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Measuring Frequency in a generator
well the other inexpensive way to do the job, (provided you have some technical skills), is purchase a small acquisition card or microprocessor eval card, suck the data into your pc and take the fft.
obviously you'd have to glue on some pre-conditioning stuff to isolate the eval board, but you could do this all for less than 50 bucks and have some fun to boot.
then again it might be easier to just purchase a cheap dmm with this capability built in.
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Measuring Frequency in a generator
"(provided you have some technical skills)"
jd, technical skills is the operative term here. Let's see, first you have to get the signal down to about 4 volts, then you have to convert from analog to digital. Store that time series. Figure out the bit order that your specific A-D converter puts out so you can make sense of the numbers. Write a fast Fourier transform program unless you have already spent the money on Matlab or some such program. Oh hell, this doesn't have to be fast, a simple discrete FT will work for this one time problem. Then by knowing the frequency of the digital sampling, figure out what frequency the FT'd peak represents.
Jd, I've got to hand it to you, your plan would work. And it would probably be fun. Sounds alot like my job though.
Dave
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Measuring Frequency in a generator
Sounds about right shortmag. At least that is how we did it 23 years ago to an apple 1. Naturally the cost was about 5X the 50$ back then.
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Measuring Frequency in a generator
Peters, maybe I'm living in the past too. I remember doing stuff like this on an intel 286.
Is there a $50 board that comes with the FFT and display software?
Dave
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Measuring Frequency in a generator
I stand behind my comment about oscilloscope grounding and safety. Since there are many safety-concious individuals here, I've pasted a link and quote from a major manufacturer of oscilloscopes. Ann--There is no need to risk someone's life with bad advice because you got lucky.
Reference:
http://www.tek.com/Measurement/cgi-bin/framed.pl?Document=/Measurement/App_Notes/Technical_Briefs/tds3000-float/eng/limitations.html&FrameSet=oscilloscopes
Quote:
"Most oscilloscopes have their "signal common" terminal connected to the protective grounding system, commonly referred to as "earth" ground or just "ground." This is done so that all signals applied to or supplied from the oscilloscope have a common connection point. This is usually the oscilloscope chassis and is held at (or very near to) zero volts by virtue of the third-wire ground in the power cord for AC-powered equipment. It also means that, with few exceptions, all measurements must be made with respect to "earth" ground. This constrains the typical oscilloscope (at least in a single measurement) from being used to measure potential differences between two points where neither point is at earth ground."
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Measuring Frequency in a generator
Shortmag; I think in total with software and computer it was near 100X, but that was for fast accusition and predated the 8088 let alone the 286. I think the unit we had which was built out of boards with cassette programming and no display was 2X times this, although it was retired when we built the Apple. Sounds like old history now.
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Measuring Frequency in a generator
hey dave,
i'm an rf eng by trade, but have recently been mucking around with the avr micro-p from atmel.
for about 60-70 bucks you can purchase an eval board that comes with a micro that has a 10 bit a-d with an 8 port mux, 3 counter-timers, pwm capability, uart and serial port communication and 32 config i/o's.
i'm actually evaluating the part for use as a controller in a pll application.
with a wall-wart, resistive divider, an rc low pass filter for anti-alias, pc and excel spread sheet --- one now has the capability of low freq spectral analysis of the generator.
matlab is my preferred post analysis tool, but if you’re handy with c code or visual basic you’re good to go.
i might be pointing out the obvious, but if one wants real time measurement of the period vs the spectral content the use of the real time counters combined with the built in analog comparator is the way to go.
and if one gets real ambitious consider using the micro in a control loop to set the speed of the motor, thus the freq output from the generator --- now we’re talking fun!
i’m actually amazed at the power of these inexpensive parts ---- i’m old enough to remember the days of the PDP-8’s and 11’s in college and getting a big smile when I printed out my name on the teletype!
have fun ---
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Measuring Frequency in a generator
jdc, Its good to know that the tractor board has some expertice in radio frequency engineering. I teach a class in hands-on MRI and today we just happen to be building simple RF receive only coils for the scanner. They'll have to lay out the copper tape in a license plate sized rectangle, and tune them with the proper capacitors to 63.8 MHz. Then they build a blocking network so they don't burn up when they're subjected to the main RF pulses. About half of them have some soldering experience so the end results are sometimes ugly. They always work though, many times better than the factory coils with pretty covers.
Dave
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Measuring Frequency in a generator
O.K. you guys, enough with the I.Q. jousting) I'm not an engineer and know notthing about DMMs and have wanted one for some time now. So from the link of the earlier post for Time Motion Tools I found one that wasn't $300.00 or more. What is your opions? The first link is the one I bought because they did not have in stock my first choice (link #2). DM15XL Should I have waited for my first choice?
David
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Measuring Frequency in a generator
The one you bought should work well. It's also almost half the cost of the cheapest on the other page. I have two cheap analog meters that I use and they've always done everything I needed them for so this one would be a real cadillac for me. The real engineers here might have better feedback for you.
Dave
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