Subject:      Interpreting spectrum analyser plots
From:         Marconi <radio@irational.org>
Date:         1998/07/24
Message-ID:   <35B8BE5F.6565@irational.org>
Newsgroups:   alt.radio.pirate 


Perhaps it would be useful for you to familiarise yourself with the
interpretation of spectrum analyser plots.  Let's take
http://www.irational.org/sic/radio/bc500meg.gif for example.  You will
see that I have set the analyser controls so that the fundamental of
this exciter is at the top of the screen.  I have then selected a
relative scale on the left axis.  This measures dBc down from the
carrier.  The next most significant component occurs at +98MHz above the
fundamental - it's the 2nd harmonic.  This signal is -60dBc down on the
fundamental.  In power terms, 60dBc is 1 million times less power.
Seeing as the fundamental is at 1W (30dBm), the level of this harmonic
is 1 millionth of a watt, or 1 microwatt (1uW) or -30dBm.  This power is
not going to bring down any aircraft.  All the other harmonics and
sub-harmonics are even smaller than this.
The solid green block at the bottom of the picture is the analyser noise
floor.  If I'd used trace averaging, this block would have collasped to
a single horizontal line.  The level of this noise floor is at -80dBc,
so we can see any unwanted frequency component down to -80dBc.

As for http://www.silcom.com/~dschmidt/fmtxtxt.html, lets look at
http://www.silcom.com/~dschmidt/veron1.jpg for example.  The noise floor
of this measurement is about 55dBc down on the fundamental.  So in my
measurement I can see another 25dB down.  Not that this matters in this
particular case, as the http://www.silcom.com/~dschmidt/veron1.jpg
picture is perfectly good enough to show the levels of the harmonics and
3f/2 tones.