EMRFD Message Archive 1876
Message Date From Subject 1876 2008-08-06 11:11:37 wb7dmx scope test 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
the picture is under spectrum analyzer and my call
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?
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;
and from HP; the original AN-150;
and the update for the digital age;
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
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
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.
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