Understanding LMR SDR 1.1 Circuit

Johnson Francis | April 9, 2026 | Amateur Radio (Ham Radio) | No Comments

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 “dead” radio.

Main Input: Should be 12V – 13.8V DC.
5V Rail (AMS1117-5.0): Output of this first regulator powers the Arduino Nano and logic ICs.
3.3V Rail (AMS1117-3.3): This regulator takes the 5V output and drops it to 3.3V specifically for the Si5351A clock generator.
- If the Si5351 isn’t getting exactly 3.3V, it may fail to initialize via I²C, and you will have no Local Oscillator (LO).

2. The Clock Generator (Si5351 & 74AC74)

This block generates the quadrature (0° and 90°) signals needed for the Tayloe Mixer.

Si5351A: Check for a signal at CLK0. Note that for an SDR, the Si5351 often outputs 4 times the operating frequency.
74AC74 (Johnson Counter): This IC divides the Si5351 clock by 4 to produce the I and Q clock phases.
- Troubleshooting 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’t work.

3. The Tayloe Mixer (74HC4051)

The heart of the RX/TX conversion.

Input/Output: In RX mode, RF comes from the antenna through the Band Pass Filter (BPF) to the mixer.
Blown mixer ICs can occur due to high RF deck leakage or static.
Check: Verify the bias voltage at the mixer inputs. It should typically be half the supply voltage (Vcc/2 approximately 2.5V).

4. Audio Pre-amplifier (NE5532 / TL084)

Since the LMR SDR 1.1 is an I/Q radio, the “intermediate frequency” is actually baseband audio.

NE5532: Usually handles the initial low-noise amplification of I and Q signals before they go to your PC soundcard.
Troubleshooting: * Check for DC offset at the output pins (Pins 1 and 7 for NE5532).
- Inject a 1kHz tone at the mixer output and see if it appears at the “Line Out” jack.
- Phase Imbalance: If your SDR software shows a massive “center spike” or mirrored signals that won’t calibrate out, check for mismatched resistor values (1K or 10K) in the I and Q paths.

5. Control & Switching (Arduino Nano)

The Nano manages band switching and the I²C bus.

SDA/SCL Pins: Check for activity on the I²C lines (A4/A5 on Nano) during frequency changes.
PTT Logic: Ensure the +TX and +RX lines are toggling correctly. Most LMR SDR versions use a transistor (like 2N3906) to switch the 12V rail between the RX and TX stages.

Common Component Values for Reference

Component	Function	Expected Value/State
C13, C31	Electrolytic Caps	100uF (Power Filtering)
R15, R20	Audio Path	330 ohms (Output isolation)
L1, L2	RF Chokes	100uH (Clock decoupling)
Q1 (BS170)	PA Driver	Check for 50mW output

Quick Fix: If you can see signals on the SDR waterfall but cannot hear audio, the issue is almost always the 3.5mm jack wiring or a dead NE5532. If the waterfall is completely “flat” regardless of the antenna, focus on the Si5351 and the 3.3V regulator.