04-10-2016, 11:58 AM
(04-10-2016, 05:59 AM)Diabolical Artificer Wrote: Blimey, opened up a can of worms with my question. : ) I've followed most of it, though transmission line theory is a bit heavy going.If I have it right though in answer to my original query, I can use any combination of BNC test leads for AF use as long as they don't have old style thick pin types and the length of the cable isn't too long.
The only longish cable's I have for use at home are about 2M long and are of decent quality Klotz balanced twisted pair. I havn't experienced any HF roll off or other issues, but would'nt expect to.
I also have some 10M mic cables which I havn't used yet, so I'll keep reading this thread with interest. Thanks for all you input.
Andy.
Hi Andy,
Post #2 says it all really. Don't worry about cable type or transmission line theory - it's not an issue at audio frequencies in a workshop. But do watch for loading effects
I'll say a few more things about it, as it's the biggest problem I see every day in my job:
In a workshop environment, the problem occurs most often when a signal source of some type includes a level meter (or some other form of output level indication, such as markings on a dial). Usually, these readouts are calibrated on the assumption the output is correctly loaded, so the output voltage will be higher than the meter says if you don't.
So if you don't take the time to measure the actual output with a 'scope, then you could be somewhat confused! Typically, if you're measuring the gain of an amplifier, it will appear to have more gain than you expect, but that's only because the signal level is higher than the meter on the generator suggests.
The function generators I use most at home are uncalibrated - just a simple rotary control - so I know that I have to measure their output directly. I actually prefer that.
Also, my preferred generators are 50 ohms, so loading effects are less of an issue for typical audio applications. So if they did have an accurate meter, the error between reported and actual would be minimal to insignificant. For example, 50 ohms feeding 10k amounts to a loss of 0.04dB. But 600 ohms feeding 10k is half a dB, which might be significant.
Today, a lot of generators are microprocessor based, so you adjust the output level by typing in numbers or turning a rotary encoder, reading the value from the screen. So again, what do these numbers mean? If you ask for 1V, that might be 1V before the output impedance (a.k.a. "EMF"), or 1V at the output terminals, when correctly terminated. Again, just to reiterate, if it's the former case, then for low-Z out going into hi-Z in, these numbers are virtually identical in most situations. But if it's the latter, the actual output could be double the expected value.
Believe me, this subject catches out a lot of people in many walks of life.
This is a good overview of the subject: https://www.youtube.com/watch?v=tClE8s6RZdg
That video mentions 'scope probes as another "trap". And another thing to watch for is RMS verses peak to peak. Signal generators tend to quote volts RMS, but function generators usually quote peak to peak. Of course, there are variations. Some allow you to change the readout type; many don't. Obviously, on an analogue 'scope, pk-pk is what we measure. A digital 'scope will offer measurement options for either (and a lot more besides). A DMM might measure RMS, in which case it'll usually say "True RMS" in big letters on the front. Or it might be "Average-dection, RMS reading", which is fine for measuring sine waves, but non-sinusoidal waveforms will give different numbers compared to a "True RMS" meter. And DMMs have much less bandwidth than a 'scope*, so knowing the upper limit of your meter is really important.
In fact, knowing your gear is the most important rule in any workshop
* Just in case someone wants to pipe up at the back, yes, there are exceptions. E.g., I have an old Fluke 8920A which has a bandwidth of 20MHz, and 'scopes with less bandwidth exist.
Finally, if you're curious about learning more about transmission line theory, this video is another good practical demonstration of what can happen: https://www.youtube.com/watch?v=PPgxFd97taY
Hope this helps,
Mark
