One sideband louder than two?

[K6JEK]

Would I be louder if I spent my 1500 W PEP on a carrier + one sideband instead of on a carrier and two sidebands?

Jon

[WD8BIL]

Limit negative peaks to 90% and drive positive peaks beyond 100% !!
Keep it real DSB AM!!

[w3jn]

It's a wash.  Unless the receiver is filtering out one sideband only, the receiver will see a 3dB decrease in recovered audio.

[WA3VJB]

Moreover, by running only 2/3 of a complete AM signal you deprive us of the option of tuning to the side that has the least amount of interference.

[K6JEK]

Moreover, by running only 2/3 of a complete AM signal you deprive us of the option of tuning to the side that has the least amount of interference.

So it would make sense when one side is being obliterated and you know it -- deliberate SSB QRM right on the net frequency, for example. Then I might as well send a carrier + the other sideband, full bore, right? 

My homebrew rig can't do this but my old CE 100V + linear can.

Jon

[WA3VJB]

So it would make sense when one side is being obliterated and you know it -- deliberate SSB QRM right on the net frequency, for example. Then I might as well send a carrier + the other sideband, full bore, right?

My homebrew rig can't do this but my old CE 100V + linear can.

Jon

This happens quite frequently. Idiot slopbucket will try to qwerm Ashtabula Bill or somebody with a zero-beat signal, say, by uttering their catcalls on LSB. Just nailing the receiver down to 4Kc and tuning to the high side on the R390A brings it back crystal clear no pain.

Please keep your AM signal intact. Do not neuter a sideband.

[Pete, WA2CWA]

Moreover, by running only 2/3 of a complete AM signal you deprive us of the option of tuning to the side that has the least amount of interference.

So it would make sense when one side is being obliterated and you know it -- deliberate SSB QRM right on the net frequency, for example. Then I might as well send a carrier + the other sideband, full bore, right? 

My homebrew rig can't do this but my old CE 100V + linear can.

Jon

I find this very handy when I'm working 20 meter AM or an AM crowded 40 meter band.

[WA1GFZ]

SDR software removes this kind of crud like cutting a worm out of an apple.
Last night a moron on 160 was 3 KHz away. Stock radio could hear him but sdr removed it like a zit.

[Tom WA3KLR]

Does 1500 W PEP SSB-AM equal 1500 W PEP DSB-AM?

Yes and no.  1500 Watts PEP DSB AM = 1500 Watts PEP SSB AM, but the SSB AM signal will have much audio distortion.  They are not truly equivalent.  But the peak-to-peak recovered audio is the same. 

There was another posting a few weeks ago on an acronym for sideband-
equivalent AM.  I don’t recall the exact term.  But it reminded me that in only one engineering textbook (which I believe I have in my library) have I ever seen a dissertation on the distortion in SSB-AM.  So far I can’t locate the paragraphs.

I did some empirical tests to show the distortion of ssb-am.

We usually do signal testing with single-tone sine wave modulation with a 1000 Hz. tone of some specified modulation level.  For most work, the level is usually restricted to less than 80 % modulation, because most r.f. signal generators can’t be trusted to do a good job of generating 100 % modulation.

If we are talking about generating a 1000 Hz. sine wave modulated AM signal, for example, via a “SSB AM” signal, we are saying we will have a carrier signal and a second signal of lesser r.f. level.  The frequency difference of these 2 r.f. signals will be 1 kHz., to generate the AM signal.  (In a ssb exciter, your 1000 Hz. audio is converted to a single r.f. signal along with the re-injected or unbalanced carrier signal 1000 Hz. away.)

To generate a 50 % AM signal with ssb-AM, the second signal is 6 dB lower in level than the “carrier” signal.  To generate a 100 % modulated AM signal, both signals must be equal.  Here the distortion is most apparent.  This signal is identical to the 2-tone signal generated by an ideal ssb exciter fed with 2 audio tones of matched level!  Some of you, at this point, may be able to visualize the classic 2-tone signal’s envelope used for testing linear amplifiers with a SSB exciter.  It is not identical to a 100% modulated DSB-AM signal!  There is a well-defined V notch in the envelope at the “zero envelope point” or “negative 100 % modulation” point called “baselining” by many AM’ers today.  A true sine wave AM signal envelope will have a rounded shape at this “negative 100 % modulation” point, identical to the positive peak envelope shape.

