EECS on Haifei's Home

High-Precision Time Measurement—Analysis of TDC / Time Tagger Principles and Architecture

hfwang132@gmail.com (hfwang132) — Sun, 02 Nov 2025 12:53:03 +0800

1. What Is a TDC / Time Tagger?

A Time-to-Digital Converter (TDC) is essentially an extremely precise timer (at the picosecond level) that assigns a high-precision timestamp (Timestamp) to every input pulse signal.

Therefore, a TDC is also called a Time Tagger, where “Tag” means assigning a timestamp.

TDCs / Time Taggers are central to experiments such as Hong-Ou-Mandel interference and fluorescence lifetime measurements; it can be said that quantum optics experiments can hardly do without them.

Impedance Matching / Signal Integrity Crash Course (Lab Edition)

hfwang132@gmail.com (hfwang132) — Sat, 25 Oct 2025 16:46:08 +0800

I. Introduction

Recently, I found that many colleagues are not very familiar with impedance matching. They only know that “the impedance of (commonly used SMA/BNC) transmission lines is 50 ohms,” but do not know what this really means. The following situations often occur:

A 50-ohm signal source connected to a scope with a 1M-ohm input:

Huh, why did the signal become twice as large?

A high-frequency square wave connected to a scope with a 1M-ohm input:

Huh, why is the overshoot so severe? Is there something wrong with this signal source?

The output signal of an MCU / DAC / FPGA connected directly, without a driver, to a 50-ohm load / 50-ohm oscilloscope input:

Huh, why did a signal that was supposed to be 3V become only 30 mV?

All of the above result from unfamiliarity with impedance matching and transmission-line theory.

HW & SW design of the AXI Quad SPI IP core

hfwang132@gmail.com (hfwang132) — Sat, 01 Jul 2023 17:49:31 +0800

1 Basics

SPI has three-wire mode and four-wire mode. The three-wire mode consists of three wires - SS (Slave Select), SCK (SPI Clock), and MOSI (Master-In-Slave-Out). The four wire mode has an extra line called MISO (Master-In-Slave-Out).

Signals	Full Name
SS	Slave Select
SCK	SPI Clock
MOSI	Master-Out-Slave-In
MISO	Master-In-Slave-Out

SPI clock has four modes: CPOL=0/1 and CPHA=0/1. CPOL stands for clock polarity – clock low or high when in idle. CPHA stands for clock phase – data valid at 0 degree or 180 degrees.

Access the Internet from evaluation boards via PC

hfwang132@gmail.com (hfwang132) — Mon, 12 Jun 2023 23:22:45 +0800

Windows

Set a network adapter that can access the internet in the Control Panel and share it with Ethernet. The IP address of Ethernet will change to 192.168.137.1 (this is the default behavior in Windows).

Then, set the gateway as 192.168.137.1 in the terminal of the development board:

sudo route add default gw 192.168.137.1

Set the IP address as 192.168.137.x, where x is any value except 1 and 255 (gateway address and broadcast address):

Partitioning and formatting SD cards on Linux

hfwang132@gmail.com (hfwang132) — Mon, 12 Jun 2023 22:43:36 +0800

To partition and format an SD card in Linux, follow these steps: First, connect the SD card to your PC. Then, use the fdisk command in the bash command line to partition the SD card. Finally, use the mkfs command to create a file system (format) on the SD card.

The main commands are as follows:

First, use sudo fdisk -l to confirm which device in /dev corresponds to the SD card.