Hello,
I think what we read on a spectrum analyzer is dBm for the specified
range.

dBm/MHz is the PSD.

Lets say if the signal bandwidth is occupying 500Mhz (UWB) and has a
PSD of -30dBm, then what is my power in the entire channel?

(-30dBm)*500 = (1microW)*500 = 500 microWatts = -3dBm

So do I see the -3dBm signal level on the spectrum analyzer in the
whole bandwidth?

Please correct me if I am wrong.


Thanks.

A spectrum analyzer has filters with particular bandwidths. What you
see displayed is the power passed by the filter. If you have the
analyzer set for a 5kHz resolution bandwidth, what you see, then, is
the power in (nominally) 5kHz of spectrum. To make things accurate,
you need to understand the shape of the filter, as well. In addition,
you can't necessarily use that number to find power spectral density in
other than a gross sense unless you know that the power is distributed
more or less evenly across that piece of spectrum. That is, if you use
5kHz resolution bandwidth and the signal has 99% of its power in a 1Hz
segment within that 5kHz, the signal's actual power spectral density
(at its center frequency) is much higher than the indicated power per
5kHz.

Many modern spectrum analyzers will display power spectral density
directly for you, taking into account the filter shapes and such.
Also, beware that resolution bandwidth and video bandwidth (on
analyzers that have both) are not the same thing.

Cheers,
Tom

wrote:
Hello,
I think what we read on a spectrum analyzer is dBm for the specified
range.

dBm/MHz is the PSD.

Lets say if the signal bandwidth is occupying 500Mhz (UWB) and has a
PSD of -30dBm, then what is my power in the entire channel?

(-30dBm)*500 = (1microW)*500 = 500 microWatts = -3dBm

So do I see the -3dBm signal level on the spectrum analyzer in the
whole bandwidth?

Please correct me if I am wrong.


Thanks.

To add to what Tom wrote,

Ordinary spectrum analyzers sweep the filter across the band of
interest, so what you see is not only the power in that filter
bandwidth, but the power that's at each frequency at the moment the
filter is there. Movement of the filter frequency constitutes modulation
of the observed signal, which in itself creates sidebands. So what you
see on a spectrum analyzer is heavily influenced by the relationship
between the nature of the modulation causing the bandwidth spreading and
the sweep rate of the analyzer. With some broadband signals, you can get
vastly different displays by changing the resolution bandwidth and sweep
rate, or with different filter shapes.

This is covered in detail in _Modern Spectrum Analyzer Theory and
Applications_ by Morris Engelson (Artech House, 1984). I highly
recommend it to anyone needing to measure and interpret complex
waveforms on a spectrum analyzer.

Roy Lewallen, W7EL

K7ITM wrote:
A spectrum analyzer has filters with particular bandwidths. What you
see displayed is the power passed by the filter. If you have the
analyzer set for a 5kHz resolution bandwidth, what you see, then, is
the power in (nominally) 5kHz of spectrum. To make things accurate,
you need to understand the shape of the filter, as well. In addition,
you can't necessarily use that number to find power spectral density in
other than a gross sense unless you know that the power is distributed
more or less evenly across that piece of spectrum. That is, if you use
5kHz resolution bandwidth and the signal has 99% of its power in a 1Hz
segment within that 5kHz, the signal's actual power spectral density
(at its center frequency) is much higher than the indicated power per
5kHz.

Many modern spectrum analyzers will display power spectral density
directly for you, taking into account the filter shapes and such.
Also, beware that resolution bandwidth and video bandwidth (on
analyzers that have both) are not the same thing.

Cheers,
Tom

wrote:
Hello,
I think what we read on a spectrum analyzer is dBm for the specified
range.

dBm/MHz is the PSD.

Lets say if the signal bandwidth is occupying 500Mhz (UWB) and has a
PSD of -30dBm, then what is my power in the entire channel?

(-30dBm)*500 = (1microW)*500 = 500 microWatts = -3dBm

So do I see the -3dBm signal level on the spectrum analyzer in the
whole bandwidth?

Please correct me if I am wrong.


Thanks.

Roy,

"Roy Lewallen" wrote in message
...
This is covered in detail in _Modern Spectrum Analyzer Theory and
Applications_ by Morris Engelson (Artech House, 1984).

Does Engelson's idea of "modern" include digitizing spectrum analyzers, which
have already taken over most of the mid- to high-end of test equipment and --
as with digital scopes -- will most likely soon be in all but the lowest of
the low-end of instruments?

Thanks,
---Joel

Joel Kolstad wrote:
Roy,

"Roy Lewallen" wrote in message
...
This is covered in detail in _Modern Spectrum Analyzer Theory and
Applications_ by Morris Engelson (Artech House, 1984).

