Radio Modulation why SSB and not DSB Question - Page 1
Forum section: electronics / beginners

author: C
author_rank: Super Contributor
timestamp: January 24, 2014, 04:34:08 pm
content: I see the reasons for the change from AM to SSB as the following list. What i am missing is why there is not more DSB these days.From AM to SSB1. AM 10k band width: Two sidebands same information but inverted in freq, High Power carrier.2. DSB 10k band width: Two sidebands same information but inverted in freq, High Power carrier removed or suppressed. For same power level more power can be put in sidebands.3. SSB 5k band width: One sideband. For same power level more power can be put in One sideband. Also a saving of 5k in band width, so 2X user increase passable.Now SSB is hard to do with getting the carrier back. Done wrong and you have a frequency shift in the information from what was transmitted.Now back at step 2 there is redundant information due to the two sidebands. So noise in one sideband should not have as much of an effect.For step 2Dual IF stripsYou could process each sideband separately but this does take more electronics.Second frequency inverted copy.What if you just made a second copy of both sidebands with one copy inverted in frequency with respect to the other. As you then tune the receiver there will be a point where the information of the two copies will add while what is in only one sideband is canceled or at just the 1X level. Is something I am not seeing preventing this? I see this add part also enabling an AFC circuit for fine tunning.First thinking would say that DSB would require 2X the bandwidth and there fore 2X less users. But what if you have a second transmitter on the same Frequency but shifted in frequency a bit? For easy numbers lets have both transmitters transmit a 1K tone. At the same frequency the 1k tones in the sidebands would be on top of each other from the two transmitters. But as you shifted the second transmitter in frequency the two would no longer be on top of each other. I see the second transmitter now being increased noise if you are trying to receive the first transmitter due to the 2X add of the two copies when tuned on frequency.So while first thoughts are DSB is 2X bandwidth and 2X less users, if the frequency inverted copy works it could allow many more users and|or more information in the same bandwidth of a transmitter .The Question is what am I missing that makes this hard to do?If it's not hard to do, it is probably in use someplace, Where?C

author: w2aew
author_rank: Super Contributor
timestamp: January 24, 2014, 04:50:42 pm
content: I think the thing you're missing is that in order to achieve this "signal addition" or "signal subtraction/cancellation", the signals much be matched in phase or exactly 180 degrees out of phase, respectively. That would not happen in the scheme you propose. (adding two signals that are frequency inverted from each other does NOT cancel them out).There IS a method of SSB generation that doesn't use filtering called the phasing method. In this method, a pair of DSB signals are generated using quadrature LOs and phase shifting the baseband. These two DSB signals can then be added to create one sideband and cancel the other. Subtracting them does the opposite and generates the other sideband.The benefits of SSB over DSB are really two fold in terms of effectiveness. All of the power is put into one sideband instead of split over two sidebands. Plus, on the receiving end, the receiver can have half the BW, thus half the noise - all this benefits SNR in the receiver and makes it easier to recover weak signals.

author: C
author_rank: Super Contributor
timestamp: January 24, 2014, 07:21:36 pm
content: I was thinking at the transmitter just a balanced mixer and transmit that.You could think of this as AM with a reduced carrier or no carrier. Just DSB here as you leave out the filter to remove a sideband for SSB.Receiver side receives this and feeds it to to two balanced mixers. one Lo lower, one Lo higher. Therefore the two outputs are Frequency inverted.So at the transmitter you have two sidebands with the same information that are inverted in frequency. The receiver with the two mixers makes this 4 sidebands. So at IF a 2X add should be possible and when going to baseband a second 2X add.You then have a 4x Add for the selected while noise or even an additional transmitter at a very small frequency difference would be closer to 1X.If I am following what you are saying then the Balanced mixers would have to destroy the phase relationship between the Upper & Lower sidebands.Or the two different LO's frequencies would have to change the phase when the outputs of the two mixers is the same frequency.Now you may be adding in filters when not needed and I can see these adding problems. But here signal selection is due to the frequency inversion and the addition of the two signals. Granted that the two LO's would have to have the proper frequency difference from the received signal.Now at extreme range DSB should be worse then SSB, but even here there should be some add out of the noise due to the inverted frequency add.So if you could, could you tell me where is the phase getting clobbered with the above?Thinking more on this I guess there could be a difference between output of the transmitter mixer and the path to the input of the two receiver mixers but even here I can see a way to cancel this.So sorry still not seeing problem areaCC