I used a HP606B and HP8640B r.f. generators with a coupler/splitter transformer to combine generator outputs.  The receiver is a Kenwood TS-430 in LSB mode for distortion reference only and AM mode with 6 kHz. bandwidth filter.  Coupler output to TS-430. 

I did some recovered speaker audio THD distortion measurements on the signals generated:
  a.  TS-430 in LSB mode – one carrier signal only, tuned for 1000 Hz. tone received, a healthy speaker level is needed for the HP 331A distortion meter.  I measured 0.7 % THD.  (A well-designed receiver could produce a much lower distortion level than this.   This measurement shows the basic receive audio distortion level the -430 can give.)
  b.  TS-430 in AM mode – one signal from HP8640B generator only , 50 % modulated DSB-AM signal; 0.7 % THD.
  c.  TS-430 in AM mode – two r.f. generator’s carriers adjusted for 50 % modulated ssb-am envelope; recovered audio is 10 % THD.
  d.  TS-430 in AM mode – two r.f. generator’s carriers adjusted for 100 % modulated ssb-am envelope; recovered audio is 14 % THD.

A E.E. math geek could calculate the exact theoretical audio THD with the SSB-AM signals at 50 % and 100 % modulation.  In general, as the modulation level is reduced from 100%, the distortion will drop. As the modulation level approaches 0 %, the THD will approach 0 % !

In practice I surmise that, transmitting ssb-am, without heavy speech clipping the average peak modulation is below 50 % and the resulting distortion level is not noticed by most people to be much worse than many other AM signals.

Below are 3 photos of the ssb-am signal and DSB –AM signal envelopes:
1. 75 % DSB-AM
2. 75 % SSB-AM
3.  100 % SSB-AM

[WA3VJB]

Tom, was that the "exhalted carrier" AM discussion?

I'm trying to pull up the thread too. At first I thought it was the type the Drakes produced, but that was controlled carrier, where the audio level swung the carrier up and down.

[Pete, WA2CWA]

Tom, was that the "exhalted carrier" AM discussion?

I'm trying to pull up the thread too. At first I thought it was the type the Drakes produced, but that was controlled carrier, where the audio level swung the carrier up and down.

Was it this one:

http://amfone.net/Amforum/index.php?topic=6919.0

[Tom WA3KLR]

There was one other relevent Technical Forum topic, Feb. 1st.  This is one I was trying to think of:

"What is AME???

http://amfone.net/Amforum/index.php?topic=6907.0

- - - - - -

The maximum % of modulation achievable does depend on the circuit implementation.  Theoretical limit is 100 % as I said, but if the carrier comes from the unbalanced balanced modulator, the maximum mod. level will probably be 71 % as John says.  If the carrier is re-injected by a separate path, as in the FT-101EE (IIRC), 100 % can be obtained, like in my photo.

[k4kyv]

It can be demonstrated grapically with vectors.  SSB with carrier produces "quatrature distortion", a combination of amplitude and phase modulation.  With DSB AM, the quadrature distortion of USB cancels the quadrature distortion of LSB, to produce pure distortion-free AM, with no phase modulation component.  The only way to make a SSB with carrier signal reasonably distortion free is to reduce the percentage of modulation to a low value, at which point the quadrature distortion becomes negligible. 

The same thing happens when you receive SSB with a diode detector.  You have to turn the rf gain down to clear up the distortion.  Think of the BFO as the "carrier" and the SSB signal as the sideband.  If the rf gain is turned up, the sideband signal is strong, in other words, the percentage of modulation is high, and you hear lots of distortion.  Turn down the rf gain, reducing the "percentage of modulation" of the BFO, and the distortion goes away.

The only thing SSB with carrier is good for is for the carrier to produce a reference signal for the BFO  to lock onto, or for emergency reception using a receiver without a bfo.  It is nothing more than SSB with poor carrier suppression.  It is not AM. 

[VE7 Kilohertz]

The only thing SSB with carrier is good for is for the carrier to produce a reference signal for the BFO  to lock onto, or for emergency reception using a receiver without a bfo.  It is nothing more than SSB with poor carrier suppression.  It is not AM. 

Buggar! I guess I am going to have to find a way to keep my Collins hF-80 system from knocking off one sideband. I got the exciter going today bypassed the Collins 2.75 KHz filter with a .1uF cap. Sure sounded better. My Racal though, was picking up both USB and LSB. What's up with that? Maybe it doesn't knock out one SB. Manual says it does.  Mmmmm.

