Re: Spectrum analyzer blind spots

On Dec 27, 7:59 pm, "m...@xxxxxxxxx" <m...@xxxxxxxxx> wrote:
On Dec 27, 11:16 am, Tom Bruhns <k7...@xxxxxxx> wrote:



On Dec 26, 6:29 pm, "m...@xxxxxxxxx" <m...@xxxxxxxxx> wrote:

On Dec 26, 5:14 pm, dpl...@xxxxxxxxxxxx (Dave Platt) wrote:

For example, I have heard frequency analysis of spread spectrum and
FHSS is not possible with standard SA's,

You can almost certainly do certain types of analysis of those sorts
of signals with a standard swept-frequency analyzer, but may have
trouble looking at the signals in detail.

This is due to the fact that a traditional SA uses a swept LO, mixer,
and narrow filter to scan a whole range of frequencies... in effect
it's a sampled system.  Since it only looks at one small slice of the
spectrum-under-examination at any given moment, it will miss whatever
happens outside of its filter position and bandwidth.  For example, a
FHSS signal might be bouncing around in ways which would always (or
almost always) cause its carrier and sidebands to be where the SA
wasn't looking at that instant.  If you want good frequency resolution
when you do see something, you have to use a narrow filter, and a
relatively slow sweep.

The same problem could exist for any pulsed modulation, not just
spread-spectrum.

A spectrum analyzer which digitizes a whole spectrum-slice and then
uses FFT to do the analysis would not have this particular sampling
problem, but might well have other limitations depending on how
continuously it can capture and process and display data.

Certain other types of SS might show up just fine and be easy to
analyze on a swept-frequency analyzer.  For example, an OFDM signal
carrying continuous intelligence would show up like gangbusters since
all of its carriers would be "on" and modulated.

                                     and that current models are
designed not to display certain frequencies used by the government..

Haven't heard that.  I suppose it's possible, but it seems rather
futile.  All you'd need to get around this would be an external
oscillator and mixer... and that's just how my old Systron-Donner
extends its response up to multi-GHz range (external mixers, to
combine the incoming signal with harmonics of a reference oscillator
that the SA provides).

I imagine that some commercial high-speed analyzers do have
discontinous frequency coverage, but I'd expect that's a money-saving
design issue (concentrate on the specific bands for which there are
commercial customers) and not one intended to hide anything
deliberately.

--
Dave Platt <dpl...@xxxxxxxxxxxx>                                   AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
     boycott any company which has the gall to send me such ads!

The FFT by default gives you the whole spectrum. You can choose not to
display the whole specturm. To just sample a range, you need a chirp-
z.

To be really clear here, a spectrum analyzer doesn't have to have a
source. A network analyzer does, but a spectrum analyzer can strictly
just do spectral analysis.

I'm not entirely convinced the analog spectrum analyzer is totally
blind to spread spectrum, or vastly inferior to FFT. Since you can set
the sweep rate of the bandpass, given a slow enough sweep, some of the
spread spectrum energy should make it through the bandpass filter.
Normally the sweep rate is chosen based on the group delay of the
bandpass, but there has to be a way to override the sweep rate to
compensate for the delay in the DUT.

Lastly, getting back to the FFT analyzer, would it be possible to set
up a waterfall display and see the spread spectrum energy move around..
Perhaps more likely to be seen with a frequency hopper than direct
sequence.

Re the last paragraph, of course that's true of frequency-hopping
spread spectrum, but not of spread spectrum done through modulation
with a (pseudo)random signal.  That is very difficult to distinguish
from noise, and if the amplitude is low, can very easily be missed or
mistaken for a source not carrying any useful information.

For a system capable of not only finding but identifying interesting
signals from an antenna (for example), seehttp://cp.literature.agilent.com/litweb/pdf/5989-2837EN.pdf.  This
system  can look at and display a lot of spectral bandwidth quickly
and continuously.  A color spectral map with frequency on one axis and
time on another (kind of a "super waterfall") allow tracking lots of
interesting things.  I'm quite sure there are no "holes" in its
coverage.

Cheers,
Tom

Direct sequence IS the use of psuedo random modulation,http://en.wikipedia.org/wiki/DSSS

I haven't seen that particular HP snooping box, but I saw a similar HP
setup. [Clearly the NSA and governments buy those racks.] The receiver
I saw had GPIB on it. It had a mess of varactor diodes for use in an
agile front end narrow band filter.

Well, it's not HP, it's Agilent. They don't have anything you'd call
a front end narrow band filter, though the N6830 that's mentioned has
a set of very low distortion preselection filters for HF reception.
For example, since the AM MW broadcast band is commonly not very
interesting but has lots of large signals, there's a switchable filter
specifically to kill it by at least 50dB. I suppose anything tuned
with varactors would have way to much distortion to be useful in such
a system.

Cheers,
Tom
.

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