author: w2aew
author_rank: Super Contributor
timestamp: January 24, 2014, 08:18:55 pm
content: I guess I really don't follow what it is you are trying to achieve. By transmitting DSB-SC (double sideband, suppressed carrier), you've already killed one of the advantages of SSB - you're occupying (slightly more than) double the BW required for just a single sideband - limiting the available space for other users. Putting together a receiver with dual/offset IF paths in order to achieve some sort of sideband cancellation sounds a lot more complicated, with little/no advantage, compared to a traditional single IF chain and product detector.I guess I'm just not seeing your vision...

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 24, 2014, 08:30:27 pm
content: I don't see it either. SSB vs DSB has 3 benefits:- Put all the power in one sideband where all the information you need is, 3dB gain.- Half the bandwidth so less noise and interference on the receiving side.- Half the bandwidth so more space for other stations.What you are trying to do.... ehm I have no idea.

author: AG6QR
author_rank: Frequent Contributor
timestamp: January 24, 2014, 08:48:38 pm
content: Quote from: C on January 24, 2014, 04:34:08 pmTrue in theory, but in practice it's really not that hard to match frequencies, with modern crystal controlled oscillators and phase locked loops.The vast majority of the time, when I hear an SSB signal on the ham bands, I just tune to the nearest whole kilohertz and everything is fine. Sometimes people use frequencies that are 500 hertz away from a whole kilohertz step -- that's easy enough to recognize and handle, as well. I listen to and occasionally participate in several SSB nets on the ham bands, and if I simply set my frequency dial to the published net frequency, that's always enough to hear good, intelligible speech from everyone who is transmitting.Occasionally I have heard people on odd frequencies that aren't right on a multiple of a 500 Hz. I assume they're using older radios, either without a crystal oscillator, or with one that has drifted. I can always understand them, but they sound a bit better if I tweak the frequency a bit. That's easy enough to do.I have used old Hallicrafters vacuum tube receivers with fully analog tuning and wildly drifting oscillators. Even without an accurate frequency scale, it's still easy to tune them by ear to receive fully intelligible speech via SSB.It helps that SSB is normally used for voice only. Speech can tolerate a minor frequency shift without losing intelligibility.DSB offers half the power efficiency, half the signal to noise ratio, and uses twice the radio frequency spectrum, to solve a "problem" that doesn't exist. That's why it's not in common use.

author: C
author_rank: Super Contributor
timestamp: January 24, 2014, 09:17:56 pm
content: DSB has redundant information due to the two sidebands.Granted a waste for the idea of SSB and power, but this could be a great benefit from what I see.For ease say current SSB has a 5k bandwidth. So in 20k you could have 4 conversions. Try to add more and you fail.First thoughts you would think DSB at 10k would just allow 2 conversions in 20kBut if you can cancel out the off frequency conversions could you not add a third conversion at an offset of 5k from the first two. And two more conversions with a shift of 2.5k.You now have 5 conversions in 20k bandwidth and could possibility do more and all five are still transmitting a 5k information band. The transmitter output filtering would not need to be very good. To fit in the 5k bw SSB needs great filters to prevent effecting the adjacent channel.As you said in your first, Phase must match to get get the two sidebands to add. At the theory level even a few Hz would allow a second conversation to happen.Going a bit further, some ham radios now convert the IF to digital. The question then becomes, is the digital processing of a IF good enough to do this just in the digital domain.With a ham's SSB bandwidth this would not work for voice directly but what about a bunch of DSB signals with a small frequency shift between each one?Seeing the vision now?C

author: Nerull
author_rank: Frequent Contributor
timestamp: January 24, 2014, 09:34:54 pm
content: SSB is normally used by HAMs when range is more important than anything. Since you're tossing out the main benefit of SSB - more efficent use of transmitter power - I really don't see why you would use this over FM.