Paul
VE7KHz

[Bacon, WA3WDR]

Paul, I think you bypassed the sideband filter that passes the one sideband and rejects the other.  If so, you solved the problem!  The DSB with adjustable carrier that you get from the balanced modulator is perfectly good, and it can produce perfect AM.  Just tune the amplifiers right, and don't clip the peaks.

Um, but you should use compression or peak limiting to keep from overmodulating.  Overmodulation from a balanced modulator is worse sounding than overmodulation from a regular plate modulated rig, and it is very easy to do.  In fact, it is difficult not to overmodulate any transmitter that has extra modulation capability.  However, overmodulation will not cause splatter in that setup, as long as the peaks are not flattening.

[VE7 Kilohertz]

Hi Bacon,

Thanks! Excellent info. i am going to review the manual today to find out what I have done. I have strapped the jumpers like the manual says, which should give 16KHz wide, no SSB filter signal, but it's not, which is why I bypassed the filter with the cap.

Will definately be using compression ahead of the exciter. Will be firing the PA today so may just be on the air tonight with a new rig.

Cheers

Paul
VE7KHz

[k4kyv]

Paul, I think you bypassed the sideband filter that passes the one sideband and rejects the other.  If so, you solved the problem!  The DSB with adjustable carrier that you get from the balanced modulator is perfectly good, and it can produce perfect AM.  Just tune the amplifiers right, and don't clip the peaks.

One other thing to be careful about is to make sure the reinserted carrier is exactly in phase with the original one.  Any phase error will distort the AM signal.  In fact, if the carrier is shifted exactly 90 degrees, the result will be pure phase modulation, with zero amplitude variation!

As I understand it, Maj.Armstrong's prototype FM transmitter consisted of a balanced modulator with reinserted carrier in quadrature (shifted 90º) with the original.  The resulting phase modulated signal was then converted to true FM by applying the appropriate pre-emphasis curve to the audio input to the transmitter.

I once modified a Drake TR-7 to bypass the sideband filter so that the AM output was true DSB.  That was a problem with that rig; the phase of the reinserted carrier bore no relationship to that of the original suppressed carrier.  I found a  small interstage rf transformer in the rf carrier amplifier chain.  I observed that adjusting the trimmer that resonated the transformer shifted the phase as you tuned through the resonant peak.  So, using a scope, I carefully adjusted the transformer for  the  proper AM waveform.  Amazingly, just as the reinserted carrier was adjusted close to the proper phase, it sort of locked in, and did not show any tendency to drift, kind of the same way a regenerative receiver locks onto an AM carrier with just the right amount of feedback when it is tuned zero-beat.  The AM output from the rig suddenly sounded 1000% better and the result was much more punch than the stock bogus SSB-with-carrier pseudo-AM.

[Bacon, WA3WDR]

As I understand it, Maj.Armstrong's prototype FM transmitter consisted of a balanced modulator with reinserted carrier in quadrature (shifted 90º) with the original.  The resulting phase modulated signal was then converted to true FM by applying the appropriate pre-emphasis curve to the audio input to the transmitter.

Yes, that is the Armstrong method of Frequency Modulation.  The problem was that you could not get more than about 0.5 modulation index from this quadrature technique, which meant 50 Hz audio would produce 25 Hz deviation from the modulator, and you would have to multiply by 3,000 to get 75 KHz deviation.  This forced Armstrong to start at very low frequencies, and then the selectivity of the low frequency stages fouled up the FM sidecurrents at high audio frequencies.  This is why he developed block conversion and amplification to repeat an FM signal - the modulator distortion was too high to run through it multiple times, plus the complexity of such huge multiplier stages was greater than that of a block conversion.

Good point on inserted carrier phase.  If it gets shifted compared to the DSB, you get some mix of PM and AM, and envelope modulation will be reduced and distorted, depending on the degree of phase shift, and the negative peaks will not be able to reach 100% negative.  Higher audio levels will cause a doubling back of the negative peaks in the positive direction in the signal envelope, which will sound terrible on an envelope detector, but still 100% negative will not be reached.  Not a good way to control overmodulation!

[k4kyv]

Yes, that is the Armstrong method of Frequency Modulation.  The problem was that you could not get more than about 0.5 modulation index from this quadrature technique, which meant 50 Hz audio would produce 25 Hz deviation from the modulator, and you would have to multiply by 3,000 to get 75 KHz deviation.

