Help: correct use of a spectrum analyzer (interpreting signal power measurements with various RBWs)
- From: varsakelis@xxxxxxxxx
- Date: Thu, 7 Feb 2008 23:54:30 -0800 (PST)
Hello,
I am trying to use a spectrum analyzer to ID various RF signals in an
urban environment, and measure their power. I am having trouble
interpreting some of my measurements, and I would greatly appreciate
any help. Here's what happens:
1) I start out by sweeping the range from 0.5GHz to 6.5GHz, using
large RBW
and VBW (50MHz in my SA - it's a portable Spectran 6080 unit).
The trace I see contains 3 very prominent peaks. They
are centered at 1.8GHz (-22dBm), at 4.08GHz (-15dBm), and at 6.13GHz
(1.1dBm!), and each is approximately 200MHz wide at its "base".
2) I "zoom in" on the largest of the 3 peaks (select a 6.13GHz center
frequency, and a span of 200MHz), and cut the RBW down to 3MHz or
1MHz. Now, the large peak that used to be there has disappeared
completely (why?).
If I now set the spectrum analyzer to "pulsed mode" for detecting
pulsed signals, then I see instead a series of roughly 10 peaks,
however their magnitudes are much lower than the large peak I saw when
measuring with the larger RBW, e.g., -22dBm instead of +1.1dBm before
(why such a huge difference?).
These smaller peaks do not stay at fixed frequencies but "move around"
with each successive sweep(why?).
Slower sweeps reveal many more peaks than fast sweeps do (why?).
Finally, out of the two magnitudes of the frequency peaks (the large
values seen when measured with the 50MHz RBW, versus the smaller peaks
when using 3MHz RBW), is either one supposed to be close to reality?
I thought that the large dBm value (1.1dBm) obtained with the 50MHz
RBW filter in place might correspond to the total power within the
filter's bandwidth which then appears "broken up" over many peaks when
using the narrower RBW, but that doesn't seem right because there
aren't that many of the smaller -22dBm peaks to justify a total of
1.1dBm.
Thanks in advance to anyone who may be able to shed some light.
Dimitris
.
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