Simple SDR Receiver Using 2x NE612 - Dual Conversion, Superheterodyne (0.1–30 MHz)
This project demonstrates a simple yet highly effective SDR (Software Defined Radio) receiver based on a dual-conversion superheterodyne architecture using two NE612 mixer ICs.
SDR (Software Defined Radio) is a radio system in which most of the functions of a classic radio (filtering, demodulation, tuning, AGC) are performed by software instead of hardware. The hardware serves mainly as a receiver and signal converter, and then the computer processes the signal. The main advantage is flexibility: with the same device you can listen to AM, SSB, FM, DRM and other signals just by changing the software. The disadvantage is that it requires digital processing and often a computer, but therefore allows much greater precision and capabilities than classic radios.
In this project I will present you a simple way to build a superheterodyne type SDR radio, which despite its simplicity, gives excellent results and according to its characteristics is comparable to many modern SW radios. The original circuit diagram is presented on several forums but I think the source is Andreas Stefan, DL5MGD. For the sake of simplicity, I omitted several parts of the original diagram, such as the low-pass filters on the VFO and RF input, and I bring the signal from the second mixer directly to the microphone input of the PC sound card. First, I made the original project and I can tell you that the difference between it and this minimal version of mine is almost imperceptible. That is quite ok for this kind of experimental SDR receiver, because my main goal when making it was a compromise between minimal design and maximum results.
Let me briefly explain the principle of operation: The antenna collects a wide range of RF signals that are mixed in the first mixer with the signal from the local oscillator that generates a frequency 455 kHz higher than the specific frequency that we want to separate from the RF signal. This is our IF frequency (|RF – LO| = 455 kHz). Next is a 455 kHz filter that passes only a narrow band around 455 kHz and rejects all other mixed products. This is the key to selectivity and a clean signal. Then the signal goes to the second mixer in which a conversion from 455 to 12 kHz is performed so that the sound card can read and process this signal appropriately. So this is a classic dual conversion superhet receiver.
This project is sponsored by PCBWay . From concept to production, PCBWay provide cutting-edge electronic design solutions for global innovators, Including hardware design, software development, mechanical design, product testing and certification. PCBWay engineering team consists of experienced engineers in electronics, embedded systems, and product development. They successfully delivered hundreds of projects across industries such as medical devices, industrial automation, consumer electronics, smart home, and IoT.
Here's what the inside of this box looks like, which houses the hardware part of the radio, just two NE612 chips and a few passive elements.
It does not contain any coils and does not require any tuning. The Local Oscillator signal is generated by my latest Smartphone Wi-Fi VFO, the construction of which is described in one of my previous videos. For the needs of this superhet receiver I added a new option to the VFO project - Offset frequency - in this case +455KHz which is activated by pressing a virtual button on the smartphone.
At the end of the text is given the latest modified code.
The output of this SDR front-end is processed by the "Sdradio" free software by Alberto I2PHD which you can download on the given page .
Now let's see how the device performs in real conditions. I will use my long wire antenna mounted on the roof. As I mentioned at the beginning, the reception is surprisingly good for such a simple receiver, and on all bands of the SW range
And finally a short conclusion. This project shows that a powerful SDR receiver doesn’t need complex hardware—just smart design and software processing. With minimal components, you can achieve surprisingly high performance across the entire shortwave band.