Didn't he use something in the vicinity of 40 mHz (40,000 kHz) for his first experimental broadcasts?  I think that's why the old FM band was something like 42-50 mHz. Dividing 40 mHz by 3000,  means he had to start out in the vicinity of 13 kHz.  That wouldn't accomodate 2nd order sidebands above about 6500~.

The original DSB signal could start out higher, say a more manageable frequency like 100 kHz.  Multiply that by 3000,  up to 300 mHz, then heterodyne it with a 260 mHz  local oscillator to produce the 40 mHz PM signal.  Then you would have to apply de-emphasis to the audio for uniform FM deviation.

[Bacon, WA3WDR]

Didn't he use something in the vicinity of 40 mHz (40,000 kHz) for his first experimental broadcasts?  I think that's why the old FM band was something like 42-50 mHz. Dividing 40 mHz by 3000,  means he had to start out in the vicinity of 13 kHz.  That wouldn't accomodate 2nd order sidebands above about 6500~.

The original DSB signal could start out higher, say a more manageable frequency like 100 kHz.  Multiply that by 3000,  up to 300 mHz, then heterodyne it with a 260 mHz  local oscillator to produce the 40 mHz PM signal.  Then you would have to apply de-emphasis to the audio for uniform FM deviation.

Yes, that would work.  I'm not sure why Armstrong didn't do that.  It may have been a patent issue.

I believe that the 13 KHz issue was exactly the problem.  Actually I think Armstrong was using a little less than a 3000 multiplier, and a little more than 0.5 modulation index (at the lowest frequencies), and his modulator was working around 16 or 17 KHz. Second order sidecurrents at the modulated generator frequency were pretty much irrelevant above a few hundred Hz, because the necessary modulation index dropped off with increasing audio frequency.  At 5000 Hz, 25 Hz deviation would have been a modulation index of only 0.005, and the Armstrong method did not produce any second order sidecurrents at the generator frequency anyway, because it was just DSB with 90 degree shifted inserted carrier.  This limitation is part of the reason you don't want to exceed about 0.5 modulation index at the generator frequency with this technique.  It caused distortion.

I'm guessing that he just barely squeaked by with 50-15,000 response.  Also, maybe he specified distortion at 80% mod or something.

Engineers wanted direct, high-stability crystal frequency control, which made PM to FM conversion the thing to do, one way or the other.  With crystals controlling the conversions, the 100K - 300 MHz - 40 MHz idea should have been OK.

[Bacon, WA3WDR]

Oops, darn, I hit quote instead of modify again.  Disregard.

[Tim WA1HnyLR]

Interesting topic  here. Poor mans AM one sideband with carrier. Yep thats how we do it on the other 3 WBCQ frequencies 5110,9330, and 18.910.  Slop bucket broadcasting is permitted under FeeCCee law . but must follow ITU standard which is the carrier must be at -6DB from the peak modulation value. The law is 50 Kw minimum power out be it carrier power for full DSB am or 50Kw PEP SSB. The 50Kw PEP SSB is much more cost effective than a full power AM transmitter to operate. There is a degree of distortion when recieved on a diode detector. The transmitting equipment is using old TMC exciters that have 8Khz filters. the overall audio reponse is much better than typical slopbucket.but the low end suffers a bit. The audio processing equipment is feeding the balanced modulators directly . For a while a phasing type generator using DSP audio phase shifting tecniques designed by Pete McNulty WA1SOV was in use on 9330. The audio quality was stunning .We hope to place it back into service soon. The only way to recieve poor mans AM is use of a sync detector . This is the other end of the equasion. A diode detector falls apart with any great degree of modulation. Most listeners are probably using inexpensive portable radios without sync detectors .Most of the programming on the frequencies mentioned are bible beaters. They do not seem to care about audio quality. Just as long as their "word" gets out. Such is life in another radio world in the HF spectrum .De Tim WA1HnyLR

[WA1GFZ]

sdr decodes it ok fine also

[Bacon, WA3WDR]

Carrier 6dB below peak means 100% positive modulation maximum, so 50KW PEP would be 12.5KW carrier out.  I see how that's cheaper to operate than AM with 50KW carrier out, even with the efficiency of a linear amp.  That's an interesting power regulation.

[scottdarling1221]

cool