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| Composite video issue |
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| maginnovision:
I'm not very familiar with composite video. These are two NTSC composite waveforms, the orange is the output, the yellow is the input. My question is what is causing the orange waveform to skew like it does? Could this be caused by excess capacitance? I seem to get more vertical lines in the output than are visible on the input. I'm also attaching the schematic for the circuit. The input to the circuit is the output of an RF modulator. The output is to the TV. If you need any questions answered I will answer them just looking to see what I can do to improve this before finalizing the board. Edit: Just thought I'd add. In these waveforms the input/output was a black box in the middle with a yellow border all around. So dark colors the levels are rising, light they are dropping. However the input waveform is more accurate to how it should look having proper and level peaks across the frame. One other thing... The transistors used are PZT3904 from ON semi. |
| soldar:
Looks like poor low frequency (and DC) response to me. Have you tried increasing cap values? |
| Benta:
The output confuguration is wrong for a video signal. Increase R3 to 2.2 kohm and place 75 ohms in series with the output. 220 uF is normally OK for 75 ohms, but the 10 uF might be too small. Do you have 75 ohms across the input? Note that the 75 ohms at the output will attenuate your video signal, so your buffer should have a voltage gain of 2. Using an integrated video amp is a better solution. |
| SeanB:
Input needs to be 47 or 100uF, and the pot should be 1k or less, around 100R works well. C1 polarity is also wrong, it will fail eventually, positive side must be to the base of the transistors. R5 change to 150R and it will improve things, though as this is just a unity gain stage it will not do much other than DC restoration to the signal. |
| maginnovision:
--- Quote from: soldar on January 12, 2019, 09:46:09 am ---Looks like poor low frequency (and DC) response to me. Have you tried increasing cap values? --- End quote --- I have not, I will try that. --- Quote from: Benta on January 12, 2019, 10:25:28 am ---The output confuguration is wrong for a video signal. Increase R3 to 2.2 kohm and place 75 ohms in series with the output. 220 uF is normally OK for 75 ohms, but the 10 uF might be too small. Do you have 75 ohms across the input? Note that the 75 ohms at the output will attenuate your video signal, so your buffer should have a voltage gain of 2. Using an integrated video amp is a better solution. --- End quote --- The 75 ohms in series with the input doesn't exist, I'm not sure if it's needed though. I'd actually have to take the modulator apart to check for sure. I think the circuit designer had more experience with monitors and things. Some research showed this output config is usually used there and the output devices have the 75ohm in series so I will definitely do that and the R3 change along with a larger value cap at the pot. --- Quote from: SeanB on January 12, 2019, 02:11:04 pm ---Input needs to be 47 or 100uF, and the pot should be 1k or less, around 100R works well. C1 polarity is also wrong, it will fail eventually, positive side must be to the base of the transistors. R5 change to 150R and it will improve things, though as this is just a unity gain stage it will not do much other than DC restoration to the signal. --- End quote --- C1 polarity makes sense since the waveforms are on opposite sides of 0 when viewed DC coupled. However I'm not sure which you're referring to as R5. Thanks for the input, I'll be back when I have tried these things(or switched to an actual amp). |
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