Cecil Moore wrote:
wrote:
The invention of spread spectrum is generally credited to George
Antheil and Hedy Lamarr (yes, the actress) and their patent of 1942.

What would be the difference between very wide band
suppressed carrier FM and spread spectrum?

To greatly simplify, spread spectrum is random frequency hopping to
discreate carrier frequencies and says nothing about the modulation
of the carrier.


--
Jim Pennino

Remove .spam.sux to reply.

wrote:
To greatly simplify, spread spectrum is random frequency hopping to
discreate carrier frequencies and says nothing about the modulation
of the carrier.

But frequency-hopping is only one form of spread spectrum.
How about the direct sequence form, DSSS?
--
73, Cecil http://www.w5dxp.com

Cecil, W5DXP wrote:
"But frequency hopping is only one form of spread spectrum."

Yes. No more than two frequencies are required to switch between, though
the transition produces more frequencies than the originals. Carson`s
rule is a close approximation of the required bandwidth and is used by
the FCC to determine bandwidth:
BW= 2(Peak Deviation + Highest Mod. Freq.)

FSK or frequency shift keying uses only two frequencies to represent
ones and zeros. Switching produces FM and is a form of spread spectrum
transmission defined as:
A communications technique in which many different waveforms are
transmitted in a wide band. Power is spread thinly over the band so
narrow-band radios can operate within the wide band without
interference. FSK is often done within the audio frequency band with no
radio necessarily used in transmission.

Best regards, Richard Harrison, KB5WZI

Roy Lewallen wrote:
"You really should try ro understand the context of the various
quotations from Terman."

I do.

I did not give the quotation from Terman, only its gist. Here it is:
"A useful rule is that a frequency modulated wave contains sideband
components of importance on either side of the carrier wave over a
frequency interval approximating the sum of the frequency deviation and
the modulating frequency. The total bandwidth in which most of the wave
is contained is then twice this value."

The original posting in this thread read:
"Most cell phones and wireless routers, modems, and access points that
use spread spectrum usually broadcast and receive their data on FM-radio
waves. Just out of curiosity, I ask why not use AM?"

Nothing about narrow-band FM in the question and I responded:

"---losing insensitivity to carrier level when recovering modulation."

I did not take Terman out of context to distort his meaning. Ternan was
merely restating Carson`s rule which is the same approximation the FCC
uses to determine bandwidth. I quoted this in my May 2, 3:28 pm posting.
I`ve used this to calculate bandwidth many times with no citations from
the FCC yet.

I make plenty of mistakes but I see only one I`ve made in my postings in
this thread and that is an instance where I typed FM when I meant AM and
I immediately corrected it in the next posting.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Roy Lewallen wrote:
"You really should try ro understand the context of the various
quotations from Terman."

I do.

I did not give the quotation from Terman, only its gist. Here it is:
"A useful rule is that a frequency modulated wave contains sideband
components of importance on either side of the carrier wave over a
frequency interval approximating the sum of the frequency deviation and
the modulating frequency. The total bandwidth in which most of the wave
is contained is then twice this value."

The original posting in this thread read:
"Most cell phones and wireless routers, modems, and access points that
use spread spectrum usually broadcast and receive their data on FM-radio
waves. Just out of curiosity, I ask why not use AM?"

Nothing about narrow-band FM in the question and I responded:

"---losing insensitivity to carrier level when recovering modulation."

I did not take Terman out of context to distort his meaning. Ternan was
merely restating Carson`s rule which is the same approximation the FCC
uses to determine bandwidth. I quoted this in my May 2, 3:28 pm posting.
I`ve used this to calculate bandwidth many times with no citations from
the FCC yet.

I make plenty of mistakes but I see only one I`ve made in my postings in
this thread and that is an instance where I typed FM when I meant AM and
I immediately corrected it in the next posting.

Best regards, Richard Harrison, KB5WZI

How did you conclude from this statement in Terman that the bandwidth
required by FM is twice that required by AM?

Roy Lewallen, W7EL

Roy Lewallen wrote:
"How did you conclude from this statement in Terman that the bandwidth
required by FM is twice that required by AM?"