// By mircemk, June 2026
#include <WiFi.h>
#include <WebServer.h>
#include <si5351.h>
#include <Wire.h>
Si5351 si5351;
unsigned long frequency = 7000000;
bool offsetActive = false;
const unsigned long IF_OFFSET = 455000; // 455 kHz Offset за суперхетеродин
const char* ssid = "Si5351_VFO_Final_Complete";
const char* password = "vfo12345678";
WebServer server(80);
const char VFO_HTML[] PROGMEM = R"rawliteral(
<!DOCTYPE html><html><head>
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=0">
<style>
:root {
--panel-bg: #E0AB07;
--inner-bezel: #E0AB07;
--lcd-bg: #0077c2;
--btn-band: #7f0000;
--btn-step: #27ae60;
--btn-mode: #2980b9;
--btn-mem: #8e44ad;
--gold-border: #f1c40f;
--text-color: #ecf0f1;
}
* { -webkit-tap-highlight-color: transparent; box-sizing: border-box; user-select: none; }
body { background: #000; margin: 0; display: flex; justify-content: center; font-family: 'Arial Black', sans-serif; color: var(--text-color); overflow: hidden; }
.vfo-main-frame { background: var(--panel-bg); width: 100%; max-width: 400px; height: 100vh; display: flex; flex-direction: column; align-items: center; position: relative; }
.bezel-display { background: var(--inner-bezel); width: 92%; margin-top: 15px; padding: 10px; border-radius: 8px; box-shadow: inset 4px 4px 10px #000; position: relative; }
.fs-zone { position: absolute; left: 0; top: 0; width: 30%; height: 100%; z-index: 10; cursor: pointer; }
.mem-zone { position: absolute; left: 30%; top: 0; width: 40%; height: 100%; z-index: 10; cursor: pointer; }
.reset-zone { position: absolute; right: 0; top: 0; width: 30%; height: 100%; z-index: 10; cursor: pointer; }
.display { background: var(--lcd-bg); border: 4px solid #111; padding: 10px; box-shadow: inset 0 0 25px #000; height: 125px; display: flex; flex-direction: column; justify-content: space-between; position: relative; transition: background 0.2s; }
.display.mem-active { background: #e67e22; }
.display.reset-flash { background: #e74c3c; }
#f-display { font-size: 55px; font-weight: 900; margin: 0; text-align: right; text-shadow: 2px 2px 4px #000; letter-spacing: -1px; line-height: 1; }
.display-info { display: flex; justify-content: space-between; font-size: 14px; color: rgba(255,255,255,0.9); font-family: Arial, sans-serif; }
.display-footer { display: flex; align-items: center; border-top: 1px solid rgba(255,255,255,0.2); padding-top: 5px; margin-bottom: 4px; }
#mode-label, .sig-text { font-size: 15px; font-weight: bold; color: #fff; }
.s-meter-container { display: flex; align-items: center; gap: 6px; flex-grow: 1; justify-content: flex-end; margin-left: 25px; }
.s-grid { display: flex; gap: 1px; height: 10px; width: 115px; background: rgba(0,0,0,0.3); border: 1px solid #111; }
.s-seg { flex: 1; background: #222; }
.s-on { background: #ffffff; box-shadow: 0 0 6px #ffffff; }
/* OFFSET DUGME */
.offset-btn {
position: absolute; right: 17px; top: 175px;
width: 60px; height: 60px; border-radius: 50%;
background: #3d0000; border: 3px solid #222;
color: #fff; font-size: 9px; font-weight: bold;
display: flex; align-items: center; justify-content: center;
cursor: pointer; box-shadow: 3px 3px 8px #000; z-index: 50;
}
.offset-btn.active { background: #ff0000; box-shadow: 0 0 15px #ff0000; border-color: #f1c40f; }
.bezel-knob { background: var(--inner-bezel); width: 270px; height: 270px; margin: 25px 0; border-radius: 50%; box-shadow: inset 3px 3px 10px #000; display: flex; justify-content: center; align-items: center; }
#knob { width: 240px; height: 240px; background: conic-gradient(#333, #777, #333); border-radius: 50%; border: 12px solid #1a1a1a; position: relative; will-change: transform; cursor: pointer; }