author: w2aew
author_rank: Super Contributor
timestamp: January 24, 2014, 09:43:21 pm
content: Quote from: C on January 24, 2014, 09:17:56 pmNo, still not seeing it. Any overlapping of unrelated signals will result in interference for SSB phone communications. The only time this intentional overlapping works without inteference is when there's something deterministic about the transmissions that the receiver can coherently lock onto. This is common in digital modulation schemes where long spreading codes can be used. Or, if the individual signals can be made orthogonal, such as in OFDM transmissions, then multiple signals (carriers) can be individually detected.In the case of phone SSB or DSB transmissions - I really fail to see the logic where transmitting DSB allows more spectrum usability, even if the redundant sideband can be used for some magical cancellation of a portion of the received spectrum. Because these signal are not deterministic - overlapping signals WILL cause intereference, and I still fail to see how to active cancellation of the unwanted sideband and unwanted signals can be done in any way that is less complex or easier than using traditional SSB via filtering or phasing methods.

author: AG6QR
author_rank: Frequent Contributor
timestamp: January 24, 2014, 09:59:50 pm
content: Quote from: C on January 24, 2014, 09:17:56 pmYou can't just cancel out the off-frequency signals, unless you know in advance what they are.As a simplified, but concrete example: Suppose my receiver detects equal amplitude sine waves at 7200.000 kHz, 7200.500 kHz, 7201.100 kHz, and 7201.600 kHz. And pretend it detects nothing else. What audio should it demodulate?Is it coming from one transmission of 250Hz centered at 7200.250, plus another transmission of 250Hz centered at 7201.350 ?Or is it coming from a transmission of two tones, 800Hz and 300Hz, centered at 7200.800 kHz?Or was it coming from an SSB transmission of four tones, separated by 500, 600, and 500 Hz? Or was it a DSB transmission of similar tones, where the opposite sideband was outside of the receiver passband? Or was it four distinct CW tones?Or maybe the 7200.000kHz and 7201.600kHz represented a tone of 800 Hz centered at 7200.8 kHz, while the 7200.5 kHz and 7201.1 kHz were part of another transmission centered somewhere else, where the other sideband was either suppressed or outside the receiver passband?How will your radio decide?That particular pure tone example wasn't necessarily very realistic, but when it gets a mush of a whole lot of different frequencies simultaneously, what algorithm does it use to reject those that came from a transmission it wasn't interested in?In reality, the receiver will be detecting lots of noise, and frequencies won't be discrete sine waves. The two sidebands won't be equal in amplitiude. In normal AM broadcast (two sidebands plus carrier), the two sidebands rarely arrive equally well. That's why synchronous detection, like the Sony ICF-2010 popularized, is so nice to have -- you can listen to the audio derived from the clean sideband, while suppressing the noisy sideband.

author: w2aew
author_rank: Super Contributor
timestamp: January 24, 2014, 10:34:18 pm
content: AG6QR - good example.

author: C
author_rank: Super Contributor
timestamp: January 25, 2014, 12:07:26 am
content: Hi AG6QRYes I know what you are saying about tunning.In the 70's I had access to a radio by Collins that cost back then in the 10k range that you would probably love to have on you bench even today.Quotebear in mind my math is bad so could be errors.so two IF's one inverted with respect to each outerIF one: a simple Frequency shift of the following7200.000 kHz, 7200.500 kHz, 7201.100 kHz, 7201.600 kHzIF two a simple frequency inversion and shift7201.600 kHz, 7201.100 kHz, 7200.500 kHz, 7200.000 kHz,Now notice 7200.000 is aligned with 7201.600 at both ends of the listRadio is tuned to 7200.800 kHzpossible 800 & 300 Hz tonesRadio is tuned to 7200.550 kHzpossible 550 Hz toneRadio is tuned to 7200.250 kHzpossible 250 Hz toneRadio is tuned to 7201.350 kHzpossible 250 Hz toneRadio is tuned to 7201.050 kHzpossible 550 Hz toneIf my math is good the above would be possible but remember the two sideband tones would have to be in phase for them to add to give the above tone(s)The other signals at the radio setting above would not add.If you can do a sum you can also do a difference. And the difference could be used to remove some of noise.Even a minor error in tunning would make the inverted copies of it's self not align in frequency.This may be the problem with this idea as you would need really precise tunning.I know more bandwidth used per user sounds the wrong way to go for more users in a band and longer distance.The simple thing is it does not take much to make SSB or FM unusable and there is problems for both at the reduced overlapped spacing I am taking of here.At the extreme range I can see the possibility of the two signals with one inverted with respect to the other and adding them pulling out the signal from the noise due to the redundant information in the signal.Spread spectrum uses more bandwidth to allow more security and more users in a band.I just have not heard of this version being used, DSB with less than bandwidth spacing for channels.Would not be that hard to do from what I see in an analog radio.C