Does Engelson's idea of "modern" include digitizing spectrum analyzers, which
have already taken over most of the mid- to high-end of test equipment and --
as with digital scopes -- will most likely soon be in all but the lowest of
the low-end of instruments?

No, the book was written before those came about. I fudged a little by
qualifying my posting with "conventional" spectrum analyzer to exclude
newer technologies, but it sounds like "conventional" is rapidly
advancing to become the newer types.

I got out of the SA development world just as digital techniques were
beginning to develop -- my last patent (# 5,629,703), in fact, dealt
with a way of reducing distortion in an A/D converter intended for use
in a SA-like instrument. At that time, we were anticipating doing a
conventional sweeping down-conversion, then digitizing it at a 25 MHz
IF. Predictably, the digitization point has been moving toward the front
of the instrument since then.

I haven't followed the technology since, but I'm sure the new ones use
sampling techniques combined with an FFT, which is another way of
imperfectly representing what the real spectrum is like. (Actually, you
can never perfectly represent the spectrum of a real waveform, because
any spectrum which is finite in frequency span has to have existed for
an infinite time. Any modulated waveform is way outside this category.)
Sampling produces its own, different, sorts of artifacts which are
different from the ones produced by sweeping, although there are some
similarities. You have to be at least as careful, and maybe more so, in
interpreting a sampled waveform as one which is from a swept filter.

I'd be surprised if someone hasn't written an equivalent book to cover
the new type instruments, although a lot of the material in Morris'
books is still valid and teaches a lot about the nature of spectra.
Morris was the driving force behind Tektronix's entry into the spectrum
analyzer market, and the group's chief engineer and architect for a long
time. He'd retired by the time I joined the group, and I only met him
when he taught a couple of one-week courses based on his books. He's
truly one of the experts in the field.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Joel Kolstad wrote:
Roy,

"Roy Lewallen" wrote in message
...
This is covered in detail in _Modern Spectrum Analyzer Theory and
Applications_ by Morris Engelson (Artech House, 1984).

Does Engelson's idea of "modern" include digitizing spectrum analyzers, which
have already taken over most of the mid- to high-end of test equipment and --
as with digital scopes -- will most likely soon be in all but the lowest of
the low-end of instruments?

No, the book was written before those came about. I fudged a little by
qualifying my posting with "conventional" spectrum analyzer to exclude
newer technologies, but it sounds like "conventional" is rapidly
advancing to become the newer types.

I got out of the SA development world just as digital techniques were
beginning to develop -- my last patent (# 5,629,703), in fact, dealt
with a way of reducing distortion in an A/D converter intended for use
in a SA-like instrument. At that time, we were anticipating doing a
conventional sweeping down-conversion, then digitizing it at a 25 MHz
IF. Predictably, the digitization point has been moving toward the front
of the instrument since then.

I haven't followed the technology since, but I'm sure the new ones use
sampling techniques combined with an FFT, which is another way of
imperfectly representing what the real spectrum is like. (Actually, you
can never perfectly represent the spectrum of a real waveform, because
any spectrum which is finite in frequency span has to have existed for
an infinite time. Any modulated waveform is way outside this category.)
Sampling produces its own, different, sorts of artifacts which are
different from the ones produced by sweeping, although there are some
similarities. You have to be at least as careful, and maybe more so, in
interpreting a sampled waveform as one which is from a swept filter.

I'd be surprised if someone hasn't written an equivalent book to cover
the new type instruments, although a lot of the material in Morris'
books is still valid and teaches a lot about the nature of spectra.
Morris was the driving force behind Tektronix's entry into the spectrum
analyzer market, and the group's chief engineer and architect for a long
time. He'd retired by the time I joined the group, and I only met him
when he taught a couple of one-week courses based on his books. He's
truly one of the experts in the field.

Roy Lewallen, W7EL

Indeed, we've been making spectral analysis equipment using FFT
techniques since the early 80's or even a little longer. I believe the
HP3577 from that era uses digital IF techniques, though I never got
familiar with its "guts." Things have come a very long way since then.
Just as Roy says, there's a whole different set of things to
understand to get accurate, meaningful measurements out of them. Even
knowing what I do, I still get surprised by the instrument's response
to particular signals sometimes.

With respect to signals in finite bandwidths having to have existed
forever, in theory that's true, but in practice, noise from sources
beyond the signal of interest will always overwhelm those portions of
the signal that are very far from its main power, for a huge proportion
of practical signals. We'll keep pushing for higher spurious-free
dynamic range and lower noise floors in the instruments, but it's
probably fair to say they'll never be perfect, and signals are
essentially always polluted by external noise sources as well, if only
thermal noise in external resistances.

