<span style=\"mso-spacerun: yes;\"> <\/span>But how can we do this?<span style=\"mso-spacerun: yes;\"> <\/span><\/b><o:p><\/o:p><\/p>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><b><a href=\"https:\/\/blogger.googleusercontent.com\/img\/a\/AVvXsEigHaEN0qb8LKWyiqlTOHL-iyvzc0Awqwzp4DVoFviJR-dc-YDh1NtQ0S4aAp1lZU_U9CEM8E0WSer8ORBQ9-TzcgczI9wsv3Zv6wOAFSnOIqpVvagnZmnOQg63Bpx0sRbI8d2PBSgdXyn236Elgaxsctw6v18srx4MFz-1w2vBW7K4V4h8eH9zqkTb\" style=\"margin-left: 1em; margin-right: 1em;\"><img loading=\"lazy\" decoding=\"async\" alt=\"\" data-original-height=\"416\" data-original-width=\"670\" height=\"247\" src=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2023\/03\/image_1753094066.png\" width=\"398\" \/><\/a><\/b><\/div>\n<p><b><br \/><\/b><b><u>Possible solution #1:<span style=\"mso-spacerun: yes;\"> <\/span>Steel screw with tighter pitch on the turns.<\/u><\/b><\/p>\n<p class=\"MsoNormal\"><b>Just using a steel screw slows the tuning rate down.<span style=\"mso-spacerun: yes;\"> <\/span>In a normal PTO we increase the inductance (and reduce the frequency) by gradually introducing a ferrous material that increases the inductance of the coil, pushing the frequency of oscillation down.<span style=\"mso-spacerun: yes;\"> <\/span>But our brass screw is non-ferrous.<span style=\"mso-spacerun: yes;\"> <\/span>This means that putting it into the core does not change the permeability of the coil.<span style=\"mso-spacerun: yes;\"> <\/span>The permeability of brass is the same as that of air. <o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b>What does happen, <span style=\"mso-spacerun: yes;\"> <\/span>however, is that introducing the brass screw into the coil causes currents to flow in the screw.<span style=\"mso-spacerun: yes;\"> <\/span>These are called eddy currents. <span style=\"mso-spacerun: yes;\"> <\/span>In effect they become shorted secondary coils. <span style=\"mso-spacerun: yes;\"> <\/span>And they have the effect of lowering the inductance of the coil \u2013 this is why the frequency of the oscillator increases as we screw in the brass screw.<o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b>When you use a steel screw you get both effects: As you screw it in, eddy currents flow in the screw, reducing the inductance and increasing the frequency of oscillation.<span style=\"mso-spacerun: yes;\"> <\/span>But you are also introducing ferrous material \u2013 this pushes in the opposite direction, increasing induction and lowering the frequency of oscillation.<span style=\"mso-spacerun: yes;\"> <\/span>I think the eddy current effect dominates, but the increase in permeability pushes in the opposite direction.<span style=\"mso-spacerun: yes;\"> <\/span>This means that with a steel screw you have to use more turns to cover the same frequency range. And that is what we want. <o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b>For example, using the same coil, with screw of the same thread pitch (the same nuts), with both screws ten turns in, one turn of the brass screw moved the inductance .014 uH.<span style=\"mso-spacerun: yes;\"> <\/span>The same single turn of the steel screw only moved the inductance .005 uH. <span style=\"mso-spacerun: yes;\"> <\/span>So just because of metallurgy, the steel screw will lead to a lower (better) tuning rate. I used a Hillman 45479 screw that is steel with a Zinc (anti-corrosive) coating. <o:p><\/o:p><a href=\"https:\/\/www.amazon.com\/Hillman-Group-45479-Phillips-Machine\/dp\/B00JDU0PZI\">https:\/\/www.amazon.com\/Hillman-Group-45479-Phillips-Machine\/dp\/B00JDU0PZI<\/a> and be sure to get the correct nuts:<a