THVD8000: OOK modulation

Hi Chester,

The OOK modulation increases the frequency of the data.  This allows for it to be coupled to a line via a capacitor while power is coupled via an inductor.  Because the data is very high in frequency, the capacitor looks low impedance (so the data passes through without distortion) and the inductor looks high impedance (so that it does not interfere with the data and the data signals do not transfer to the power rail as noise).  Because the power is very low in frequency, the inductor looks low impedance (so there is minimal power loss) while the capacitor looks high in impedance (so that the power voltage does not transfer to the signaling IC and cause damage).

Without OOK modulation, it is generally not practical to send RS-485 signals through a capacitor or other AC-coupling barrier.  This is because the RS-485 data can be very low in bit rate and can have long static periods (due to idle time on the bus or due to multiple consecutive identical bits within a frame).  Because of the low frequency, the series capacitor would affect the signal integrity strongly and distort the signal waveforms.

Please let me know if this is not clear or if you have further questions on the device.

Regards,
Max