Disturbance in Video Signal

[gauravkothari23]

hi all...
i am working on a project where i am implementing a small robotic vehicle with video camera.
i have attached my camera to the vehicle and output of the camera is directly connected to TFT Screen and TFt screen and camera has nothing to do with the controller.
Now i have connected my wheel motors with the controller 8051 which i am driving it using L298. My Input source in 18V 5 Amps SMPS, where again converting the same to 12V using LM2596 Module.
12V is being connected to camera, TFT display and motors. where again using a regulator 7805 furthur emitting it to the controller.
this is a simple circuit i have designed.
but my problem is when the motors are in off state, then is receive a good display on TFT screen without any disturbance, but as soon as the motors starts even at a very nominal speed the video quality goes OFF.. i can see white patches on the screen scrolling from top to bottom and vise versa.
can anybody pleas let me know what the problem is.

 

[FvM]

The problem is careless design. You need power supply filters, common mode chokes, well considered ground and video signal cabling. What's the video signal interface? Analog or digital?

 

[gauravkothari23]

The problem is careless design. You need power supply filters, common mode chokes, well considered ground and video signal cabling. What's the video signal interface? Analog or digital?

The SMPS has inbuilt filters and about cabling the video signals, i am using CCTV Cameras and my video out is analog.

 

[FvM]

The SMPS has inbuilt filters

As expectable. Strongest interference sources are LM2596 switcher and motors. Need to isolate the video circuit from the motor supply, e.g. using separated grounds and filters.

 

[gauravkothari23]

As expectable. Strongest interference sources are LM2596 switcher and motors. Need to isolate the video circuit from the motor supply, e.g. using separated grounds and filters.

Using separated GND wont be possible for me because i have only one input source.
Can you please help me, how can i isolate or what filters has to be added for the same without seperating the GND.
I have added 470uF Electro Cap at each outputs of LM2596, but it doesn't works, i agree it wont be enough.

 

[FvM]

No problem to have grounds connected at a supply star point. Show the present wiring.

 

[Audioguru]

A 470uF capacitor works as a filter only at low frequencies because at high video frequencies it is an inductor. Also use ceramic high frequency capacitors.

 

[gauravkothari23]

No problem to have grounds connected at a supply star point. Show the present wiring.

This is the basic block diagram i have attached. do i need to add LC filter circuit between camera and LM2596 output.

- - - Updated - - -

A 470uF capacitor works as a filter only at low frequencies because at high video frequencies it is an inductor. Also use ceramic high frequency capacitors.

Do i need to add ceramic capacitors at video line and GND..??

 

[FvM]

The diagram doesn't show the actual wiring. You e.g. need to avoid motor current flowing through the video ground. Depending on the cable connection, this may well happen.
.

 

[gauravkothari23]

The diagram doesn't show the actual wiring. You e.g. need to avoid motor current flowing through the video ground. Depending on the cable connection, this may well happen.
.

How can i resist the motor current flowing to the video GND, because the source i have is only 1 through SMPS.

 

[FvM]

Common ground at the supply isn't necessarily a problem, but you need to avoid ground loops involving the video circuit.

E.g. have a common LC filter for the monitor and camera supply, use separate supply cables behind the filter, avoid shorts between motor and video ground.

 

[gauravkothari23]

Common ground at the supply isn't necessarily a problem, but you need to avoid ground loops involving the video circuit.

E.g. have a common LC filter for the monitor and camera supply, use separate supply cables behind the filter, avoid shorts between motor and video ground.

Thanks,
The things are working perfect. i have just initially separated the GND in 2, one for camera, video and display and other for rest of my circuit.
but will implement LC filter circuit for all of them in my final design and will make sure the GND is separated.

 

[BigBoss]

It might be a EMI in air.Have you ever screened the Camera-TFT cable ?? Or have you changed the position of the cable ?? A Ferrite bead can be used on this cable.
Because AFAIK, LM 2596 uses a a big ferrite wounded coil and this can create EMI on other circuits by magnetic coupling.
Electrolytic Caps have high ESR at high frequencies.Therefore A Common Mode Filter can be used following Polyester Capacitor for each port.
There maybe also other reasons but we don't know cabling and PCB connections.

 

[Audioguru]

Do i need to add ceramic capacitors at video line and GND..??

Capacitors from the video line to ground will kill the video.
Since the 470uF capacitor is useless at high frequencies then it needs a ceramic high frequency capacitor parallel with it to filter away the high frequency interference.