A summary about the horizontal stripes:
1. They are not noise caused by the TEC switching. If I set the switching to be very slow (once a second) the stripes stay at the same intensity for the whole 1 second. Any electrical noise caused by the switching would not last that long.
2. They feel more like the gain or offset or some voltage is slightly changing, hence the darker/brighter stripes. Or some capacitive effect???
3. I measured voltages with oscilloscope to try to find any variations that happen at the same frequency as the stripes. I did not find anything but it is very difficult to detect small voltage variations on large base voltages.
4. If we use higher frequency the stripes start losing their sharp transition edges. Unfortunately we don't have proper PWM on the pin controlling the TEC and we cannot manually change the TEC pin fast enough during the frame reads. The stripes are clearly visible even if the TEC is switched on and off at every line change (1000 lines in 2 seconds).
5. The intensity of the stripes can be reduced if the cold finger is electrically connected to the back of the sensor by using an electrically conductive thermal paste, like Arctic Silver 5. However, they are still present.
6. If the cold finger and the back of the sensor is grounded the camera will stop working.
So, unless someone figures out what is causing the stripes I don't think it is possible to use the current TEC pin to have manual PWM during the frame reading.
Note that Rim said:
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The peculiarity lies in the fact that it is necessary to properly connect the power wires supplying the "+" and "-" peltiers to the source of the power switching transistor, otherwise there will be some banding of the image. It is necessary to drive these wires separately from the input connector and it is better to use a separate connector for powering the cooling power section.
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Not sure how that would help us.
Anyway, Cam90 uses a different pin for the TEC control and that pin has PWM. TEC is switched at about 500Hz and the stripes are not visible. So the only option for us would be to use the humidity sensor pin (which supports PWM), cut traces on the board and wire it to the MOSFET that switches the TEC. Or alternatively wire the humidity sensor pin to a separate MOSFET that is not mounted on the board - this would mean that no traces need to be cut.
