Nguyễn Khánk's Wiki

# Analog & Digital

# Analog

Analog means “continuous” — both in time and value.

An analog signal can take any value within a range, at any moment in time.

It’s smooth, infinitely detailed, and mirrors physical reality directly.

All physical quantities (sound, light, temperature, pressure, radio waves) are continuous signals.

# Examples

Analog in Real Life

Field Desc
Sound Sound waves are analog changes in air pressure.
Electric signal Microphone output is an analog voltage changing with sound.
Temperature sensor Produces continuous voltage depending on heat.
Old TV/radio Uses continuous frequency and amplitude to send info.
Light intensity A dimmer switch changes brightness smoothly.

Analog in Modern Technologies

Area Role of Analog
Audio recording Capturing sound as voltage variations
Wireless communication Radio waves (analog carriers)
Sensors / IoT Convert physical signals to voltages
Power electronics Control continuous voltage/current
Display / Audio output Convert digital data to analog for human senses

→ The human world (ears, eyes, touch) only understands analog signals, so every digital system must eventually go analog at the output.

# Characteristics

  • Continuity: Infinite possible values
  • Realism: Closely represents physical world
  • Noise sensitivity: Easily affected by interference
  • No quantization error: No rounding — exact wave
  • Difficult to copy: Quality degrades with duplication

Analog signals can carry infinite detail, but because they are physical (voltage, current, wave), they are vulnerable to noise, distortion, and signal loss.

# Analog & Digital

# Digital

Digital means information is represented by numbers (0 and 1).

Instead of a smooth wave, the sound is sampled many times per second and stored as a series of values.

# Technologies

Technology Analog Part Digital Part
Wi-Fi Uses 2.4GHz or 5GHz analog radio waves Transmits digital packets (0s/1s) via modulation (OFDM)
Bluetooth Uses analog radio wave Sends digital data via frequency hopping
RFID Tag communicates by analog electromagnetic field Data in the tag is digital
Mobile (4G/5G) Uses analog carrier wave Transmits digital symbols encoded (QAM, OFDM)
FM Radio (traditional) Entirely analog (sound modulates frequency) No digital data (except RDS in modern systems)
Aspect Analog Digital
Nature Continuous wave Discrete steps (0,1)
Representation Real-world signal Encoded numbers
Quality Natural, full detail Precise but sampled
Noise Sensitive Resistant
Copying Degrades Perfect reproduction
Processing Hard (needs analog circuits) Easy (software, CPU)

# Data acquisition system

  1. Sensor detects physical quantity → outputs weak analog signal. (Signal Acquisition)
  2. Amplifier boosts the signal to usable level. (Signal Amplification)
  3. Filter (anti-aliasing) removes unwanted high frequencies.
  4. ADC (Analog-to-Digital Converter) samples and digitizes it.oise.

TIP

According to the Nyquist theorem, you must sample at least 2× the highest frequency present in the signal.

For example, for 20 kHz audio → sample ≥ 40 kHz (so we use 44.1 kHz).

The typical human hearing range is from 20 Hz to 20,000 Hz (20 kHz).

# Analog-to-Digital Converters (ADCs)

For sound

# Step 1: Sampling

  • The analog signal (a continuous wave) is measured at regular time intervals.
  • Each measurement is called a sample.
  • The number of samples per second is called the sampling rate (measured in Hertz, Hz).

Example: CD audio uses 44,100 samples per second (44.1 kHz) → enough to capture all frequencies humans can hear (up to ~20 kHz).

# Step 2: Quantization

  • Each sample’s amplitude (signal strength) is rounded to the nearest digital value.
  • The precision depends on bit depth (e.g. 8-bit, 16-bit, 24-bit).
    • 8-bit → 256 levels
    • 16-bit → 65,536 levels
    • 24-bit → 16.7 million levels

The higher the bit depth → the more detailed the sound, less noise.

# Step 3: Encoding

  • After sampling and quantization, each sample is stored as a binary number (0s and 1s).
  • These binary values are then organized into a digital file format (like WAV, FLAC, MP3).

# Electromagnetic waves

An electromagnetic wave (EM wave) is a wave that carries energy through electric and magnetic fields that oscillate (vibrate) perpendicularly to each other and to the direction the wave travels.

Structure

           ↑ Electric Field (E)
           |
           |        ↗ Wave direction
           |       /
-----------•----------------→
           |
           ↓ Magnetic Field (B)
  • The Electric field (E) and Magnetic field (B) keep generating each other while moving forward.
  • No physical medium (like air or water) is required — EM waves can travel in vacuum.

# Sound wave vs Light wave

Property Sound wave Light / EM wave
Nature Mechanical wave Electromagnetic wave
Required physical medium Yes No
Type of vibration Molecular vibration of matter (compression & expansion) Electric field (E) and Magnetic field (B)
Frequency unit Hz Hz
Speed ≈ 343 m/s ≈ 3 × 10⁸ m/s
Frequency range 20 Hz – 20 kHz (hearable) ~4 × 10¹⁴ – 7.5 × 10¹⁴ Hz (seeable)
Wavelength cm -> m 400-700nm (super short)
Use cases Voices, music, supersonic light, Wifi, X-ray

# Frequency vs. Wavelength

Formula

v ​= λf => f = v / λ

  • v: wave speed
  • λ: wavelength
  • f: frequency
  • Higher frequency → shorter wavelength → shorter range but more data per second.
  • Lower frequency → longer wavelength → longer range but less data.

