video transmitter-doubts - RF Cafe Forums
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Post subject: video transmitter-doubts Posted: Thu May 21, 2009 8:19 am
Joined: Sun Jun 22, 2008 3:57 am
i have built a video transmitter. i am posting the circuit.
when i tune it on my mini b&w tv, it is not stable. and some times there is no picture except for a black(ribbon like) line on the white background.
i have some doubts. the second stage which should be a rf amp,also has a feedback capacitor.is it correct?
and the input signal is applied at the emitter of the first transistor and the base of the second transistor. anything wrong here? i am new to video transmitters.do i need to add a buffer amp stage.if so please tell me how to design it. please help.
here's the circuit :
http://www.circuit-projects.com/rf-radi ... itter.html
Post subject: Re: video transmitter-doubtsPosted: Fri May 22, 2009 2:57 am
Joined: Sat May 16, 2009 3:16 pm
I'm no transmitter expert either but have spent too much of my life building TV & satellite distribution networks.
The circuit looks very crude, so I wouldn’t expect it to be particularly stable but it looks correct. Q1 is an oscillator, which is being used to generate the AM video carrier and its amplitude is being modulated by the video signal applied to the emitter.
The modulated output of Q1 is coupled to Q2, which looks like a class A power amp.
If it is actually producing the power levels stated (100mW or 500mW) you are almost certainly overloading the input to your TV, which will not want more than about 10mW. A typical TV signal-level is less than 1mW. I suggest you don’t connect them directly together, if that is what you’re doing. Try adding some attenuation to your TV input. The picture you describe is typical of a TV receiver fed with too much signal.
If you’re sure the RF levels are OK then the fact that you’re able to tune into a ‘white screen’ suggests most of it is working, since you obviously have a carrier. So all that’s left is the modulation, there’s no mention on the drawing of the video level it requires. What are you driving it with?