ASK Modulation

What Is ASK Modulation?

ASK (Amplitude Shift Keying) is the signal encodingencodingData writing to NFC tags during manufacturing productionView full → technique NFC readers use to transmit data to tags over the 13.56 MHz carrier. The reader varies the carrier signal's amplitude to represent binary data. ASK is the fundamental modulation scheme for both NFC-A and NFC-B communications.

How ASK Works

The reader generates a continuous 13.56 MHz carrier wave. To transmit data, it modulates the amplitude:

• 100% ASK (NFC-A): The carrier is completely turned off during "low" periods. Signal alternates between full amplitude and zero.
• 10% ASK (NFC-B): Carrier amplitude reduces by only 10% during "low" periods. Signal remains largely intact.

100% ASK (NFC-A)

Complete carrier absence makes detection easy for tags but briefly interrupts power delivery to passive tags. Tags must store energy in internal capacitors to survive power gaps.

10% ASK (NFC-B)

Shallow modulation provides more stable power to tags but requires more sensitive receiver circuitry.

Tag-to-Reader (Reverse Direction)

Passive tags cannot generate their own RF field. Instead, they use load modulation — switching internal impedance to create small amplitude variations that the reader detects. NFC-A tags use a subcarrier at 847.5 kHz with Manchester encoding; NFC-B tags use BPSK encoding.

Engineering Trade-offs

• 100% ASK (NFC-A): Simpler tag design, higher noise immunity, but power gaps can reduce read range for power-hungry tags performing AES operations.
• 10% ASK (NFC-B): More stable power enables communication with secure microprocessor cards (e-passports, government IDs), but requires more expensive analog circuitry.

These modulation parameters are handled entirely by hardware (NFC controller and tag analog front-end) and are transparent to application software.