RF Demodulator - where to get one?

[Jeffrafa]

My friend and I are working on a science fair project and we are in need of a simple RF demodulator to go from a coaxial tv input to either a composite video line or preferrably RGB outputs. We need it to work primarily around channel 2 ( ~50 MHz input ) and we'd like it to be controlled by some sort of analog knob so that we can fine tune it into the frequency we need.

Does anyone know where we could get a demodulator like this for a reasonable price, or does anyone know how to make one that would do this for us?

Any help is greatly appreciated.

- Jeff

[Al Skierkiewicz]

Jeff,
The simplest thing to do is press a defective VCR into service. You can use it for composite out and it should have a AFC to pull in a signal off frequency. What is your specific application/. There may be something else better suited for your task. Getting to RGB is fairly difficult and requires a much more complex device.

[Jeffrafa]

We are wanting to use it for part of our van eck phreaking project ( Recieving, demodulating, and redisplaying video from the radiation emitted by computer monitors ).

We are going to recieve the signal via a tv antenna, boost it with signal amplifiers, demodulate it from rf to composite video or rgb, then input it into a monitor while supplying our h-sync and v-sync separately.

- Jeff

[Al Skierkiewicz]

Jeff,
Are you going to substitute H and V sync from the original monitor? Adding sync back to video is a difficult process unless you have some good electronic design experience.
It sounds like you are pointing the antenna at the screen to get radiation data and then amplifying it to be able to display the effect on another monitor. This will be very difficult if you are expecting to see actual video demodulated from the radiation of the screen. The radiation occurs at baseband video and can have a bandwidth of at least 10-20 MHz. Remember that the radiation is a function of brightness and electrons striking the phosphors on the face of the tube. The electron stream varies with each of the colors as each of the phosphors have a different sensitivity to electron bombardment. Red, Blue, and Green streams occur simultaneously as there are three guns in the back of the tube so there is no way to separate the radiation. A VGA or Super VGA or higher res tube does not operate at the standard sync rates that normal TV demods operate at and so even a high quality tuner will not give you any valid data to look at since it is not occuring at the same frequency of a standard tv channel.
That being said, you might be able to get a watchable signal (black and white) with another monitor and an amplifier. Feed the same sync to both monitors and a video signal to the first monitor. A loop of wire in front of that picture tube with a wideband amplifier might give enough signal to feed into the video input of another monitor. There is an electronics kit mailorder called Ramsey Electronics that used to have amplifier kits for pretty cheap prices. They had a website but I haven't gone there recently. You are looking for a wideband preamp and as I remember that had several in the under $50 price range without a box.

[Jeffrafa]

We do plan on tagging off the same source for the h and v sync lines, at least initially - we may look into doing this a different way later on if all goes well.

Are you suggesting simply inputting the amplified signal into the second monitor's video input, or still using a VCR or something to demodulate the signal?

After the wideband amplifier, should we be using some kind of filter to eliminate noise from frequencies other than the range we are aiming to capture and display? If so, where would we be able to get ahold of a filter like that, or are there plans to make one somewhere?

I found three preamps from ramsey electronics that looked like they would achieve what I was after, the PR2 broadband preamp (http://www.ramseyelectronics.com/cgi...action&key=PR2), the SA7 broadband preamp kit (http://www.ramseyelectronics.com/cgi...action&key=SA7), and the PR100 high performance 2 meter preamp kit, which doubles as a signal filter (http://www.ramseyelectronics.com/cgi...tion&key=PR100).
Based on the information about each it seems like either the PR2 or the PR100 would be what I'm after, depending on if I want the filtering incorporated or not.

Could it be simpler to initially attempt van eck phreaking on televisions instead of computer monitors since h and v sync rates are standard on TV's?

As far as using a VCR for RF demodulation - how would I go about controlling the AFC, is it something I could wire a potentiometer to in order to tune or what?

Thanks for all the assistance, Jeff

[Al Skierkiewicz]

Jeff,
The SA7 was the one I was thinking of and it's pretty cheap. You might want to shield it in a metal box but I don't think that should be a problem. There is no need for a demod since the signals you are looking for are not modulated on an RF carrier. They are just baseband radiation. By using a video monitor that is locked to the same sync as the monitor you are snooping makes it very easy to "sync' the signal you are looking for. The preamp raises it to a level that can be displayed on the second monitor. Remember that the antenna you use effects the overall gain of the system. The higher the gain the more likely you can overdrive the preamp. You need to get at least 0.5 volts of video out of the preamp for the monitor to display something. Anything over 0.8 volts will wash out to pure white do to overdriving the input video amp. An oscilloscope would be a big help to see if you are getting any signal through the preamp and at what level.

[Gdeaver]

What your trying to do could be an illegal act so I will not post links. However, with the proper goggle you will find plans, kits and how to do it.