assignment question re: impedance matching

[slipvayne]

having a bit of trouble with this question and im guessing theres a few people who know data communication on this board.

What are the results if the load is not matched in a:
a) TV transmission system
b) data communication system

thank you

[maddhatter]

What do you think the answers are?

OCAU has a no homework policy dude, good luck getting your questions answered here if you don't show us that you have at least tried them yourself

Also, I know little about electronics - but what impedance load in the system are they referring to? I would have thought variances in different parts of a system could cause any number of different problems...? The question doesn't sound complete to me.

[Symon]

It's a legitimate question.

If the load is not balanced to the transmission line you get unwanted reflections and an ineffective power transfer into the load, so your TV transmitter will not work as well as it could, and you will experience data loss and/or slow data transfer rates with your communication system.

But like madhatter said, OCAU is not here for homework, use google and wikipedia.

[Dr feelgood]

Whilst there are some very knowledgable people on OCAU, I wouldn't rely on an internet forum for technical info that can be quite precise like that.
My advice is to read up oin impedance and its relationship to signal supply voltage and current draw, then apply that knowledge to the relavent transmission system.

[slipvayne]

iits really just a general definition question. i know that in both cases there'd be more noise from interfering signals and the wanted signal will be drowned out, i was wanting to know what would be the specific symptoms for the two cases (tv transmission and data communication).

thanks to symon i now know that. and i didnt know about the no homework policy, sorry

[mattz0r]

Also check out the maximum power theorem and impedance matching.

[nux]

Incorrect termination would cause the same thing for both TV and Data. If its a general knowledge question they would have taught you how to answer it in your subject.

[slipvayne]

they didnt teach specifically for tv and data comms systems. at least not in the subject the assignment is for

[dakiller]

I'm pretty sure for analog TV you'll get ghosting of the picture on the screen due to impedance mismatching producing reflections on the transmitted wave and the way that TV scans the picture line by line you are going to have some of the signal from one area smeared across the screen due to that signal reflection

For pure data communications? I'm not really sure, it is probably very dependent on the transmission protocol used, if it isn't bad enough that it just doesn't work at all then you'll probably get a significant amount of errors

[dohzer]

Either way, you're not going to receive the maximum energy you would with a matched system.
Some of the energy will be reflected.

[Symon]

Originally Posted by dakiller

probably get a significant amount of errors

That's exactly what happens, when you have a significant amount of reflection in a data transmission system, your data becomes corrupted and becomes flooded with errors, so much so that the data transfer rate is dropped significantly or stops entirely.

If you are talking about radio systems (such as TV), there will be a high standing wave ratio (SWR) which shows you have excessive reflection, and as such the output radiating from your antenna is significantly reduced. In the old days before transmitters had AGC's (automatic gain control) fitted, high SWR used to damage the final stage amplifiers.

[munka]

TV often ends up with a ghost image.

Data becomes flakey but this depends on speed and length of line.

[brokenback]

Originally Posted by dohzer

Either way, you're not going to receive the maximum energy you would with a matched system.
Some of the energy will be reflected.

yep in a high power Txmtr if the SWR is not right and the aerial impedance is not matched to the txmtr you get reduced power out thru wave superposition.

The power from the txmtr has to go somewhere..to the connections, lines even back to the tx output stage (a very bad case) causing heating, blowing up of connectors, txmtr (in a very bad case).

[stewpot]

I think ghosting is more a sign of multipath propagation between the TV transmitter and your antenna.

Line frequency on a TV is 15625Hz, which corresponds to a wavelength of about 20000m (or about 15000m in a transmission line with vf=0.75). To produce a ghost displaced by 20% of the screen width, the transmission line would have to be 10% of 15000m long.

[munka]

Originally Posted by stewpot

I think ghosting is more a sign of multipath propagation between the TV transmitter and your antenna.

Line frequency on a TV is 15625Hz, which corresponds to a wavelength of about 20000m (or about 15000m in a transmission line with vf=0.75). To produce a ghost displaced by 20% of the screen width, the transmission line would have to be 10% of 15000m long.

if your maths was correct then multipath propegation would have the same limitation. I would have thought we would be safe to work on the pixel frequency here (its usually a horizontal ghost after all) so you can divide your length by about 300, ie 5m. so lets say you have a 30 m line you will get a 120% ghost image which is why most ghosts appear lower and more often than not to the right.