href=\"https:\/\/www.amazon.com\/Hard-Find-Fastener-014973241704-Piece-100\/dp\/B00L1L76E0\/ref=sr_1_4?crid=UOPEF2HLAD75&#038;keywords=1%2F4-28+nut&#038;qid=1678881552&#038;s=hi&#038;sprefix=1%2F4-28+hex+nuts%2Ctools%2C71&#038;sr=1-4\"> https:\/\/www.amazon.com\/Hard-Find-Fastener-014973241704-Piece-100\/dp\/B00L1L76E0\/ref=sr_1_4?crid=UOPEF2HLAD75&#038;keywords=1%2F4-28+nut&#038;qid=1678881552&#038;s=hi&#038;sprefix=1%2F4-28+hex+nuts%2Ctools%2C71&#038;sr=1-4<\/a><\/b><\/p>\n<p class=\"MsoNormal\"><b>But there is more:<span style=\"mso-spacerun: yes;\"> <\/span>steel screws are also available with tighter (#28) thread pitches. The Hillman 45479 uses this tighter thread pitch. <span style=\"mso-spacerun: yes;\"> <\/span>This too means that more turns are needed to move through the same tuning range.<span style=\"mso-spacerun: yes;\"> Again, that is what we want. <\/span><o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b>I found that using a steel screw with #28 thread pitch allowed for the coverage of the 40 meter band in approximately 11 turns of the dial.<span style=\"mso-spacerun: yes;\"> <\/span>That is much better than what we got with the brass screw:<span style=\"mso-spacerun: yes;\"> <\/span>About 27 kHz per turn instead of the 50 kHz per turn that we got with brass. <span style=\"mso-spacerun: yes;\"> <\/span>But it is not quite good enough.<span style=\"mso-spacerun: yes;\"> <\/span>It would be better if we could use the entire range of that PTO coil form. <o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b><u>Solution Two:<span style=\"mso-spacerun: yes;\"> <\/span>Add a fixed inductor in series with the PTO coil.<span style=\"mso-spacerun: yes;\"> <\/span><\/u><o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b>After some noodling, I decided to split up the inductor:<span style=\"mso-spacerun: yes;\"> <\/span>A portion of it would remain fixed, the other portion would continue to be tunable. <o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b>I estimated that I was starting out with a coil of about 1.428 uH.<span style=\"mso-spacerun: yes;\"> <\/span>So I just put a 1 uH choke in series with the variable inductor and reduced the variable coil to about .428 uH (about 9 coil turns).<span style=\"mso-spacerun: yes;\"> <\/span>This worked, but it worked a bit too well!<span style=\"mso-spacerun: yes;\"> <\/span>It would not tune the entire 40 meter band.<span style=\"mso-spacerun: yes;\"> <\/span>So I figured I needed less fixed inductance and more variable inductance.<span style=\"mso-spacerun: yes;\"> <\/span>I found an air-cored coil in my junk box and cut it so that it measured about .650 uH.<span style=\"mso-spacerun: yes;\"> <\/span>I added turns to the variable coil, going to a total of 15 turns.<span style=\"mso-spacerun: yes;\"> <\/span>This REALLY worked well and yielded the 26 or 27 turns to tune across 40 meters that you can see in the video. <o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><b><u>TWEAKS: <\/u><\/b><\/p>\n<p class=\"MsoNormal\"><b>Later, I tweaked it a bit more: With 15 turns of #22 wire on the variable inductor, a steel screw tuned from .791 uH (screw out) to .662 uH (screw in). <\/b><b>I put one additional turn on the fixed inductor, making it .749 uH, or about 8 turns of #22 (wound tighter on a cardboard tube from a coat hanger than was the coil on the variable inductor). With these coils I could tune from 6.9772 to 7.386 MHz. That&#8217;s a bit more than we need but this allows us to keep the tuning away from the ends of the coil where tuning is more likely to become non-linear. I am able to go from 7.0 to 7.3 MHz in 23 turns of the dial. And the tuning is quite linear: The first turn from 7.0 MHz moves the frequency 12 kHz. At the mid-point of 7.150 MHz, one turn of the dial moves the frequency 12 kHz. At the high end, going down from 7.3 MHz, one turn of the dial moved the frequency 11 kHz. That, for me, is VERY linear tuning. You probably will have to adjust the coils a bit (just squeezing the turns together or spreading them apart) to get the tuning range where you want it. <\/b><\/p>\n<p class=\"MsoNormal\"><b><u>YMMV \u2013 Keep it simple!