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Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
Green Xenon wrote:
"So I ask what would be the disadvantage of using AM instead of FM for this?" Short answer is: losing insensitivity to carrier level when recovering the modulation. Advantage of AM is reduced bandwidth and simpler demodulation. Advantage of FM is that once a certain minimum carrier level is applied to the detector. the demodulated signal is independent of the carrier level. A balanced detector allows non-linear RF amplification, insensitivity to amplitude variations, and no response to noise which is amplitude (AM) in nature. Disadvantage of FM is that it requires about twice the bandwidth of FM. Best regards, Richard Harrison, KB5WZI |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
I miswrote. FM requires about twice the bandwidth of AM.
Best regards, Richard Harrison, KB5WZI |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
Richard Harrison wrote:
I miswrote. FM requires about twice the bandwidth of AM. Quoting my 1957 ARRL Handbook: "Narrow-band f.m. or p.m., the only type that is authorized for use on the lower frequencies where the phone bands are crowded, is defined as f.m. or p.m. that does not occupy a wider channel than an a.m. signal having the same audio modulating frequencies. Narrow-band operation requires using a relatively small modulation index." It goes on to say a modulation index of 0.6 is about optimum for those conditions. Is NBFM still allowed? -- 73, Cecil http://www.w5dxp.com |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
In article ,
Cecil Moore wrote: Quoting my 1957 ARRL Handbook: "Narrow-band f.m. or p.m., the only type that is authorized for use on the lower frequencies where the phone bands are crowded, is defined as f.m. or p.m. that does not occupy a wider channel than an a.m. signal having the same audio modulating frequencies. Narrow-band operation requires using a relatively small modulation index." It goes on to say a modulation index of 0.6 is about optimum for those conditions. Is NBFM still allowed? I read through the current Part 97 regs a few months ago, and concluded that it is. See 97.305 (Authorized Emission Types), and 97.307 (f) (1). The current standard allows a modulation index of up to 1, at the highest modulating frequency. This applies to all angle-modulation modes (PM and FM). The rules aren't specifically worded as "f.m. or p.m. that does not occupy a wider channel than an a.m. signal having the same audio modulating frequencies" these days - the restrictions are in terms of the modulation index, and the general requirement that one not use "more bandwidth than necessary for the information rate and emission type being transmitted, in accordance with good amateur practice." I don't recall having run across any narrowband FM on the HF bands... I think there are still a few 10-meter repeaters operating around the country somewhere, but FM HF simplex seems to be either completely dead, or used just by occasional experimenters. -- 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! |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
I don't recall having run across any narrowband FM on the HF bands...
I think there are still a few 10-meter repeaters operating around the country somewhere, but FM HF simplex seems to be either completely dead, or used just by occasional experimenters. ==================================== When the 10m band opens , here in Europe FM is quite common between 29.5 and 29.7 MHz . It is sometimes even possible to open the Boston Mass. repeater on 29.600 MHz. Frank GM0CSZ / KN6WH |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
Cecil wrote:
"Is NBFM still allowed?" Interesting question but I was responding to the question in the thread tille. FM usually occupies about twice the bandwidth of AM. Spread spectrum ordinarily occupies more bandwidth. Spread spectrum was patented by Nikola Tesla in 1903. FCC now codifies rules for spread spectrum in Wi-Fi, Bluetooth, cordless phones, etc. Terman says about twice the bandwidth is required for FM as compared with AM on pages 589 and page 590 of his 1955 opus. The signal for spread spectrum is a form of FM but usually the signal is transmitted on a bandwidth considerably larger than the frequency content of the original information says Wikipedia. Best regards, Richard Harrison, KB5WZI |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
Frank, GM0CZ / KN6WH wrote:
"It is sometimes even possible to open the Boston, Mass. repeater on 29.600 MHz." FM is almost essential to repeater operation due to the inherent automatic gain control of the FM mode. Once the weak signal point called the "FM improvement threshold" is exceeded, further carrier strength increases have no effect on the recovered modulation. The pip squeak is as loud as the power house. Remodulate that on the repeater output. Best regards, Richard Harrison, KB5WZI |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
Richard Harrison wrote:
