Having a close look at the integrated circuits in LMR SDR 1.1

| March 31, 2026 | Amateur Radio (Ham Radio) | No Comments

I have bought five integrated circuits along with the LMR SDR 1.1 PCB. Thought of having a look at them and learning their main functions.

HD74LS145P

It is a is a BCD-to-Decimal Decoder/Driver integrated circuit (IC). It is designed to take a 4-bit Binary Coded Decimal (BCD) input and activate one of ten decimal outputs. This IC comes in a 16-pin Dual In-line Package (DIP). The 16 pins are arranged as follows (top view, with the notch on the left):
Inputs: Pins 12, 13, 14, 15 (BCD inputs A, B, C, D). Outputs: Pins 1–7 and 9–11 (Decimal outputs 0–9).
Power: Pin 16 is (+5V) and Pin 8 is GND.

Had a look at HD74LS145 linked to Arduino Nano in LMR SDR 1.1 Schematic. It is providing outputs for 10 bands from pins 1 to 10 (160m, 80m, 40m, 20m, 17m, 15m, 12m and 10m). That covers the usual HF bands including the WARC bands. It decodes 4-bit Binary Coded Decimal (BCD) signals coming from the microcontroller Arduino Nano into ten separate decimal outputs for each of the amateur radio bands, functioning as a band selection control. Because it features high-performance, open-collector n-p-n output transistors capable of sinking up to 80mA, it is used to directly drive the relays for the onboard Band Pass Filters (BPF) and Low Pass Filters (LPF).

SN74AC74N

The SN74AC74N is a dual positive-edge-triggered D-type flip-flop in a 14-pin PDIP (Plastic Dual-In-line Package). It contains two independent flip-flops, each with its own data (D), clock (CLK), preset (PRE), and clear (CLR) inputs, along with complementary outputs. In LMR SDR 1.1 it serves as a quadrature clock generator.

Its primary function is to take a high-frequency clock signal from the oscillator Si5351 and divide it down to produce two square wave signals that are exactly 90 degrees out of phase with each other. These “I” (In-phase) and “Q” (Quadrature) clock signals are then used to drive a Mixer for frequency down-conversion or up-conversion. There are three more ICs and the Si5351 mounted as a surface mount device on reverse side of the board. Keep tuned for updates!