rigma Posted February 25, 2004 Share Posted February 25, 2004 gerbache, I am in Lebanon about 20 mi east of Nashville. Quote Link to comment Share on other sites More sharing options...
gerbache Posted February 25, 2004 Share Posted February 25, 2004 As for why the meter is showing a voltage when you set it on AC, I'll bet that the voltmeter is calculating some form of RMS voltage. When you break down what RMS means, it stands for the root of the mean squared. Put more simply, you take a signal, square the whole thing, then take the square root of the average value. Of course, when you square a negative number, it flips up and becomes positive. Now, lets consider a square wave. If a wave is going from +5V to -5V and we square it, both parts will become +25V^2. So in essence, it is now a constant flat line, so the average value is exactly +25V^2. When we take the square root, we get back to +5V, which is why we can say that there is an AC voltage of 5 volts on it. When you use a DC voltmeter to measure the voltage, it will report to you the basic average value over time, which in this case will be simply 0 volts because the signal is switching between +5 and -5. If you're interested, as soon as I can come up with some free time, I can try to put together some drawings to help explain this stuff better. Quote Link to comment Share on other sites More sharing options...
sfogg Posted February 25, 2004 Share Posted February 25, 2004 "When you use a DC voltmeter to measure the voltage, it will report to you the basic average value over time, which in this case will be simply 0 volts because the signal is switching between +5 and -5. " Another way to see this is the case is to use an analog voltmeter set to DC and input square waves of a low frequency into the meter. If you set the square waves to 5hz or so you can watch the analog meter flip flop between a positive and negative DC voltage. It is really obvious at 1hz. With most digital meters you can maybe see this at lower frequencies too if the meters sampling rate is fast enough. If you have a digital meter with a very high sampling rate ($$$) you can see it alternating between + and - DC values on square waves at much higher frequencies. Shawn Quote Link to comment Share on other sites More sharing options...
3dzapper Posted February 25, 2004 Share Posted February 25, 2004 Shawn, That is what I was taught in school. DC is a varing DC wave form made up of the sum of all frequencies. Like white light. Other than that until I saw Marks diagram I wouldn't know the difference between AC and Dc coupling. What's that you say "read the manual"? C'mon what fun would that be? Rick Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 Mark, When are you going to start charging for these lessons Craig Quote Link to comment Share on other sites More sharing options...
Painful Reality Posted February 26, 2004 Share Posted February 26, 2004 Mr Painfull Do you even own a scope ?? ---------------- Of coarse! Not one but two: a Tektronix 2213A 60 MHz and a BK 20 Mhz (I think it's a 2120). The BK is for sale actually. Quote Link to comment Share on other sites More sharing options...
sheltie dave Posted February 26, 2004 Share Posted February 26, 2004 Craig, don't get too upset. We are dealt the same hand in the radiation field every day. We have techs who take calibrated equipment out in the field, spend four hours suveying a truck, come back in, write up all the paperwork, and then I review it and do some number crunching. I have the onerous task of rejecting about 60% of all field work because varying radiation backgrounds prevent being able to achieve a valid reading. You have the same situation. Your measurements are only as good as your equipment, and distortion is a real MF to track. To have a valid cardinal reading for output distortion, you need to have a total internal (background)machine error of between 10% to 50% of the distortion you are trying to measure. ANSI standards are pretty unforgiving when it comes to calibrations. For a calibrated, perfectly operating O-scope, the most significant errors in the error chain will be your contact leads and the readout. I hope you aren't drinking too much coffee when you are scoping the Mac Learning can be done through reading, seeing, and doing. Adult learners usually require all three, though I am an exception. I also have to be donked over the head so the learning can be implanted in the empty caverns that once held my brain! Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 Dave, I'm not upset but the information here is actually incorrect by a couple people. I just do not have the talent in written word or technical background to get my point across ! This is very frustating so I would rather just let it go. It all is pretty meaningless in the end. The result that matters is my work sounds excellant Why is completely secondary. Craig Quote Link to comment Share on other sites More sharing options...
Painful Reality Posted February 26, 2004 Share Posted February 26, 2004 Oh Lord... Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 Do I here Kelly ?? Oh Lord ! Believe what you want but much of the information in this thread is not completely true. Quote Link to comment Share on other sites More sharing options...