I believe you can find some information on the Agilent website about
factors to consider when making FFT-based spectral measurements. I
probably have a pdf or two hiding somewhere in my archives covering
some of this, too.

Modern analyzers are backed up by some very impressive software that
can help you analyze all sorts of things about modulation, signal
variation with time, etc. It goes FAR beyond simple spectral displays
of FFT results.

One of the key features of FFT-based spectral analysis is that you
don't miss as much as a swept analyzer with narrow IF does. For
example, if you have a signal that keys on for a millisecond out of
every second or so at 100MHz, it might indeed be radiating a broad
spectrum but there's observable energy only within a few kilohertz of
100MHz. An analyzer sweeping 50-150MHz with a 1kHz resolution
bandwidth will take quite a while to do its sweep, several seconds, and
may very well not be "looking at" 100Mhz, or close enough to it, at the
time the pulsed signal comes up to be able to see it. But an FFT-based
analyzer that's sampling fast enough to capture the whole 50-150MHz
band at once can easily find the 100MHz signal every time it pulses on.
The FFT is, quite literally, a large bank of parallel filters/energy
detectors at evenly spaced frequencies. In combination with a
"windowing" function, the effective shape of those filters can be
adjusted.

Cheers,
Tom

Hi Roy,

That's a lot of extra detail; thanks!

"Roy Lewallen" wrote in message
...
Morris was the driving force behind Tektronix's entry into the spectrum
analyzer market, and the group's chief engineer and architect for a long
time. He'd retired by the time I joined the group, and I only met him when
he taught a couple of one-week courses based on his books. He's truly one of
the experts in the field.

Was Dennis Rosenauer around while you were still at Tek? Last I heard (a
couple of years ago), he was working on their spectrum analyzers. As you're
probably aware, Tek pretty much dumped spectrum analyzer development for
awhile in the '90s, and something like 3-4 years ago hired Elaine May to be
the new product group manager... she having been layed off when *Agilent*
decided to dump *their* spectrum analyzer development! Sheesh. Rumor had it
she was trying to get a a bunch of her former co-workers from Agilent to join
her up at Tek.

These days Tek and Agilent seem to have realized that SAs are rather
important, albeit with Agilent pursuing a more "traditional" architecture
(very high frequencies, mix down to IF, digitize there) but Tektronix
utilizing their high-speed ADCs from their scopes to just digitize directly
and compute FFTs -- this approach limiting them to lower frequencies with
noticeably less dynamic range, but giving them the advantage of being able to
trigger on arbitrary masks withoug ever "missing" one.

---Joel

P.S. -- Planning to visit Swaptober Fest on the 21st?

Joel Kolstad wrote:
Hi Roy,

That's a lot of extra detail; thanks!

"Roy Lewallen" wrote in message
...
Morris was the driving force behind Tektronix's entry into the spectrum
analyzer market, and the group's chief engineer and architect for a long
time. He'd retired by the time I joined the group, and I only met him when
he taught a couple of one-week courses based on his books. He's truly one of
the experts in the field.

Was Dennis Rosenauer around while you were still at Tek? Last I heard (a
couple of years ago), he was working on their spectrum analyzers. As you're
probably aware, Tek pretty much dumped spectrum analyzer development for
awhile in the '90s, and something like 3-4 years ago hired Elaine May to be
the new product group manager... she having been layed off when *Agilent*
decided to dump *their* spectrum analyzer development! Sheesh.
....

Sheesh, indeed. Where did you hear _that_? Lots of shrinking happened
back then (and not just in Agilent, for sure), but it would certainly
be a stretch to say that they "dumped their spectrum analyzer
development." That's a pretty gross misrepresentation of what went on.

Cheers,
Tom

Hi Tom,

"K7ITM" wrote in message
ps.com...
Sheesh, indeed. Where did you hear _that_?

Various people at Tektronix. Perhaps they were trying to portray Agilent as
being somewhat more "shrunken" than they really were based on how downsized
they'd just been?

The new Agilent MXA series analyzers look really nice (we've had a demo). I'm
glad to see that the Agilent spectrum analyzer folks realized that, if you're
going to have an instrument that's based on a PC architecture, people want a
*good* PC architecture to back it with -- I'm thinking of the MXA's XGA LCD,
some reasonably modern CPU, etc.; it's *snappy*. Compare to some of Agilent's
network analyzers... we have an N5230A that's only 9 months old, and it has a
640x480 LCD *with a dead pixel* and a whopping 500MHz CPU. Ouch! My
understanding is that the current ones still have the same old LCD, but a
1.1GHz CPU now (woo hoo!)

Thanks for the extra information,
---Joel