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Questions about SSB + Carrier AM
I have noticed that some SSB tube transmitters only send a single side
band plus the carrier in AM mode. Does this provide a lower quality signal for reception? What are the advantages and drawbacks versus DSB plus carrier AM transmitters? It seems like DSB is the preferred mode for the AM crowd. Some receivers, such as my SX-101a, will demodulate only one side band which must be selected. Does this provide lower fidelity than full wave AM detectors? Does this compromise its performance as an AM receiver? Dan |
SSB+Carrier is known as vestegial sideband and has an efficiency
somewhere between AM and SSB. However, unless that modeswitch is designated VSSB, I'd look to readjusting your carrier balance control. -- Gregg *It's probably useful, even if it can't be SPICE'd* http://geek.scorpiorising.ca |
geojunkie ) writes:
I have noticed that some SSB tube transmitters only send a single side band plus the carrier in AM mode. Does this provide a lower quality signal for reception? What are the advantages and drawbacks versus DSB plus carrier AM transmitters? It seems like DSB is the preferred mode for the AM crowd. Some receivers, such as my SX-101a, will demodulate only one side band which must be selected. Does this provide lower fidelity than full wave AM detectors? Does this compromise its performance as an AM receiver? Dan One reason it's done this way in ham equipment is that it's cheap. Just unbalance the balanced modulator, and you've got some sort of AM for use with AM receivers. It takes a resistor off the mode switch, which is really easy to implement. If you went and bypassed the sideband filter, you'd get into more complicated switching (because you'd be dealing with actual signals at the IF frequency). In boatanchor era SSB equipment, AM is thrown in only for compatibility, since it dates from a time when there was still significant AM activity. Hence, that it may not be the best scheme (a low level signal going through various stages of linear amplification) is irrelevant because anything more complicated would be costly, for equipment that is really intended as SSB equipment. A fallout from sending a single sideband, besides less complicated circuitry, is that the signal chain doesn't have to amplify the eliminated sideband, so more of the output signal is wasted; if both sidebands were sent there would be a loss of actual output signal. There was of course a period when AM pretty much disappeared from SSB transmitters and transceivers, since there was little point in it since AM was fading, and by that time, fewer with SSB wanted to talk to those with AM. It's reappeared, to some extent, in recent years, likely more fallout from manufacturers wanting to have as many bells and whistles. In AM days, virtually all of it was generated at a high level (ie plate or screen modulation of the final amplifier), and then there was no option but to send out both sidebands. It's only when more complicated equipment came along with SSB that many amateurs had equipment that could anything but a signal with a carrier and two sidebands. But with an SSB transmitter, given sufficient switching schemes, one could generate AM (a carrier with two sidebands), DSBsc (Double SideBand with suppressed carrier), SSBsc (Single SideBand with suppressed carrier) and SSBc (I don't think that's the proper way to designate it, but Single SideBand with carrier). In other words, there was never any real need to send the extra sideband except for simplicity in the transmitter, and the carrier came along for the ride (and of course it allowed for simpler reception). But since the two sidebands are exact images of each other, one is redundant, yet the AM transmitter has to devote some of its power to sending that redundant sideband. And since the extra sideband is redundant, there is absolutely no loss of "fidelity" if it's not sent, or the receiver ignores it. You can receive an AM signal fine with an SSB filter, tuning issues aside. You will have not lost one bit of actual content by stripping off the extra sideband with an SSB filter (at the transmitter or receiver). Of course, one traditionally used a wider passband in AM days, while with SSB once you had the need for a filter to strip off the unwanted filter one might as well use it to limit the bandwidth of the signal, so one might notice some loss of "fidelity" when using an SSB filter. But this is due to the bandwidth of the filter, not because one stripped off one of the sidebands. Michael VE2BVW |
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A carrier plus one sideband can sound very good:
witness the CHU time signals at 7335 KHz. (USB + carrier). That "A.M. Equivalent" mode used to be denoted as A3H, whilst SSB (no carrier) was A3J and standard A.M. was A3. (The official nomenclature describing the modulation modes has become more complicated.) Someone listening with a 6 KHz IF bandwidth receiver with envelope detector will hear a bit less recovered audio from an "A.M. equivalent" transmitter than from a standard A.M. transmitter, as if there were a smaller percentage of modulation. Someone listening with a 3 KHz wide receiver would not notice the difference between A.M. equivalent and standard A.M., because his receiver is in effect converting all sgnals to A.M. equivalent. One difference: he wouldn't be able to switch sidebands to dodge QRM. Regards, Ed Knobloch |
Gregg wrote:
SSB+Carrier is known as vestegial sideband and has an efficiency somewhere between AM and SSB. Vestigial sideband is used for the Visual signal in NTSC TV transmission. The term "Vestigial" comes from the fact that part of the lower sideband is transmitted to reduce the phase shift caused by the tuned circuits used to remove most of the lower sideband. In the very early days of TV they couldn't build accurate filters to remove the full lower sideband without causing video problems, so a compromise was made in allowing a small part of the lower sideband to be transmitted. However, unless that modeswitch is designated VSSB, I'd look to readjusting your carrier balance control. Gregg -- We now return you to our normally scheduled programming. Michael A. Terrell Central Florida |
Not exactly right.
