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From: Jeff Liebermann on 22 Jan 2007 19:24
On 22 Jan 2007 12:14:21 -0800, miso(a)sushi.com wrote:

>I don't know if there are RF spectrum
>analyzers with such intelligence, but I don't see why not.

There are plenty that have enough intelligence to grind out true
power. I can't afford any of these. I can't even afford one with
obsolete IEEE488 bus.

>In datacom (again, back to voice band communications), some modems used
>guard tones to indicate the line contained data, not voice. The CCITT
>specification is based on energy, so making such measurements (energy
>in a bandwidth) is a desirable feature. That is, the tone had to be a
>certain value relative to the data energy, which was spread out.

Sorta. *ALL* modems 1200 baud and up include a scrambler. What the
scrambler does is evenly distribute the energy across the entire
bandwidth, thus reducing the peak power, and thus making it easier to
transmit. The required dynamic range is also reduced. OFDM and DMT
DSL modems do the same thing by splitting the data up into many
discrete carriers. The total energy in the bandpass is the same
whether scrambled or de-scrambled, but the peak power is quite
different.

However, this has nothing to do with what I was mumbling about. I was
questioning the original comment that one could just read the tx power
from a spectrum analyzer. With spread spectrum, it's not that easy.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl(a)comix.santa-cruz.ca.us
# http://802.11junk.com jeffl(a)cruzio.com
# http://www.LearnByDestroying.com AE6KS
From: miso on 22 Jan 2007 19:50

Jeff Liebermann wrote:
> On 22 Jan 2007 12:14:21 -0800, miso(a)sushi.com wrote:
>
> >I don't know if there are RF spectrum
> >analyzers with such intelligence, but I don't see why not.
>
> There are plenty that have enough intelligence to grind out true
> power. I can't afford any of these. I can't even afford one with
> obsolete IEEE488 bus.
>
> >In datacom (again, back to voice band communications), some modems used
> >guard tones to indicate the line contained data, not voice. The CCITT
> >specification is based on energy, so making such measurements (energy
> >in a bandwidth) is a desirable feature. That is, the tone had to be a
> >certain value relative to the data energy, which was spread out.
>
> Sorta. *ALL* modems 1200 baud and up include a scrambler. What the
> scrambler does is evenly distribute the energy across the entire
> bandwidth, thus reducing the peak power, and thus making it easier to
> transmit. The required dynamic range is also reduced. OFDM and DMT
> DSL modems do the same thing by splitting the data up into many
> discrete carriers. The total energy in the bandpass is the same
> whether scrambled or de-scrambled, but the peak power is quite
> different.

Without the scrambler, it is possible to create data sequences that you
can't "clock recover."

>
> However, this has nothing to do with what I was mumbling about. I was
> questioning the original comment that one could just read the tx power
> from a spectrum analyzer. With spread spectrum, it's not that easy.

Hey, at least you have an analyzer that goes out to wifi bandwidth. ;-)
RF gear really holds it's value, more than audio test equipment.


>
> --
> # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
> # 831-336-2558 jeffl(a)comix.santa-cruz.ca.us
> # http://802.11junk.com jeffl(a)cruzio.com
> # http://www.LearnByDestroying.com AE6KS

From: Jeff Liebermann on 22 Jan 2007 20:29
On 22 Jan 2007 16:50:37 -0800, miso(a)sushi.com wrote:

>Without the scrambler, it is possible to create data sequences that you
>can't "clock recover."

Well, yeah. It does depend on the coding protocol and whether the
clock signal and synchronization is encoded into the data. It's kinda
easy with 802.11, where the preamble takes care of any sychronization
issues.

>> However, this has nothing to do with what I was mumbling about. I was
>> questioning the original comment that one could just read the tx power
>> from a spectrum analyzer. With spread spectrum, it's not that easy.

>Hey, at least you have an analyzer that goes out to wifi bandwidth. ;-)
>RF gear really holds it's value, more than audio test equipment.

Wrong. My pile of test equipment mostly quits at 1GHz. I use down
converters (mixers) to deal with anything on 2.4 and 5.7Ghz. I am
borrowing a Wiltron 610C with 6215D 1 to 4GHz plugin and VSWR bridge
which is VERY useful for antennas. As I recall, I recommended that
you look into one of these for antenna testing. However, I have to
return it to the owner next week. Grumble. Mostly, I borrow what I
need.

RF test iron does hold it's value better than audio, but only because
it requires more expensive calibration and repair exercises. If it's
working, it's quite valueable. If it's non-functional, it total
garbage. Also, lots of analog audio test equipment is being obsoleted
by DSP based digital processing test equipment.

At this time, I have more test equipment to repair, than is
functional. I also have 2 or more of everything as I have the bad
habit of buying two or more junk units, and conglomerating them into
one that works. It's often cheaper to buy a complete unit for parts,
than to try to find the individual parts.


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl(a)comix.santa-cruz.ca.us
# http://802.11junk.com jeffl(a)cruzio.com
# http://www.LearnByDestroying.com AE6KS
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