.controls-container { width: 94%; display: flex; flex-direction: column; }
.grid { display: grid; gap: 6px; width: 100%; grid-template-columns: repeat(4, 1fr); }
.btn { border: 3px solid var(--gold-border); border-radius: 8px; color: #fff; font-weight: 900; font-size: 15px; padding: 11px 0; text-align: center; cursor: pointer; box-shadow: 3px 5px 8px #000; text-transform: uppercase; }
.b-band { background: var(--btn-band); }
.b-step { background: var(--btn-step); font-size: 20px; padding: 12px; grid-column: span 4; margin: 12px 0; }
.b-mode { background: var(--btn-mode); }
.b-mem { background: var(--btn-mem); font-size: 13px; margin-top: 6px; }
</style>
</head><body>
<div class="vfo-main-frame">
<div class="bezel-display">
<div class="fs-zone" onclick="toggleFS()"></div>
<div class="mem-zone" onclick="startMem()"></div>
<div class="reset-zone" onclick="clearAllMem()"></div>
<div class="display" id="main-display">
<div class="display-info"><span id="band-label">40M HAM</span><span id="step-label">100Hz</span></div>
<h1 id="f-display">07.000.000</h1>
<div class="display-footer">
<span id="mode-label">USB</span>
<div class="s-meter-container">
<span class="sig-text">Sig:</span>
<div class="s-grid" id="s-grid"></div>
</div>
</div>
</div>
</div>
<div id="offset-led" class="offset-btn" onclick="toggleOffset()">OFFSET</div>
<div class="bezel-knob"><div id="knob"></div></div>
<div class="controls-container">
<div class="grid">
<div class="btn b-band" onclick="setBand(531000)">MW</div>
<div class="btn b-band" onclick="setBand(1810000)">160</div>
<div class="btn b-band" onclick="setBand(3500000)">80</div>
<div class="btn b-band" onclick="setBand(7000000)">40</div>
<div class="btn b-band" onclick="setBand(14000000)">20</div>
<div class="btn b-band" onclick="setBand(18068000)">17</div>
<div class="btn b-band" onclick="setBand(21000000)">15</div>
<div class="btn b-band" onclick="setBand(24890000)">12</div>
</div>
<div class="btn b-step" id="step-btn" onclick="nextStep()">STEP: 100Hz</div>
<div class="grid">
<div class="btn b-mode" onclick="setMode('AM')">AM</div>
<div class="btn b-mode" onclick="setMode('USB')">USB</div>
<div class="btn b-mode" onclick="setMode('LSB')">LSB</div>
<div class="btn b-mode" onclick="setMode('FM')">FM</div>
<div class="btn b-mem" id="m1" onclick="handleMem(1)">M1</div>
<div class="btn b-mem" id="m2" onclick="handleMem(2)">M2</div>
<div class="btn b-mem" id="m3" onclick="handleMem(3)">M3</div>
<div class="btn b-mem" id="m4" onclick="handleMem(4)">M4</div>
</div>
</div>
</div>
<script>
var freq = 7000000; var lastAngle = 0; var curRot = 0; var isDrag = false; var lastSent = 0;
var steps = [10, 100, 1000, 5000, 10000, 100000]; var stepIdx = 1;
var isMemMode = false; var isOffset = false;
const sGrid = document.getElementById('s-grid');
for(let i=0; i<20; i++) { let d=document.createElement('div'); d.className='s-seg'; sGrid.appendChild(d); }
function toggleOffset() {
isOffset = !isOffset;
document.getElementById('offset-led').classList.toggle('active');
fetch('/setOffset?state=' + (isOffset ? 1 : 0));
}
function sendFreq() {
let now = Date.now();
if (now - lastSent > 50) { fetch('/set?f=' + freq); lastSent = now; }
}
function updateUI() {
document.getElementById('f-display').innerText = Number(freq).toLocaleString('de-DE').replace(/,/g, '.');
updateBandLabel();
}
function move(e) {
if (!isDrag) return;
let ev = e.touches ? e.touches[0] : e;
let r = document.getElementById('knob').getBoundingClientRect();
let ang = Math.atan2(ev.clientY - (r.top + r.height/2), ev.clientX - (r.left + r.width/2)) * 180 / Math.PI;
let d = ang - lastAngle;
if (d > 180) d -= 360; if (d < -180) d += 360;
curRot += d;
freq += Math.round(d) * (steps[stepIdx] / 10);
