HDMI redriver circuit

[Kepco]

Hi ,
Attached is an HDMI (1.485gbps) redriver/retimer circuit. It takes in DP++(DDI) input and produces HDMI signals going to connector. We are trying to troubleshoot this circuit. We are seeing some lines on the monitor. When scoped the output HDMI signal is about 3Vpp. This voltage should not be more than 1.2Vpp as per re-driver specification. The eye on the signal is closed but somehow the display comes up but with lines. We have used this circuit successfully in past on another design and the difference between the two is the 1.1V regulator U3. The output voltage of the U3 is good 1.1V measured with scope. This is what we know have done so far.

1. 3.3V and 1.1V supply look good.
2. All the parts appear to be correctly populated.
3. X-rayed the board and solder on the re-driver IC looks fine.
4. Input signal to re-driver produces very nice eye pattern.
5. Removing the ESD protection diode does not change the output signal.
6. I2C bus works as monitor recognizes monitor over it.

Any comments on things that we have missed looking into are appreciated.

Thx

 

[iop95]

What is bandwidth of scope used to see U3 output?
Why used 16V cap for C44? I think need to use caps with smallest ESR and add one or two more.
If modification of C44 has no effect, try to chance FB2 and FB3 with higher values / different models, that works at and above 2MHz.

 

[Kepco]

Thanks iop95 for your comments. See response below.

What is bandwidth of scope used to see U3 output?
(We are using the 2GHz scope. The signal frequency is 742MHz. It is not ideal but it is good enough to show input side eye diagram on U3. Those are very clean eye patterns.)

Why used 16V cap for C44? I think need to use caps with smallest ESR and add one or two more.
(This part came from an older design. I tried adding four of these caps in parallel with no improvement in U3 operation. We have done some more testing where we completely remove the 1.1V switcher from the circuit and tried to feed voltage from external battery ( 1.5V with series 1 ohm for 1.1V and 3.2V from two series batteries for 3.3V) by removing ferrite bead FB2 and FB3 to rule out ripple noise of the 3.3V and 1.1V regulator as an issue. The display worked but still no eye opening)

If modification of C44 has no effect, try to chance FB2 and FB3 with higher values / different models, that works at and above 2MHz.
(OK. We can try it.)

 

[aap2]

Is your fuse blown RT1?

 

[zappedagain]

Have you verified your signal integrity with the new PCB layout? If the layout was copied did the fabricator use a proper stack up to meet the transmission line requirements?

Z