author: atw60444
author_rank: Contributor
timestamp: January 25, 2014, 12:33:29 am
content: Quote from: AG6QR on January 24, 2014, 08:48:38 pmThat's a strange concept to me. I always tune SSB signals for the most natural sounding voice. I don't really take much notice of the actual frequency displayed.But I did start out in the days when running VFO's were much more common 1981. Granted I should think most recent transceivers have very good frequency stability and accuracy. And for nets it's best to leave your dial in place and just tune the receiver incremental tuning when someone is uncomfortably off frequency.de G4VFT

author: AG6QR
author_rank: Frequent Contributor
timestamp: January 25, 2014, 12:51:46 am
content: Quote from: C on January 25, 2014, 12:07:26 amBut consider this: The transmitted RF is the only thing between the transmitter and the receiver. Those four transmitted RF signals are at different frequencies. Different frequency waves can never remain in phase with one another, nor can they remain out of phase. They will continuously alter their phase relationship at a rate equal to the difference between the frequencies.Quote from: C on January 25, 2014, 12:07:26 amI thought the initial motivation for this was to eliminate the tuning requirement of SSB (which isn't much of a problem to begin with).Backing up a bit, study the information theory works of Claude Shannon. Basically, to transmit an audio signal of, say, 3 kHz bandwidth, you must use 3 kHz bandwidth in the transmission medium. You can't squeeze two high bandwidth signals into a narrow or overlapping bandwidth section of your transmission medium and expect to separate them at the other end. Regardless of what scheme you use.You can compress the audio to use less bandwidth, but you'll lose some information by doing so.

author: AG6QR
author_rank: Frequent Contributor
timestamp: January 25, 2014, 01:17:27 am
content: Quote from: atw60444 on January 25, 2014, 12:33:29 amThere is absolutely nothing wrong with your method. It's what I used to do (before digital displays), and it still works very well. It's the only method that works well if people are at somewhat random frequencies.But I've found that, in practice these days, I'm nearly always tuning to an even kilohertz step, or at least very close. For me, it's a bit of a shortcut to just start out on the exact kilohertz step, and only deviate from it if the audio doesn't sound quite right.73

author: G0HZU
author_rank: Super Contributor
timestamp: January 25, 2014, 02:20:09 am
content: QuoteAre you guys sure about this? W2AEW, AG6QR and PA0PBZ appear to be making SSB appear a lot better than it really is?It's late and I'm tired and I've not looked at DSBSC vs SSB for a few years (and I rarely use either of them) but I always thought the S/N ratio after the receiver demodulation was the SAME for SSB vs DSBSC if the transmitter has the SAME average transmit power. This assumes both systems are demodulated correctly.However, the transmitter typically needs to cope with twice the PEP for DSBSC but I don't see where these various 3dB system gains come from?For me the main benefit of SSB is reduced channel BW (half the bandwidth so twice as many stations allowed per band) and reduced distortion when there is selective fading. Also with SSB the transmitter needs to cope with a lower PEP for a given S/N at the receiver and you don't need such a complex demodulator with SSB.But in terms of overall efficiency in terms of average Tx power versus the resultant S/N ratio after the receiver demodulator then SSB and DSBSC are about the same? This assumes a level playing field with a flat noise spectrum across the DSB bandwidth.QuoteThe information isn't redundant if you make correct use of it in the DSBSC receiver. One can argue that it is duplicated/inverted information but it is still 'needed' information if you want to demodulate DSBSC in the classic (correct?) manner.The problem is that it is quite difficult to do this and get good performance especially if there is some selective fading in the DSBSC signal. So for me the main downsides of DSBSC are the complexity of the receiver and the wide channel bandwidth and the need for a transmitter that has to cope with a higher PEP for the same average power as an SSB signal.