<\/u><\/b><\/p>\n<p class=\"MsoNormal\"><b>Like they used to say in the commercials:<span style=\"mso-spacerun: yes;\"> <\/span>Your Mileage May Vary. <span style=\"mso-spacerun: yes;\"> <\/span>There are many ways of doing this.<span style=\"mso-spacerun: yes;\"> <\/span>The objective is smooth tuning across the 40 meter band.<span style=\"mso-spacerun: yes;\"> <\/span>I think that by varying the pitch of the variable coil turns you could get a more linear tuning response (please let us know if you have any luck).<span style=\"mso-spacerun: yes;\"> <\/span>You might also be able to get similar results by changing the amount of capacitance in the feedback network (which is also the frequency determining element in this simple Colpitts oscillator).<span style=\"mso-spacerun: yes;\"> <\/span>But remember that simplicity and a low parts count were also our objectives in this.<span style=\"mso-spacerun: yes;\"> <\/span>This mod adds only 1 part (the fixed inductor), requires the removal of some turns from the main tuning cap, and perhaps the replacement of the brass screw with a steel #28 screw and nuts. <span style=\"mso-spacerun: yes;\"> <\/span><o:p><\/o:p><\/b><\/p>\n<p class=\"MsoNormal\"><span style=\"mso-spacerun: yes;\"><\/span><\/p>\n<div class=\"separator\" style=\"clear: both; text-align: center;\"><span style=\"mso-spacerun: yes;\"><a href=\"https:\/\/blogger.googleusercontent.com\/img\/a\/AVvXsEjamxt-xIFykhl-JDTalczeLe7qB33cF6uct3E0YDLwrLBXcC9EFt5Ssovl_rc6n60sgXKIiC8vZoTKMtVbwun37jlBK93wl2vslhCyv8wdx40S5ND8z0r1JZOprRbhEJ6BKbZQjUJxR1vGLOSmfoDym3WHWlANtXdodf-HX6gzOzxihz5ndAUv31T3\" style=\"margin-left: 1em; margin-right: 1em;\"><img loading=\"lazy\" decoding=\"async\" alt=\"\" data-original-height=\"314\" data-original-width=\"419\" height=\"263\" src=\"https:\/\/www.homebrewradio.us\/blog\/wp-content\/uploads\/2023\/03\/image_1753094068.png\" width=\"351\" \/><\/a><\/span><\/div>\n<div class=\"separator\" style=\"clear: both; text-align: left;\"><b>We might present to the student this problem and our search for a solution. This would be a good example of how homebrewers work to make their rigs better and easier to use. It illustrates well the design dilemmas that can come up, and how amateurs like us can come up with solutions. <\/b><\/div>\n","protected":false},"excerpt":{"rendered":"<p>Here is the problem: For the capacitive element in the LC circuit we have essentially two 660 pF caps in series. This results in a total capacitance of 330 pf. I measured 362 pF. To get a resonant frequency of 7.0 MHz with 362 pF we need 1.428 uH. To get 1.428 uH on the &hellip; <a href=\"https:\/\/www.homebrewradio.us\/blog\/2023\/03\/14\/fixing-the-tuning-problem-in-the-high-school-direct-conversion-receiver-with-video\/\" class=\"more-link\">Continue reading<span class=\"screen-reader-text\"> &#8220;Fixing the Tuning Problem in the High-School Direct Conversion Receiver (with video)&#8221;<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":6227,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[365,12],"tags":[],"class_list":["post-6226","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-pto","category-tj-dc-rx"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/6226","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=6226"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/6226\/revisions"}],"predecessor-version":[{"id":6229,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/6226\/revisions\/6229"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/6227"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=6226"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=6226"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=6226"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}