{"id":16862,"date":"2026-04-09T18:54:28","date_gmt":"2026-04-09T13:24:28","guid":{"rendered":"https:\/\/johnsonfrancis.org\/techworld\/?p=16862"},"modified":"2026-04-09T18:54:29","modified_gmt":"2026-04-09T13:24:29","slug":"understanding-lmr-sdr-1-1-circuit","status":"publish","type":"post","link":"https:\/\/johnsonfrancis.org\/techworld\/understanding-lmr-sdr-1-1-circuit\/","title":{"rendered":"Understanding LMR SDR 1.1 Circuit"},"content":{"rendered":"<iframe loading=\"lazy\" width=\"560\" height=\"315\" src=\"https:\/\/www.youtube.com\/embed\/TygsxLNfhPc?si=b5R7vqGQ1oHLTdeL\" title=\"YouTube video player\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe>\n\n<p class=\"wp-block-paragraph\"><strong>LMR SDR 1.1<\/strong> is a popular software defined radio design by YE3CIF, being popularized in this region among amateur radio homebrewers by VU3ZOF. For convenience we will discuss it as a few functional blocks.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">1. Power Supply Rail Check<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The board uses a cascaded regulation system. Failure here can cause of a &#8220;dead&#8221; radio.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Main Input:<\/strong> Should be <strong>12V &#8211; 13.8V DC<\/strong>.<\/li>\n\n\n\n<li><strong>5V Rail (AMS1117-5.0):<\/strong> Output of this first regulator powers the Arduino Nano and logic ICs.<\/li>\n\n\n\n<li><strong>3.3V Rail (AMS1117-3.3):<\/strong> This regulator takes the 5V output and drops it to 3.3V specifically for the <strong>Si5351A<\/strong> clock generator.\n<ul class=\"wp-block-list\">\n<li>If the Si5351 isn&#8217;t getting exactly 3.3V, it may fail to initialize via I<sup>2<\/sup>C, and you will have no Local Oscillator (LO).<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">2. The Clock Generator (Si5351 &amp; 74AC74)<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"p-rc_e7cde8eb88c30478-21\">This block generates the quadrature (0\u00b0 and 90\u00b0) signals needed for the Tayloe Mixer.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Si5351A:<\/strong> Check for a signal at <strong>CLK0<\/strong>. Note that for an SDR, the Si5351 often outputs 4 times the operating frequency.<\/li>\n\n\n\n<li><strong>74AC74 (Johnson Counter):<\/strong> This IC divides the Si5351 clock by 4 to produce the<em> I<\/em> and <em>Q<\/em> clock phases.\n<ul class=\"wp-block-list\">\n<li><em>Troubleshooting <\/em>can be done by using an oscilloscope or a high-frequency probe to ensure you have square waves at pins 5, 6, 8, and 9. If these are missing, the mixer won&#8217;t work.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">3. The Tayloe Mixer (74HC4051)<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The heart of the RX\/TX conversion.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Input\/Output:<\/strong> In RX mode, RF comes from the antenna through the Band Pass Filter (BPF) to the mixer.<\/li>\n\n\n\n<li>Blown mixer ICs can occur due to high RF deck leakage or static.<\/li>\n\n\n\n<li><strong>Check:<\/strong> Verify the bias voltage at the mixer inputs. It should typically be half the supply voltage (Vcc\/2 approximately 2.5V).<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">4. Audio Pre-amplifier (NE5532 \/ TL084)<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Since the LMR SDR 1.1 is an I\/Q radio, the &#8220;intermediate frequency&#8221; is actually baseband audio.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>NE5532:<\/strong> Usually handles the initial low-noise amplification of <em>I<\/em> and <em>Q<\/em> signals before they go to your PC soundcard.<\/li>\n\n\n\n<li><strong>Troubleshooting:<\/strong> * Check for DC offset at the output pins (Pins 1 and 7 for NE5532).\n<ul class=\"wp-block-list\">\n<li>Inject a 1kHz tone at the mixer output and see if it appears at the &#8220;Line Out&#8221; jack.<\/li>\n\n\n\n<li><strong>Phase Imbalance:<\/strong> If your SDR software shows a massive &#8220;center spike&#8221; or mirrored signals that won&#8217;t calibrate out, check for mismatched resistor values (1K or 10K) in the <em>I<\/em> and <em>Q<\/em> paths.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">5. Control &amp; Switching (Arduino Nano)<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\" id=\"p-rc_e7cde8eb88c30478-23\">The Nano manages band switching and the I<sup>2<\/sup>C bus.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>SDA\/SCL Pins:<\/strong> Check for activity on the I<sup>2<\/sup>C lines (A4\/A5 on Nano) during frequency changes.<\/li>\n\n\n\n<li><strong>PTT Logic:<\/strong> Ensure the <strong>+TX<\/strong> and <strong>+RX<\/strong> lines are toggling correctly. Most LMR SDR versions use a transistor (like 2N3906) to switch the 12V rail between the RX and TX stages.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Common Component Values for Reference<\/h3>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td><strong>Component<\/strong><\/td><td><strong>Function<\/strong><\/td><td><strong>Expected Value\/State<\/strong><\/td><\/tr><\/thead><tbody><tr><td><strong>C13, C31<\/strong><\/td><td>Electrolytic Caps<\/td><td>100uF (Power Filtering)<\/td><\/tr><tr><td><strong>R15, R20<\/strong><\/td><td>Audio Path<\/td><td>330 ohms (Output isolation)<\/td><\/tr><tr><td><strong>L1, L2<\/strong><\/td><td>RF Chokes<\/td><td>100uH (Clock decoupling)<\/td><\/tr><tr><td><strong>Q1 (BS170)<\/strong><\/td><td>PA Driver<\/td><td>Check for 50mW output<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Quick Fix:<\/strong> If you can see signals on the SDR waterfall but cannot hear audio, the issue is almost always the <strong>3.5mm jack wiring<\/strong> or a dead <strong>NE5532<\/strong>. If the waterfall is completely &#8220;flat&#8221; regardless of the antenna, focus on the <strong>Si5351<\/strong> and the <strong>3.3V regulator<\/strong>.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>LMR SDR 1.1 is a popular software defined radio design by YE3CIF, being popularized in this region among amateur radio homebrewers by VU3ZOF. For convenience we will discuss it as a few functional blocks. 1. Power Supply Rail Check The board uses a cascaded regulation system. Failure here can cause of a &#8220;dead&#8221; radio. 2. [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":16863,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-16862","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-amateur-radio-ham-radio"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Understanding LMR SDR 1.1 Circuit - Johnson&#039;s Techworld<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/johnsonfrancis.org\/techworld\/understanding-lmr-sdr-1-1-circuit\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Understanding LMR SDR 1.1 Circuit - Johnson&#039;s Techworld\" \/>\n<meta property=\"og:description\" content=\"LMR SDR 1.1 is a popular software defined radio design by YE3CIF, being popularized in this region among amateur radio homebrewers by VU3ZOF. 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