13.56MHz RFID tag ISO14443 Type A ASK Modulator Problem

Hi,

I am having a problem with my simulation result when I designed the OOK Manchester capacitive load modulator for 13.56MHz RFID tag according to ISO14443 Type A standard. The followings are my circuit design for the modulator and the simulated result by using Mentor Graphic software TSMC 0.18um technology.

Circuit for Capacitive Load Modulator for ISO14443 Type A RFID tag

Circuit for DATA_43A_SYMBOL

* The crossOR_gate symbol is a normal CMOS 2 input XOR gate
Inverter symbol is a normal CMOS inverter

Simulated waveform for Capacitive Load Modulator for ISO14443 Type A RFID tag

*DATA_PASS is the data signal that the tag wants to send to the reader
DATA is the data signal after encoding (according to ISO14443 type A standard)
The output waveform from the modulator can be seen from V(RF1)

Zoom In of Simulated waveform

So, my problem is I am not sure whether my result is correct or not. If is wrong because the carrier frequency is missing when the DATA signal is "0', then how should I solve this problem? TQ[/img]