author: Rory
author_rank: Frequent Contributor
timestamp: January 25, 2014, 03:19:54 am
content: G0HZU must have read this:http://www.g4jnt.com/Poisson_Shannon_Am.pdfJ.P. Costas, "Poisson, Shannon and the Radio Amateur", Proceedings of the IRE, December 1959, p. 2058QuoteInteresting read.

author: w2aew
author_rank: Super Contributor
timestamp: January 25, 2014, 03:25:04 am
content: When I say the other sideband is redundant, I mean that it redundant in terms of the 'message content', not in terms of being redundant as far as demodulation is concerned.Here's how a think about the efficiency gain. A given RF final amp design in a transceiver has a limit in the maximum PEP output. For SSB vs. DSBSC, this same power is now concentrated over half the BW. So, in the receiver, there is half the amount of total integrated noise because the receiver BW can be half of what's needed for DSBSC. Hence, a gain in SNR in the receiver.

author: G0HZU
author_rank: Super Contributor
timestamp: January 25, 2014, 03:42:34 am
content: There is no 3dB advantage in terms of signal to noise with SSB. For a given average signal power at the transmitter the s/n will be the same at the receiver despite the lower bandwidth of SSB.This is because the Dsb sidebands are demodulated coherently but the channel noise is not. So the 3dB boost in coherent demod offsets the 3dB extra noise in the wider bw of DSB.

author: C
author_rank: Super Contributor
timestamp: January 25, 2014, 04:09:00 am
content: QuoteIf I AM modulate a transmitter, Is not mixing it back down to 0 one way to get back the Audio, A direct conversion receiver. For this to work with AM then the sidebands phase must be ok or you would get not get audio. I have worked with 1, 2,3,4 conversion receivers and always get audio if audio was the original modulation. So while what you state above is true, I still see the relationship holding between them for sidebands at lease when mixed back to 0.So If I feed an AM RF signal to two mixers at the same time such that One LO is higher than the RF while the Second LO is lower than the RF by the same amount such that the difference frequency for both is the same frequency, is there a phase difference between the two resultant difference signals from the two mixers?When you take the two difference signals and mix them back to 0 you get audio. Is there a phase difference between the two audios that you create doing this?The Basic mixer theory I was taught was two signals in and the mixer output is the Sum, Difference and two originals. With the change the change of LO from below original and the LO above the original causing a frequency inversion in the sum and difference outputs of the two.Yes I know of Claude Shannon but where am I breaking it? I start with a signal then double the information from what is needed. Then use the double information to help in the recovery of the original. This is done all the time. What is spread spectrum if you do not call this adding redundant information and then using that redundant information for recovery.The closest signal I can think of is the FM stereo difference signal where they need the 19k pilot tone to reinsert the proper phase carrier back in the FM stereo difference so that the difference signal is in phase of the original mono signal which is the sum of the two channels.So back to what I started with,an AM or a reduced carrier version of AM or the DSB version where the carrier it gone. All have a mirror copy of the modulation above and below the carrier. Two mixers both with the same RF for input and the outputs being frequency inverted with respect to each other. The output of th two mixers fed to A simple sum anp & a simple difference amp to make use of the mirror copy that the modulation started with and that I see that should add.It will add or not, if not why not?C

author: G0HZU
author_rank: Super Contributor
timestamp: January 25, 2014, 02:20:11 pm
content: Quote from: Rory on January 25, 2014, 03:19:54 amNo, I wasn't referring to that article. I was just falling back on basic RF theory learned many, many years ago during my career as an RF design engineerIn my opinion the worst thing any novice can do if they are trying to discuss anything novel in RF is to visit a 'busy' ham radio or general electronics hobby forum for advice. This is because the amount of misinformation they will get back (from numerous well intending helpers) will massively outweigh any valid information. ...and on internet forums the laws of physics are outdone by majority rule and accepted common myths.I've found this out to my expense on forums like QRZ.com where there really is no point arguing if you are outnumberedThe net result is to end up looking a fool by majority vote even if you are technically correct.

