Hi,
The project that I am working on is to transmit video stream through visible light. The schematic of receiver circuit is shown in attachment.
Received signal which is transmitted through visible light is converted into voltage with help of OPA657 then I clamped the signal to ground level and finally buffered.
This schematic works in breadboard design.
However, In PCB design there is always ringing on signal which disturbs the signals color.
PCB layout is also in attachment.
62.5kHz ringing on signal is shown in DS1Z_QuickPrint135.png.
Is there any suggestions?
Thank you in advance.
The bursts of ringing are synchronous with the video signal, so they are related to that signal.
The repetition frequency of the bursts is 4 times 15.625 kHz (the PAL or SECAM line rate).
From your schematic, it appears you are using Composite Syncs to switch your DG419 to ground to act as a keyed clamper.
That is not normal practice, which is to derive a clamp pulse from your line sync pulse (and from the equalising pulses & serrations in the field group.)
Clamp pulses are of a standard duration, (somewhat shorter than that of a sync pulse) & also
usually delayed so they fall during the back porch of the signal requiring clamping.(sometimes the clamping is performed during the sync pulse duration)
Using the whole composite sync waveform will make the clamping fairly erratic.
It could be that this causes various spurious signals to appear.
If you use the burst/ back porch output you will have a closer approximation to how it is done in Broadcast TV, although the pulse duration will be a bit long.
If you do use this signal, you will need to put a 4.433MHz trap in series with your connection to pin (1) of the DG419, so you don't mangle the burst.
For fault finding I would suggest you remove the connection to the DG419.
You should now have a composite video signal which has lost its DC reference.
If it still has the interference, disconnect the connection to the LM1881.
Again, if that device was the cause, you will have a composite video signal without its DC reference.