Easy, I turn up the peak deviation intil twice the bandwidth is occupied
with FM as would be required for AM. It is arbitrary and depends on peak
deviation. If you make the deviation less than half the modulating
frequency, the bandwidth occupied will be about the same as required for
AM.

Best regards, Richard Harrison, KB5WZI

In article ,
wrote:

What would be the difference between very wide band
suppressed carrier FM and spread spectrum?

To greatly simplify, spread spectrum is random frequency hopping to
discreate carrier frequencies and says nothing about the modulation
of the carrier.

That is one specific form of spread-spectrum transmission (FHSS or
frequency-hopping). This form tends to be used a lot in lower-cost
consumer electronic gear (older-generation cordless phones, for
example).

There are other forms of spread spectrum in common use, and in these
forms the spreading is tied to the modulation scheme to at least some
extent. Two examples:

- Direct-sequence spread spectrum (DSSS). A digital bitstream is
XORed with a high-speed pseudorandom "spreading sequence", and the
carrier is modulated by the resulting high-speed bitstream. The
basic carrier frequency going into the modulator doesnt't change
(it's fixed on a per-channel basis) but the energy is spread out
as a very broad set of sidebands. 802.11b WLAN uses DSSS.

- Orthogonal frequency-division multiplexing (OFDM), which uses a large
number of individual carriers, typically spaced at regular
intervals, each modulated at a relatively low rate with some
portion of the information being sent. Here, too, the frequency of
each individual carrier tends to be fixed. 802.11g, DMT (discrete
multitone) DSL modems, and the venerable Telebit Trailblazer
phone-line modem use OFDM.

Very-wide-band FM (i.e. FM with a high modulation index) carries
little of its energy at the carrier frequency - most of it is in the
regularly-spaced sidebands, located at offsets from the carrier which
are multiples of the modulating frequency/frequencies. In this sense,
its spectrum use would be not dissimilar to that of an FDM spread
spectrum (although it would generally not be OFDM because the
sidebands aren't spaced in an orthogonal manner).

--
Dave Platt 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!

Dave Platt wrote:
In article ,
wrote:

What would be the difference between very wide band
suppressed carrier FM and spread spectrum?

To greatly simplify, spread spectrum is random frequency hopping to
discreate carrier frequencies and says nothing about the modulation
of the carrier.

That is one specific form of spread-spectrum transmission (FHSS or
frequency-hopping). This form tends to be used a lot in lower-cost
consumer electronic gear (older-generation cordless phones, for
example).

There are other forms of spread spectrum in common use, and in these
forms the spreading is tied to the modulation scheme to at least some
extent. Two examples:

- Direct-sequence spread spectrum (DSSS). A digital bitstream is
XORed with a high-speed pseudorandom "spreading sequence", and the
carrier is modulated by the resulting high-speed bitstream. The
basic carrier frequency going into the modulator doesnt't change
(it's fixed on a per-channel basis) but the energy is spread out
as a very broad set of sidebands. 802.11b WLAN uses DSSS.

- Orthogonal frequency-division multiplexing (OFDM), which uses a large
number of individual carriers, typically spaced at regular
intervals, each modulated at a relatively low rate with some
portion of the information being sent. Here, too, the frequency of
each individual carrier tends to be fixed. 802.11g, DMT (discrete
multitone) DSL modems, and the venerable Telebit Trailblazer
phone-line modem use OFDM.

Very-wide-band FM (i.e. FM with a high modulation index) carries
little of its energy at the carrier frequency - most of it is in the
regularly-spaced sidebands, located at offsets from the carrier which
are multiples of the modulating frequency/frequencies. In this sense,
its spectrum use would be not dissimilar to that of an FDM spread
spectrum (although it would generally not be OFDM because the
sidebands aren't spaced in an orthogonal manner).

What part of "To greatly simplify" are you having problems understanding?


--
Jim Pennino

Remove .spam.sux to reply.

wrote:
What part of "To greatly simplify" are you having problems understanding?

"Everything should be made as simple as it is,
but not simpler." Albert Einstein.
--
73, Cecil http://www.w5dxp.com