author: G0HZU
author_rank: Super Contributor
timestamp: January 25, 2014, 02:31:09 pm
content: QuoteYour description of this system seems to be very vague and inconsistent to me...Maybe you should draw up a block diagram of it. A better place to ask RF questions would be the RF section of EDA forumseg here:http://www.edaboard.com/forum26.htmlDespite it being 'just another' internet forum, the ratio of good advice to bad advice will be way better than you will get here because it's dedicated to RF and there are quite a few good and experienced posters on there

author: M0BSW
author_rank: Frequent Contributor
timestamp: January 25, 2014, 02:38:36 pm
content: Quote from: w2aew on January 24, 2014, 08:18:55 pmI agree with Alan , I just don't see what your trying to achieve ,and I've been in this game for some 40 Years.

author: G0HZU
author_rank: Super Contributor
timestamp: January 25, 2014, 03:38:59 pm
content: Quote from: M0BSW on January 25, 2014, 02:38:36 pmIt's difficult to make out the system that is being proposed because it is described inconsistently. I don't know if it's aimed and ham use or commercial use. But you 'can' use DSB to multiplex two separate AM signals by using quadrature LOs in the transmitter. You can then separate them out from each other in a suitable coherent receiver. So you get two different signals wrapped up inside the bandwidth of a single DSB transmission. (eg QAM)Obviously, this requires both transmissions to be generated together from the same transmitter site so this is more likely to have commercial use than ham use.

author: Rory
author_rank: Frequent Contributor
timestamp: January 25, 2014, 07:08:25 pm
content: Quote from: G0HZU on January 25, 2014, 02:20:11 pmWise advice.After years of being a know-it-all, having no friends left in the business and now seeing my grown son spouting expert nonsense about something he knows absolutely nothing about, I've decided that I myself know next to nothing, have very little original ideas and haven't really got anything useful to contribute that others can't do better. I usually end up embarrassed by my stunning lack of knowledge. The best I can do is research the question at hand for my own edification and if I can, point the questioner to a credible source.

author: C
author_rank: Super Contributor
timestamp: January 25, 2014, 11:02:25 pm
content: The Use is greater usable use of the frequency spectrum due to interference rejection.Here is a list of blocks.1. The transmitter: An example would be Your signal generator output with AM or DSB modulation.The remaining parts(blocks) are in the receiver.The signal generator is connected to the TWO mixers, the desired output is one output being a frequency inverted copy of the other output with the carrier in each output at the same frequency.2a. A mixer inputs (#1, #3a )2b. A mixer inputs (#1, #3b)The two mixers which both need a LO signal. Use a frequency with the same difference from the carrier(the signal gen freq in this example) with One above the carrier and One Below the carrier.3a. LO Freq = #1+x output to #2a { I note that this LO may need to be phase locked to the carrier }3b. LO Freq = #1-x output to #2b { I note that this LO may need to be phase locked to the carrier }The outputs of the two mixers feeding TWO amp's4. A sum amp with input( #2a+#2b ) [Signal output]5. A difference amp with input( #2a-#2b )[ interference & noise output]And the simple Question of Will the two sidebands of the signal generator add in the sum amp(#4) with the carrier at the same frequency in both inputs?The two LO's with different frequencies could change the mixer outputs. Instead of phase locking the LO's to the input carrier signal, could it be necessary to phase lock to the carrier in the mixer output signal?RoryHad not seen that link, ThanksFrom Rory's linkQuoteIf I have the name correct the above 3 db is due to a product detector.The product detector uses a LO of the carrier frequency to have a difference output that folds around 0 Hz. The 3 db gain is from folding the two sidebands around 0 HzThe above blocks is a Simple Idea, Do what a product detector does But with a IF input & a IF output & NO detector. To get the 3 db process gain at a IF frequency and stay at a IF Frequency requires a frequency inverted copy of the input ( the two mixers) so that you can add the sidebands.By staying at an IF frequency you can also create #5. I have not seen a product detector with an output like #5 the interference|noise. The advantage would be the two outputs(#4 & #5) that are still at a IF frequency that you could then do more processing on.Simple facts as I see themEven a small frequency overlap between two SSB transmitters in a SSB receiver pass band creates a problem in the SSB receiver.With DSB|AM and using a product detector with a frequency overlap like above you have a 3 db process gain that will cancel some of this interference that is not possible to do with SSB. The foundation of the 3 db process gain in DSB|AM is the two sidebands.Edit Added block diagramC

author: eb4fbz
author_rank: Regular Contributor
timestamp: January 27, 2014, 11:25:02 pm
content: The problem with your proposed demodulator is that both LO's (high and low) have no phase relationship. You must do some math before going further with any "invention".What you are trying to achieve is well solved using the phasing method which makes use of the Hilbert transform. You need to LO's and two mixers as in your schema, but LO's are at the same frequency but 90º away. Shift 90º again at the mixers output (Zero-IF) and sum/add both signals to get the desired sideband.This technique is also used in microwave bands for image rejection mixers (IRM), 15 to 20dB rejection is common and enough, as it's unlikely to have interference in a highly directional link. That way you can avoid expensive and lossy filters or using a high IF to cover a big span.

