TV signal into an Oscilloscope

Question

I'm plugging TV signal cable directly into an oscilloscope. TV cable looks like this:

First I connected outer shell of TV cable connector to GND of oscilloscope BNC cable and center to the center of BNC cable and observed no signal as shown in this little video: https://www.youtube.com/watch?v=QZ4LjM1p2pA&feature=youtu.be

And then I connected 'only' the center of TV connector to center of the oscılloscope's BNC cable and I observed many signals fluctuating with a maximum around 20V. And when I adjust trigger level I see only 1 signal. Here is the little video recording: https://www.youtube.com/watch?v=tET0UnLCc04&feature=youtu.be

I'm confused at 2 points:

1) Isn't any signal must be in a loop? Why I see no voltage when I connect inner and outer shell as in 1st video. And why I see a signal when I only connect the center to the center or oscilloscope? I mean to me it is like reading voltage from a voltmeter by only connecting 1 pole of a battery. I couldn't figure out why it is like that here..

2) Why am I seeing many signals (in 2nd video) and when I trigger I see only 1 signal? I couldn't show in the video but when I change the time scale I see a kind of sinusoid as well. Is that the carrier signal?

BTW the scope I'm using is an analog 10 MHz Oscilloscope.

Answer 1

A TV antenna signal as a very low amplitude and a very high frequency. No chance at all to see it on your scope.

The all-present 50 or 60 Hz mains hum will easily picked up. This is probably what you see when you connect only one lead.

Answer 2

Note that there is a difference between a RF (radio frequency) TV signal received off the air, and the video (baseband) signal. It seems you are connecting to the antenna socket, so you are trying to see the RF signal. As @Wouter van Ooijen sais, there is zero chance of seeing that on a 'scope.

The RF signal will be at least 50MHz up to 900MHz in frequency (i.e. VHF and/or UHF). This is what the antenna will receive, at one or two microvolts at most.

After being amplified and detected by the tuner in the TV, the baseband signal will be from almost 0 up to about 6MHz, at about 1 volt peak-to-peak. You can clearly see that on a 'scope if you look at the video output (the yellow socket) of a DVD player or VCR.

In 1987 I built a circuit to decode video from an encoded channel here in South Africa, using exactly that - an oscilliscope connected to the "video output" of a VCR tuned to the correct channel, and a PLL and some other parts :)

Please see http://en.wikipedia.org/wiki/Television_channel_frequencies regarding the RF frequencies

Please see http://en.wikipedia.org/wiki/Composite_video regarding the baseband signal