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.
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 :
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.
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?