Cecil wrote: "Is NBFM still allowed?" Interesting question but I was responding to the question in the thread tille. FM usually occupies about twice the bandwidth of AM. Spread spectrum ordinarily occupies more bandwidth. Spread spectrum was patented by Nikola Tesla in 1903. FCC now codifies rules for spread spectrum in Wi-Fi, Bluetooth, cordless phones, etc. Terman says about twice the bandwidth is required for FM as compared with AM on pages 589 and page 590 of his 1955 opus. The signal for spread spectrum is a form of FM but usually the signal is transmitted on a bandwidth considerably larger than the frequency content of the original information says Wikipedia. Best regards, Richard Harrison, KB5WZI The invention of spread spectrum is generally credited to George Antheil and Hedy Lamarr (yes, the actress) and their patent of 1942. -- Jim Pennino Remove .spam.sux to reply. |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
Jim Pennino wrote:
"The invention of spread spectrum is generally credited to George Antheil and Hedy Lamarr (yes the actress) and their patent of 1942." Yes, I believe Lamarr and her husband encoded several tones in sequence using a player-piano roll. The system was used in WW-2. Poor old Tesla only invented the a.c. power system, wireless power transmission, and demonstrated his remote control of a boat using spread spectrum to avoid interference more than 40 years before Hedy Lamarr`s patent. T.A. Edison did his best to deny Tesla credit for anything. That`s commerce. Best regards, Richard Harrison, KB5WZI |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
Richard Harrison wrote:
FM usually occupies about twice the bandwidth of AM. Thanks for adding "usually" to your original statement. I was just pointing out that if the FM peak deviation is equal to the maximum modulation frequency, then the FM signal occupies the same bandwidth as AM. The S/N ratio advantage usually enjoyed by FM over AM occurs when the FM peak deviation is *greater than* the maximum modulation frequency. FM seems to have been the original "spread spectrum" mode. :-) -- 73, Cecil http://www.w5dxp.com |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
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Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
Richard Harrison wrote:
Jim Pennino wrote: "The invention of spread spectrum is generally credited to George Antheil and Hedy Lamarr (yes the actress) and their patent of 1942." Yes, I believe Lamarr and her husband encoded several tones in sequence using a player-piano roll. The system was used in WW-2. Poor old Tesla only invented the a.c. power system, wireless power transmission, and demonstrated his remote control of a boat using spread spectrum to avoid interference more than 40 years before Hedy Lamarr`s patent. T.A. Edison did his best to deny Tesla credit for anything. That`s commerce. Best regards, Richard Harrison, KB5WZI George Antheil and Hedy Lamarr were never married. Antheil was a neighbor. The didn't encode "serveral tones in sequence on a player-piano roll", they used a piano roll to produce frequency hopping between 88 different frequencies with the intention of making radio guides torpedoes hard to detect or jam. The first practical use (after technology caught up with the concept) was by the Navy during a blockade of Cuba in 1962 based on the work started in 1957 by Sylvania Electronic Systems. While Tesla and several Europeans had their fingers in various frequency hopping schemes, the general consensus is that Antheil and Lamarr were the first originators of the basis for spread spectrum radio systems as used today. Both Antheil and Lamarr were given a special award in 1997 in recognition of that fact. Other than that, you mostly got it right. -- Jim Pennino Remove .spam.sux to reply. |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
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. |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
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Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
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! |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
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 |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
In article ,