if (freq < 100000) freq = 100000;
updateUI();
document.getElementById('knob').style.transform = 'rotate(' + curRot + 'deg)';
sendFreq();
lastAngle = ang;
}
let knob = document.getElementById('knob');
knob.addEventListener('mousedown', function(e) { isDrag = true; let r = knob.getBoundingClientRect(); lastAngle = Math.atan2(e.clientY - (r.top + r.height/2), e.clientX - (r.left + r.width/2)) * 180 / Math.PI; });
knob.addEventListener('touchstart', function(e) { isDrag = true; let r = knob.getBoundingClientRect(); lastAngle = Math.atan2(e.touches[0].clientY - (r.top + r.height/2), e.touches[0].clientX - (r.left + r.width/2)) * 180 / Math.PI; e.preventDefault(); }, {passive: false});
window.addEventListener('mouseup', () => isDrag = false);
window.addEventListener('touchend', () => isDrag = false);
window.addEventListener('mousemove', move);
window.addEventListener('touchmove', move, {passive: false});
function toggleFS() { if(!document.fullscreenElement) document.documentElement.requestFullscreen().catch(e=>{}); else document.exitFullscreen(); }
function setBand(f) { freq = f; updateUI(); sendFreq(); }
function setMode(m) { document.getElementById('mode-label').innerText = m; }
function nextStep() { stepIdx = (stepIdx + 1) % steps.length; let labels = ["10Hz", "100Hz", "1KHz", "5KHz", "10KHz", "100KHz"]; document.getElementById('step-btn').innerText = "STEP: " + labels[stepIdx]; }
function startMem() { isMemMode = true; document.getElementById('main-display').classList.add('mem-active'); }
function clearAllMem() { localStorage.clear(); document.getElementById('main-display').classList.add('reset-flash'); setTimeout(() => location.reload(), 300); }
function handleMem(id) {
if(isMemMode) { localStorage.setItem('m'+id, freq); location.reload(); }
else { let s = localStorage.getItem('m'+id); if(s) { freq = parseInt(s); updateUI(); sendFreq(); } }
}
function loadSavedMem() { for(let i=1; i<=4; i++){ let s = localStorage.getItem('m'+i); if(s) document.getElementById('m'+i).innerText = (parseInt(s)/1000000).toFixed(3); } }
function updateBandLabel() {
let b = document.getElementById('band-label');
if (freq >= 7000000 && freq <= 7200000) b.innerText = "40M HAM";
else if (freq >= 531000 && freq <= 1602000) b.innerText = "MW BROADCAST";
else b.innerText = "GEN";
}
setInterval(() => {
fetch('/getS').then(r => r.text()).then(v => {
let segs = document.querySelectorAll('.s-seg');
let act = Math.floor((v/100)*20);
segs.forEach((s,i) => { if(i<act) s.classList.add('s-on'); else s.classList.remove('s-on'); });
});
}, 250);
loadSavedMem(); updateUI();
</script>
</body></html>
)rawliteral";
void updateSi5351() {
unsigned long outFreq = frequency;
if (offsetActive) outFreq += IF_OFFSET;
si5351.set_freq(outFreq * 100ULL, SI5351_CLK0);
}
void setup() {
Serial.begin(115200);
Wire.begin(21, 22);
pinMode(32, INPUT);
analogReadResolution(12);
analogSetAttenuation(ADC_6db);
si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0, 0);
updateSi5351();
WiFi.mode(WIFI_AP);
WiFi.softAP(ssid, password);
WiFi.setTxPower(WIFI_POWER_19_5dBm); // Максимално засилување на Wi-Fi
server.on("/", []() { server.send(200, "text/html", VFO_HTML); });
server.on("/set", []() {
if (server.hasArg("f")) {
frequency = server.arg("f").toInt();
updateSi5351();
server.send(200, "text/plain", "OK");
}
});
server.on("/setOffset", []() {
if (server.hasArg("state")) {
offsetActive = server.arg("state").toInt() == 1;
updateSi5351();
server.send(200, "text/plain", "OK");
}
});
server.on("/getS", []() {
int val = analogRead(32);
int percent = map(val, 0, 1200, 0, 100);
if(percent > 100) percent = 100;
server.send(200, "text/plain", String(percent));
});
server.begin();
}
void loop() { server.handleClient(); }