author: G0HZU
author_rank: Super Contributor
timestamp: January 28, 2014, 12:41:08 am
content: Yes, I kind of gave up when I saw the poorly produced block diagram. I wasn't sure if LO High referred to high in frequency or a shift in phase. (eg are the two LOs simply in quadrature?)The diagram needs to include frequency and phase information. At the moment I can kind of see an IQ demodulator in the block diagram but only if I add my own information about frequency and phase that isn't provided.If the LOs are actually at different frequencies then I can't see how it can be a DSB demodulator. Presumably, there's another stage after this?

author: vk6zgo
author_rank: Super Contributor
timestamp: January 28, 2014, 04:45:36 am
content: Over the years,I have seen many articles putting a similar point to that of the OP,in many cases by people with many more years in RF work.It usually boils down to needing a fairly complex demodulator circuit to obtain a phase locked carrier to reinsert at the detector.This carrier is normally synthesised by taking samples of both sidebands.The sidebands,which are assumed to be still complete & in phase,are recombined with the carrier to givethe required demodulated signal.There is no requirement for a quadrature phase difference any more than in a normal AM detector.Thiscanwork quite effectively with a VHF signal over a "line of sight" path,or in Line Transmission systems,but falls down in HF use.One fairly well known problem with full carrier AM is "selective fading".This is where there is a different path of propagation for one sideband,compared to the other.In an AM detector,the sidebands can be so far out of phase,as to completely cancel.In HF work,DSB cannot,by definition,be immune to this effect,whereas SSB largely is,due to its narrower occupied bandwidth.At VHF,DSB is inferior to narrow band FM (NFM) for strong signal Commercial work.The proof of the pudding is pretty much in the eating!SSB has become the default system in HF work.At least Commercially, NFM is the default at VHF & above.

author: eb4fbz
author_rank: Regular Contributor
timestamp: January 28, 2014, 10:27:45 am
content: Another problem in your schematic is: how will you get rid of the undesired sidebands at your mixers?You will have an upper sideband from your "high LO" branch, and undesired lower sideband from your "low LO" branch.For example, you have an USB signal at 7100kHz you want to demodulate with 5kHz BW. According to your schematic, you will use a low 7100kHz LO and a high 7105kHz, right? Then the low mixer will demodulate both 7100-7105 and 7100-7095kHz and the high mixer 7105-7100 as well as the undesired 7105-7110kHz.Leaving aside that your demodulator won't work due to non phase coherent mixing, you will have noise and interference contribution from two images!

