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From: osr on 21 Mar 2010 18:58
Tim, respectfully must disagree a bit.

Sorta, depends on the method of spreading and the bandwidth. Some you
can see quite well in the near field. Or you'll see the Barker code at
startup and sync. Depends very much on the system.

Steve
From: Tim Wescott on 21 Mar 2010 19:27
osr(a)uakron.edu wrote:
> Tim, respectfully must disagree a bit.
>
> Sorta, depends on the method of spreading and the bandwidth. Some you
> can see quite well in the near field. Or you'll see the Barker code at
> startup and sync. Depends very much on the system.
>
> Steve

Well, I sorta agree with your disagreement, but only a bit.

A spread spectrum system that was _trying_ to hide in the FM broadcast
band could probably do so quite effectively. A system that is meant to
be public but happens to have qualities that would hide it (E.g. CDMA
without explicit despreading) would also hide effectively, at least from
a spectrum analyzer (and yes, CDMA isn't frequency hopping -- but still).

Granted, you'd be able to see its spectrum if you're in the near field
-- but that's getting close to my 'hook up to the transmitter' case.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Joel Koltner on 22 Mar 2010 12:49
A small addition to what othes have said:

"Tim Bently" <timbently(a)peromax.com> wrote in message
news:4ba56f04.2648078(a)news.tpg.com.au...
> In technical terms, how does a spectrum analyzer lock onto a signal
> that constantly frequency hops?

(As Tim mentions, it doesn't -- at their cores, spectrum analyzers just
display "what's there," it's only with additional software that they try to be
"smart" after follow hops or whatever.)

> What is the best type of spectrum analyzer for this purpose? Are any
> special options needed?

Tradiational spectrum analyzers literally sweep in frequency, displaying
incoming signal power vs. frequency as they go. There are some tricks you can
use, playing with sweep rates and maximum (input) hold features to let you at
least see the various frequencies in use -- or at least the frequency ranges
in use. However, the better approach (for spread spectrum) to really see
what's going on quickly is to digitize an entire chunk of spectrum over a
relatively brief interval of time (on the order of a small part of the hop
time) and then display the FFTed result on, e.g., a waterfall display. If
that "chunk of spectrum" is small (tens of kHz or less), this is easily done
with pretty much any radio receiver and a PC (you feed its audio output to the
PC's sound card) -- this is often called a "panadapter." If the chunk of
spectrum is large (which is usually the case -- "spread spectrum" being about
"spreading out" the spectrum and all :-) ), you need high bandwidth receivers.
Hence you need a somewhat more specialized receiver ... or if you're still
after a spectrum analyzer as such, a so-called "real time" spectrum analyzer
like these: http://www.tek.com/products/spectrum_analyzers/

(You can find much cheaper units avaialble if you have specific uses in mind,
such as just looking at Bluetooth -- one of the most common FHSS transmitters
people tend to own.)

> Are there any technical guides available online that deal specifcally
> with this type of application?

What are you trying to do? Tek's web site there certainly has various guides
that might help...

---Joel

From: artie on 25 Mar 2010 22:21
In article <4ba56f04.2648078(a)news.tpg.com.au>, Tim Bently
<timbently(a)peromax.com> wrote:

> As part of a university course we are interested in monitoring and
> identifying FHSS signals within the commercial broadcast frequency
> spectrum. Please excuse a few questions.
>
> In technical terms, how does a spectrum analyzer lock onto a signal
> that constantly frequency hops?
>
> Can this still be achieved if the frequency divisions and timing are
> random?
>
> What is the best type of spectrum analyzer for this purpose? Are any
> special options needed?
>
> Are there any technical guides available online that deal specifcally
> with this type of application?
>
> Once again, sorry for the several questions. I don't know how else to
> ask.
>
> Thank you for any advice.
>
> Tim Bently

What's the center of the band, and how wide is the band? You'll need a
wideband receiver with some interesting characteristics.

Google Agilent's Blackbird -- mostly sold to Three Letter Agencies,
very good at finding hoppers.
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