Post subject: video transmitter-doubts Posted: Thu May
21, 2009 8:19 am
Joined: Sun Jun 22, 2008
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 :
Post subject: Re:
video transmitter-doubtsPosted: Fri May 22, 2009 2:57 am
Joined: Sat May 16, 2009 3:16 pm
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?