Cecil Moore wrote: FM usually occupies about twice the bandwidth of AM. Thanks for adding "usually" to your original statement. I was just pointing out that if the FM peak deviation is equal to the maximum modulation frequency, then the FM signal occupies the same bandwidth as AM. I believe that you're mistaken on this point, Cecil. As I understand it, the spectrum of a frequency-modulated carrier will have energy at the carrier frequency (except at certain very specific modulation indices), and energy at offsets from the carrier which are equal to the modulating frequency and all of its harmonics. It's entirely possible (and in fact common) for an FM signal to have energy at frequencies which are further away from the nominal carrier frequency, than the maximum instantaneous deviation of the carrier would suggest. For example, a carrier which is modulated with a 1000 Hz tone, but only to the level of having a maximum peak deviation of 500 Hz from nominal, will still have sideband energy out 1000 Hz and 2000 Hz away from the nominal carrier frequency. This is *really* counter-intuitive, and I haven't been able to fully wrap my brain around the question of just how the math works... but the math says that it's true, and my own spectrum measurements show that it's true. An FCC-approved NBFM phone signal, which has a modulation index of no more than 1.0 at the highest modulating frequency, *will* have energy out further than the peak deviation would suggest. There will be some amount of sideband energy located out at twice the highest modulating frequency, and a bit at three times. The levels of these further-away-from-nominal-carrier sidebands will be relatively low - they don't start to become appreciable until you get to a higher modulation index. An AM signal being modulated by the same intelligence signal would not have any energy out at the multiples, *if* it was generated and transmitted in an entirely-distortion-free manner. My guess is that in practice, NBMF (per FCC regs), and ham-grade AM, probably have very similar bandwidths. All it would take for the AM signal to be spread out as far as the FM signal, would be a bit of nonlinear distortion in the audio chain, mixer, or amplifier chain... this will create a second-harmonic sideband. I did an introductory presentation to a local hamclub last year (with some math, admittedly simplified and approximated and covered up with moderate amounts of hand-waving) which explains some of these concepts. I included some plots of actual measured spectra, generated by an HP signal generator and captured using an old Systron Donner spectrum analyzer. http://www.radagast.org/~dplatt/hamr...modulation.pdf -- 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! |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
Dave Platt wrote:
In article , Cecil Moore wrote: FM usually occupies about twice the bandwidth of AM. Thanks for adding "usually" to your original statement. I was just pointing out that if the FM peak deviation is equal to the maximum modulation frequency, then the FM signal occupies the same bandwidth as AM. I believe that you're mistaken on this point, Cecil. Yep, you're right. A modulation index of about 0.6 yields approximately the same bandwidth for AM and FM. -- 73, Cecil http://www.w5dxp.com |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
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. |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
Richard Harrison wrote:
. . . Terman says about twice the bandwidth is required for FM as compared with AM on pages 589 and page 590 of his 1955 opus. . . . You really should try to understand the context of the various quotations from Terman. In his _Radio Engineering_, Third Edition (1947), he points out that "When the modulation index is less than 0.5, i.e., when the frequency deviation is less than half the modulating frequency, the second and higher order side-band components are relatively small, and the frequency band required to accommodate the essential part part is the same as in amplitude modulation." This is, of course, what is considered to be narrow band FM. Unlike an AM signal with its one pair of sidebands containing replicas of the modulating signal, any FM signal contains an infinite number of pairs of sidebands. However, as Terman and any other communications text points out, the relative strengths of some of those sidebands can be made to be very small by the choice of modulation index. In the case of NBFM, all but the first pair are small. That first pair are spaced the same distance from the carrier as AM sidebands, so the bandwidth is essentially the same as for AM. You can, of course, increase the modulation index which increases the bandwidth by increasing the amplitudes of higher order sideband pairs, making wideband FM. The advantage of doing this is that you can improve the signal/noise ratio of the received signal as a trade for the increased bandwidth. So FM can be as narrow in bandwidth as AM, or any greater bandwidth, all depending on the modulation index. Saying that "twice the bandwidth is required for FM as compared with AM" is simply not a correct statement and, if said by Terman, was taken out of context which surely qualified it. Roy Lewallen, W7EL |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
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Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
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 |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
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 |
Disadvantages of using AM for DSSS/FHSS Spread Spectrum?
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 |
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