Characteristics related to HDMI 1.4

AN3318

6/10

Doc ID 18263 Rev 1

Figure 7.

Differential impedance

Figure 7 shows the differential impedance measured with the “Time Domain Reflectometry”

method. This method consists of sending a pulse with a short rise time (200 ps between

10% and 90% for HDMI) and to measure the reflected pulse. This gives the impedance of

the line along the signal path.

The HDMI standard requires 100

Ω ±15% differential impedance (between 85 Ω and

115

Ω). As shown on Figure 7, TDR measurement on HSP061-4NY8 gives an impedance

between 91

Ω and 100 Ω. These values are in accordance with the HDMI standard.

The eye diagrams defined in the HDMI standard are related to bit rate of the signal and to

location (source or sink). There are more constraints on the source side, this is why we have

chosen this one on the datasheet. The duration of the eye corresponds to a bit time. This

diagram visualizes signal duration, synchronization, overshoot and capability of the signal to

move from one state to another one. The key point for protection is to be sure there is no

slow down effect. Figure 8 shows the eye diagram for a 3.4 Gbps signal. This measurement

is done directly on the HSP061-4NY8 to avoid PCB measurement effects. Measurement

shows a large safety margin between the eye pattern and the signal.

Figure 8.

Eye diagram at 3.4Gbps

tr = 200 ps (10% - 90%)

12.5

Ω /div

Z0Diff = 100 Ω

Vertical : 200 mV / div

Horizontal : 50 ps / div