Painful Reality Posted February 26, 2004 Share Posted February 26, 2004 Yes Craig, Who cares about AC coupling or DC coupling as long as you show what goes in and what goes out! And Fourier was wrong too. So is the fundamental of the cap, they never at no point let DC pass. Youse guys in white lab coats who wrote funny signs on those paper sheets. Ya kill me. Flick on those switches! Quote Link to comment Share on other sites More sharing options...
Guy Landau Posted February 26, 2004 Share Posted February 26, 2004 ---------------- On 2/26/2004 12:10:17 PM NOSValves wrote: Believe what you want but much of the information in this thread is not completely true.---------------- Like that your work sounds excellent ? Quote Link to comment Share on other sites More sharing options...
Allan Songer Posted February 26, 2004 Share Posted February 26, 2004 Tell you what---I've owned this little Fisher for a long time and I'll let everyone know how it sounds when I get it back! Quote Link to comment Share on other sites More sharing options...
gerbache Posted February 26, 2004 Share Posted February 26, 2004 What information are you thinking is incorrect? I don't think much of this is intended as an attack on you or your work, but the theoretical stuff that's been discussed is correct, at least for what I've paid attention to. Without pictures and graphs, though, it's sorta hard to picture some of this stuff, which is why mdeenan's posts have been helpful to the discussion. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 Fair enough ! I'm not talking just about the Fisher I'm talking my work in general but if you ask me it sounds wonderful. I have no clue how it sounded before I had to fix it to run it and in doing so had already modified the circuit. I'm sure it sounded great bone stock I really didn't change things that much. Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 Guy, Can you ever add anything positive to these threads ? Your motives are pretty easy to see here. Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted February 26, 2004 Author Share Posted February 26, 2004 ---------------- On 2/26/2004 12:19:41 PM painful reality wrote: So is the fundamental of the cap, they never at no point let DC pass. Youse guys in white lab coats who wrote funny signs on those paper sheets. Ya kill me. ---------------- Jeff, A Quality Cap that is doing its job correctly doesn't allow DC passed at least not in a measureable amount if it did it would induce distortion all by itself ! I suggest you do some more reading. Craig Quote Link to comment Share on other sites More sharing options...
Parrot Posted February 26, 2004 Share Posted February 26, 2004 Say what you will about FLKlipscher, at least he is funny. The same thing can't be said about Guy or Painful. If they're looking to be comedians, they shouldn't quit their day jobs. Whoops! That doesn't apply to Guy. Quote Link to comment Share on other sites More sharing options...
gerbache Posted February 26, 2004 Share Posted February 26, 2004 The cap isn't exactly letting DC pass, and the term "DC component" is probably what is causing the confusion here. You're right that a square wave centered at 0 does not have a DC component, in that the average value of the signal is indeed 0 volts. During each half-cycle, though, where the signal is flat, that part of the wave does in fact look like a DC value, albeit only for a short period of time (equal to 1/2 the period of the wave, which in the case of a 20Hz signal is 1/40th of a second). If this period is long enough, the capacitor will cause the signal to be high pass filtered, which will result in the slanting shape we've seen on an AC coupled scope. When I say it has a DC component, I'm probably throwing you off a bit. When looking at the signal as a whole, you're right that there is no DC component, but during the period of time when it is staying at a constant value (which is half of each period in a square wave), it looks to a capacitor like a DC signal. Quote Link to comment Share on other sites More sharing options...
sfogg Posted February 26, 2004 Share Posted February 26, 2004 Craig, " A Quality Cap that is doing its job correctly doesn't allow DC passed at least not in a measureable amount..." Wrong. Here is how you can measure it. Take any 'quality' cap that you own. Discharge it. Hook one side of it to a DC power source, leave the other side unconnected. Attach your voltmeter set to DC to the unconnected side of the cap and the other end to the ground on your power supply. Guess what you are going to read? DC voltage. Why? Because the cap hasn't charged up. Now attach a 1kOhm resistor from the side of the cap connected to the meter and to ground. Watch the DC voltage through the cap. When the cap fully charges you will no longer see DC through the cap. Shawn Quote Link to comment Share on other sites More sharing options...
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