author: C
author_rank: Super Contributor
timestamp: January 28, 2014, 11:07:05 pm
content: Quote from: eb4fbz on January 28, 2014, 10:27:45 amThink you are applying SSB thinking here. For AM and DSB you have two sidebands and both are desiredFor an AM or DSB transmitter,With a SSB receiver you can use USB mode or LSB mode both would give you and output.With a DSB receiver you add a possible 2X process gain.For DSB you could have a receiver three IF strips all with a product detector on their outputs .1. The USB IF strip2. The LSB IF strip3. The DSB IF strip which would be 2X bandwidth of #1 or #2 that also has a 2X possible process gain.Most additional Noise or interference you hear in #1 would not be in #2. What is not at same level in 1 & 2 does not add for the process gain in #3With a SSB transmitter you get a choice of 1 or 2 of the above. additional Noise or interference you have no way to remove based on the difference of the two sidebands as you only have one sideband to start with.The idea is to add the original USB in the high LO branch mixer output to the Original LSB in the low LO Branch mixer output which are now at the same frequency. Inference in one of the sidebands would then not add.An IF Process not a detector that would only be possible with sidebands like the ones what exist in AM & DSB.As stated many timesOne Lo is (Carrier Freq + IF Freq)Second LO is (Carrier Freq - IF Freq)This results in both mixer difference outputs with the AM carrier at the same freq in the center of each IF strip.The two frequencies for the LO's also result with one mixer output being frequency inverted with respect to the second mixer output.Not much of an artistNote that in attached drawing, all lines that start from the same spot on a box have the same phase at the arrow ends.With two different LO frequencies you can not compare the phase of the two as they are constantly changing. But here the two LO are created by the same two oscillators so what ever relationship exists is fixed.Start with a 1k test tone.AM modulator with a 1k modulation. There would be three tones in the mixer output that we would be interested in.The carrier which is at the same freq in each mixer IF OutputThe original USB tone which would a appear at +1k to the carrier in one Mixer output and -1k to the carrier in the second mixer output.The original LSB tone would do the opposite -1k to the carrier in one Mixer output and +1k to the carrier in the second mixer output.So the original 1k modulation input to the modulator is now 4 signals at two frequencies. At the same frequency we have the modulation from USB in one mixer output and the modulation from LSB in the second mixer.So the first simple question, what is the phase relationship of the two signals at the same frequency between the two mixer outputs?What is the phase relationship of the other two signals at the same difference from carrier matching frequency on the other side of the carrier of the two mixer outputs?How does the output phase change with a change in the original modulation frequency(the test tone)?So Yes very simple questions and simple answers are possible. To get from the question to the answer, not so simple.And the answers from the above would answer the question of if a Sum amp could be used at IF frequencies.C

author: G0HZU
author_rank: Super Contributor
timestamp: January 30, 2014, 12:59:51 am
content: I've not had much time to look at this but I 'think' I understand your block diagram even though it would benefit from some extra info such as typical frequency data.So... what I think I see are two separate superhet receivers but one of them has a high side LO and one has a low side LO. So one will have an inverted response.What I can't yet see is what benefit this dual system offers because I can't see how you can get anything better than you would get with a single receiver path designed to demodulate DSB coherently.Also, as I mentioned earlier (and VK6zgo also mentions) there is the problem of selective fading with DSB or AM and I think most operators would prefer the characteristics of SSB under conditions of severe fading and this will outpoint any subtle versatility in the ability to reject band noise (or station interference) in parts of the DSB bandwidth.This is because you can always 'change frequency a few kHz' to get away from interference but you can't move away from the annoyance of selective fading unless you try a completely different frequency band.

author: C
author_rank: Super Contributor
timestamp: January 30, 2014, 03:33:51 am
content: Quote from: G0HZU on January 30, 2014, 12:59:51 amCorrect one inverted to the otherso as I have said many times the with the DSB being inverted in one you have two copies of one sideband aligning with the opposite sideband.This would not help with SSB but could give options for any modulation like AM or DSB as you can now do some processing of one sideband against the other at a IF freq.Amply each mixer output enough to drive some processing.A diff amp output would be noise, interference, fading difference.A sum amp output would be signal.Each of the above (sum,diff) output would still contain both sidebands.A lot of processing options that would not exist for SSB.Quote from: G0HZU on January 30, 2014, 12:59:51 amPlay fair if selective fading happens to DSB it also happens to SSB. Your cure is to move to a new freq. that option also exists for DSB. What is the cure for SSB if you can not move? For DSB you have the option to use other sideband or more processing.For interference when you use a product detector you get a 2X gain on signal to interference that is only in one side band, but you are no longer at an IF freq.What separation of signal vs junk could you get if you can do a sum & diff of the two sidebands and still be at a IF freq.With DSB you know that both sidebands should be the same just inverted with respect to each other. Using this could you not remove noise or interference that only appears in one sideband?DSB with it's two sidebands gives you many options for correction that just do not exist for SSB.DSB takes 2x the power to transmit to match power levels at the receiver with a possible 2X gain on receive.To get the most out of DSB requires some processes that treat each sideband separately.Forgetting the transmit power difference, What is the actual range difference having one RF front end driving two ssb IF strips vs one ssb IF strip.Would this not be a fair comparison?Could this just be 3 db more gain in the RF front end?Then if you actually get the 2X or more process gain it would be just get better faster with a receive strength increase.C