# Technogies in Comparion

# TV Broadcast

Technology Frequency Band Range Note
Analog TV 30 – 300 MHz (VHF) ~60 – 100 km
Analog TV 470 – 890 MHz (UHF) ~30 – 60 km more quality
Digital TV 470 – 700 MHz (UHF) ~30 – 80 km COFDM => stable quality
Satellite TV 10 – 12 GHz (Ku band) or 3.7 – 4.2 GHz (C band) global or continent

# RFID

Technology Frequency Band Range Data Rate Typical Use
RFID ( LF) 125–134 kHz < 10 cm < 10 kbps Animal tags, access control
RFID (HF) 13.56 MHz < 1 m ~ 100 kbps Smart cards, library tags
RFID (UHF) 860–960 MHz 1–12 m ~100 kbps Inventory tracking, logistics
NFC 13.56 MHz (same as HF RFID) < 10 cm ~424 kbps Contactless payment, card emulation

# GPS

Signal Name Frequency (MHz) Used by Explanation
L1 1575.42 MHz Civilian (public GPS) phones, cars, planes, ....
L2 1227.60 MHz Military & high-precision survey, RTK (Real-Time Kinematics)
L5 1176.45 MHz Civil aviation & advanced GPS new frequency, anti noise
L3, L4 1381.05 / 1379.91 MHz Military, experimental
L6 (Japan QZSS) 1278.75 MHz Regional augmentation like Japan, India

# Wifi

Different Wi-Fi standards (like 802.11a/b/g/n/ac/ax) use different frequency bands to transmit data.

Wi-Fi Standard Frequency Band Typical Use / Note
2.4 GHz band 2.400 – 2.4835 GHz Longer range, slower speed, more interference (shared with Bluetooth, microwave ovens)
5 GHz band 5.150 – 5.825 GHz (varies by country) Higher speed, shorter range, less interference
6 GHz band (Wi-Fi 6E) 5.925 – 7.125 GHz Very high speed, many channels, needs new devices
60 GHz (Wi-Gig / 802.11ad/ay) 57 – 71 GHz Extremely high speed, very short range (used for VR, wireless HDMI)

# Others

Technology Frequency Band Range Data Rate Typical Use
Microwave oven 2.45 GHz N/A
FM Radio 88 – 108 MHz Up to ~100 km
Cellular 4G ~700 MHz – 2.6 GHz 1–10 km (per cell tower)
Wi-Fi 2.4 GHz, 5 GHz, 6 GHz, 60 GHz ~10–100 m Up to several Gbps Internet, local networking
Bluetooth (Classic/LE) 2.4 GHz ISM band ~1–100 m (depending on version & power) Up to ~2 Mbps (LE) or ~3 Mbps (Classic) Audio, wearables, IoT, peripherals

# Range

Why NFC and HF RFID have the same frequency (13.56 MHz) but different range?

NFC and HF RFID use the same frequency: 13.56 MHz, so their wavelength is the same (~22 meters). But — the communication mechanism is completely different:

Aspect HF RFID NFC
Power source The reader (antenna) sends strong electromagnetic field to energize passive tags (no battery). Both devices (like phone ↔ POS) are active, each has power.
Coupling method Inductive coupling (magnetic field dominates). Mutual inductive coupling (two powered coils exchange data).
Communication directio Usually one-way (reader → tag). Two-way (peer-to-peer).
Field strength / Power Reader uses higher power (~1–2 W). FC uses much lower power (~100 mW).
Protocol behavior Optimized for longer detection (up to ~1 m). Optimized for secure, short-range (~4–10 cm).

Friis Transmission Equation

Pr ​= Pt ​+ Gt ​+ Gr ​− Lp​

More detail:

Pr​ = Pt ​+ Gt​ + Gr ​+ 20log10​( λ / 4πd)

  • Pr: Received power (dBm)
  • Pt: Transmitted power (dBm)
  • Gt, Gr: Antenna gains (dBi)
  • λ: Wavelength
  • d: Distance (m)
  • Lp​: Path loss

Simplified Insight

Factor Effect on Range
Frequency Higher frequency → shorter range, because high-frequency waves lose energy faster and are absorbed more easily.
Transmit Power More power → longer range, but limited by safety laws and regulations.
Antenna Gain More focused antenna beam → longer effective distance.
Environment Walls and obstacles reduce range, while open outdoor space allows much farther reach.
Receiver Sensitivity Higher sensitivity → can detect weaker signals → increases range.

# Chặn sóng

Phương pháp Mô tả kỹ thuật Ứng dụng
Faraday Cage (Lồng kim loại) Dùng vật dẫn điện bao kín (như lưới đồng, thép) để hấp thụ và phản xạ sóng. Phòng thí nghiệm EMI/EMC, lò vi sóng, thang máy, phòng an toàn dữ liệu.
Vật liệu hấp thụ sóng Dùng vật liệu ferrite, carbon hoặc foam đặc biệt để hấp thụ năng lượng RF thay vì phản xạ. sssGiảm nhiễu sóng trong thiết bị điện tử, phòng đo sóng.
Sơn hoặc màng chắn sóng Sơn chứa hạt kim loại hoặc phủ film dẫn điện trên tường/kính để chặn sóng radio Bảo vệ phòng họp, phòng y tế (MRI), hoặc khu nghiên cứu.
Directional Antenna Design Dùng anten định hướng để giảm phát tán sóng ra ngoài vùng cần thiết. Quản lý sóng Wi-Fi, radar, hoặc truyền thông riêng tư.
Frequency Filtering (Lọc tần số) Dùng mạch lọc hoặc tường chắn chọn lọc tần số muốn cho qua. Bảo vệ thiết bị khỏi nhiễu tần số không mong muốn.
Security Jamming the transmission of electromagnetic energy to degrade, deny, or deceive an adversary’s use of the electromagnetic spectrum (radios, radars, GPS, communications, etc.) DONT

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