1876	2008-08-06 11:11:37	wb7dmx	scope test
Message	Date	From	Subject
I posted a picture of the ftt display of a 50 Mhz generator HB, and what I see is what really impressed me. the generator was set for 48.0 Mhz and the display shows a very clean output, and the freq reading is right on. and the harmonics are way down. the picture is under spectrum analyzer and my call bob WB7DMX
1877	2008-08-06 11:23:15	Ken Evans	Re: scope test
Bob, I cannot find a listing for minimum PC requirements for it. Can you help? Ken W4DU _____
1878	2008-08-06 12:38:49	wb7dmx	Re: scope test

1879	2008-08-06 15:15:05	kerrypwr	Re: scope test
There are a couple of "standard works" on spectrum analysis; from R&S; http://wireless.ictp.it/school_2005/download/rohdeschwarz/SpectrumAnalysysis.pdf and from HP; the original AN-150; http://www.hpmemory.org/an/pdf/an_150.pdf and the update for the digital age; http://www.home.agilent.com/agilent/editorial.jspx?cc=US&lc=eng&ckey=459160&nid=-11143.0.00&id=459160 Comments on your display. If the vertical scale is calibrated in dBm, you have +10 dBm input; that is a lot for an analyser. Remember that it's basically a receiver that works with very small signals; we would consider +10 dBm into a receiver as pretty high. High inputs give odd displays; mixing/intermod processes occur in the analyser. Add 10 or 20 dB attenuation and see if some of the non-integer harmonics disappear. The principle is that, if all "spikes" on the display change by the amount of attenuation, the analyser is not overloaded; it's a good check. Also increase the number of bins; this should make the "spikes" more discrete. Your display at present has a "bin-width" of about 1 MHz which probably accounts for the small error in frequency readings. Fun stuff; measure everything you can find!! But keep the input power low!!
1880	2008-08-06 15:39:55	bob bailey	Re: scope test
thank you very much, that is a lot of information. got it all saved. right, I understand about the lower power thing, I was just playing around with it and the sig generator, been testing many things and looking at the results of the different settings. starting to get a feel of it now. this is all a new experience for me, so much to learn. again thank you for the links bob WB7DMX
1883	2008-08-06 17:52:39	Glenn Dixon	Re: scope test

1886	2008-08-06 19:20:30	kerrypwr	Re: scope test
Yes, Glenn is correct; I managed to confuse myself and forgot the difference between swept-superheterodyne analysers (with which I am familiar) and FFT analysers.
1888	2008-08-07 01:36:42	Jason Milldrum	Re: scope test
Glenn, you are absolutely correct that the FFT function on a scope is quite a different beast from a true spectrum analyzer, but newer SAs now usually integrate FFT technology for a variety of reasons. The SAs that we make here at Tek are essentially a hybrid design. The front end is a superheterodyne system that has a low frequency final IF that feeds into a digital FFT acquisition system (when I say "low frequency", I mean VHF range). It's not a swept system, but it still looks a lot like a traditional SA. Two of the biggest downfalls of a scope FFT system that I can think of right off of the top of my head are the dynamic range and the bandwidth. The dynamic range of the FFT on my TDS1012 is about 50 dB if I'm lucky. I'd be surprised to see if the USB scope could do any better than that. This is due to the fact that these digital scopes pretty much all use 8-bit ADCs. In a "real" SA, you've got a dynamic range of 70-80 dB (maybe more). In our instruments I believe that we use 12-bit and 14-bit ADCs. The bandwidth issue can also be problematic if you don't remember to take it into account. On the TDS1012, I can crank the span of the FFT display to something like 2 GHz, but there's no way I'm getting an accurate measurement at most frequencies, since the bandwidth of the device is only 100 MHz. With a true spectrum analyzer, you should have good amplitude accuracy no matter where the device lets you measure. Regardless of this, it's still very handy to have a scope FFT display. You can still get some good, useful measurements out of it, as long as you understand its limitations. 73, Jason Milldrum, NT7S ===== Amateur Radio Station NT7S - <http://www.nt7s.com> NT7S Blog - <http://www.nt7s.com/blog> qrp-l.org Flickr Group - <http://flickr.com/groups/qrp-l/>
1889	2008-08-07 07:35:54	wimmie262000	Re: scope test

1890	2008-08-07 09:49:17	bob bailey	Re: scope test
sorry, I do not have the equipment or the knowledge to answer your questions. this stuff is all new to me and I am trying to learn it