When only one side band and carrier are transmitted instead of two side bands (regular AM signal) you loose 6 db of signal recovery. There is 3 db less power in the AM side band because of only one side band. There is another 3 db of loss in the detector because it is not receiving coherent signals from the audio. In double side band AM both side bands combine in the detector to give a 6 db increase in output. The second problem with it is that when you use a standard diode detector in the receiver (as most AM detectors are), you can not fully modulate the transmitter to 100% modulation or you will end up with high distortion in the receiver. Without both side bands in the diode detector the second harmonics between the carrier and the one side band become very high and it sounds distorted on high audio levels. You must keep the modulation percentage low to avoid distortion in the receiver. With both side bands this cancels out in the detector. An SSB transmitter transmitting carrier and both side bands works just as well as a plate modulated transmitter. But some of the SSB transmitters like the Collins KWS1 did only transmit one side band with carrier. It was easier to do as someone else mentioned. It does sound ok if set up properly but it does not have the quality or punch that it would have with both side bands for the reasons mentioned above. 73 Gary k4FMX |
Behold, Michael A. Terrell signalled from keyed 4-1000A filament:
Gregg wrote: SSB+Carrier is known as vestegial sideband and has an efficiency somewhere between AM and SSB. Vestigial sideband is used for the Visual signal in NTSC TV transmission. The term "Vestigial" comes from the fact that part of the lower sideband is transmitted to reduce the phase shift caused by the tuned circuits used to remove most of the lower sideband. In the very early days of TV they couldn't build accurate filters to remove the full lower sideband without causing video problems, so a compromise was made in allowing a small part of the lower sideband to be transmitted. Hi Mike, Quite true! However there were several SW broadcasters experimenting with VSSB in order to free up the congested SW bands. -- Gregg *It's probably useful, even if it can't be SPICE'd* http://geek.scorpiorising.ca |
On Mon, 02 Feb 2004 21:30:52 GMT, Gregg wrote:
Behold, Michael A. Terrell signalled from keyed 4-1000A filament: Gregg wrote: SSB+Carrier is known as vestegial sideband and has an efficiency somewhere between AM and SSB. Vestigial sideband is used for the Visual signal in NTSC TV transmission. The term "Vestigial" comes from the fact that part of the lower sideband is transmitted to reduce the phase shift caused by the tuned circuits used to remove most of the lower sideband. In the very early days of TV they couldn't build accurate filters to remove the full lower sideband without causing video problems, so a compromise was made in allowing a small part of the lower sideband to be transmitted. Hi Mike, Quite true! However there were several SW broadcasters experimenting with VSSB in order to free up the congested SW bands. Greg, I think that all are either AM DSB with carrier or SSB with carrier. There is no reason that I know of to transmit only part of one side band for broadcast or communications. Other than in TV of course as was pointed out. The SW broadcasters use SSB with carrier in order to squeeze into tighter frequency assignments. TV stations eliminate part of one side band for the same reason, to protect the adjacent channel. But they do not abruptly eliminate the video signal because of the phase shifts caused in the filtering. This is where the vestigial side band comes in. 73 Gary K4FMX |
Behold, Gary Schafer signalled from keyed 4-1000A filament:
On Mon, 02 Feb 2004 21:30:52 GMT, Gregg wrote: Behold, Michael A. Terrell signalled from keyed 4-1000A filament: Gregg wrote: SSB+Carrier is known as vestegial sideband and has an efficiency somewhere between AM and SSB. Vestigial sideband is used for the Visual signal in NTSC TV transmission. The term "Vestigial" comes from the fact that part of the lower sideband is transmitted to reduce the phase shift caused by the tuned circuits used to remove most of the lower sideband. In the very early days of TV they couldn't build accurate filters to remove the full lower sideband without causing video problems, so a compromise was made in allowing a small part of the lower sideband to be transmitted. Hi Mike, Quite true! However there were several SW broadcasters experimenting with VSSB in order to free up the congested SW bands. Greg, I think that all are either AM DSB with carrier or SSB with carrier. There is no reason that I know of to transmit only part of one side band for broadcast or communications. Other than in TV of course as was pointed out. The SW broadcasters use SSB with carrier in order to squeeze into tighter frequency assignments. TV stations eliminate part of one side band for the same reason, to protect the adjacent channel. But they do not abruptly eliminate the video signal because of the phase shifts caused in the filtering. This is where the vestigial side band comes in. 73 Gary K4FMX Phase shifts - gotcha! Thanks :-) -- Gregg *It's probably useful, even if it can't be SPICE'd* http://geek.scorpiorising.ca |
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