| Products > Test Equipment |
| Question: Cheap pure sine wave genertor 1khz? |
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| innkeeper:
--- Quote from: cncjerry on January 12, 2020, 08:43:34 pm --- One thing I didn't see was the DSA you are testing? I just bought an HP 3562a DSA and that is good for a boat anchor but the best it will do in range is 120db. It is listed at 80db but there is a way I read someplace to get it to 120db. It must be averaging or something or maybe that was the minimum signal. I've only had it a few days. --- End quote --- I am looking for my manuals with the application n notes. i know it has 150 DB of measurement range. but the operators manual does say 80db dynamic range. not sure how that plays out doing thd....if its limited to 80db damn if that is the case that sucks.. but well might have to find another method when i get down to low thd readings, i know there are some tricks, but..thats all in the application notes not in the operators manual. I hope to play with it a bit again this week |
| cncjerry:
I went back and looked at my DSA and pulled out the section, attached 2nd, that speaks to the dynamic range in an automatic function that I wasn't familiar with. I also attached the noise floor but have to figure out how that compares on the DSA. I don't know what DSA you are using, but I know some DSAs have lower noise floor and if you want to look at the 3rd harmonic, for instance, you just focus the analyzer there. You can't see the entire spectrum but you can calculated the THD by summing the harmonics. This guy has a well formatted formula for THD calcs: http://www.onmyphd.com/?p=thd.total.harmonic.distortion He also has the thd+noise calc there. I'm surprised the HP analyzers go that low automatically calculating THD. I really want one, just haven't been able to find one locally. Jerry |
| peter-h:
I would use two EPROMs to get 16 bit words, drive them from a synchronous counter, and feed the 16 bits of data into a 16 bit audio DAC :) I designed such a thing for a friend of mine's (!) final year univ. project in 1978. We used fusible link PROMs back then, only 8 bits of data so not smooth, but frequency changes are instant and there is no amplitude bounce. If you use a good DAC, no missing codes, you can get THD below 0.005%. Probably less by filtering the output. Won't be dirt cheap admittedly. I believe there is a way of connecting up shift registers in a feedback arrangement which gives you a sin(x) output, but you still need a DAC. |
| Fungus:
--- Quote from: peter-h on January 13, 2020, 06:12:33 pm ---I would use two EPROMs to get 16 bit words, drive them from a synchronous counter, and feed the 16 bits of data into a 16 bit audio DAC :) --- End quote --- Did you build the counter yourself using D-type flip-flops? :popcorn: These days we have microcontrollers that are a lot cheaper than EPROMs and can do the counter, the feeding of the DAC, and everything. |
| trobbins:
There are good external USB soundcard/interfaces around - I have a 15 year old EMU 0404 USB - and although I haven't used one, it seems stock product like FocusRite 2i2 are now pretty cheap new. REW software allows insertion of selected harmonics (2 to 9) in to the test signal from its tone generator, with settable magnitude and phase. REW also provides bandwidth limiting and distortion component measurements of the loopback spectrum. Using my EMU0404 USB as DUT with loopback, I did some testing last year and had 0.0006% 2nd and THD at 389.6Hz test tone when limiting bandwidth to just include 2nd harmonic. I can then null that out (0.0001%) with -104dB of added 2nd H with 90 deg phase shift. I effectively did the same for higher order harmonics. This method will work for any typical soundcard, so the generation of a very low harmonic distortion waveform is not limited to the raw performance of the soundcard itself. As such, a cheap or even zero-cost, and quick setup can be prepared by most. |
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