Synthesizer stability: 1 part in 108 per day.
(TMC photo courtesy Jptronics.org)
SOME NOTES ON SINGLE SIDEBAND MODES
Within the Single Sideband umbrella, there are variants of operation. Basically, they are:
1. USB Upper side band only
2. LSB Lower side band only
3. ISB Independent side bands
4. VSB Vestigial side band.
5. DSB Double side band
USB and LSB are the most common SSB sub-modes where the carrier is virtually eliminated and one of the side bands is filtered out completely while the other sideband is modulated with intelligence. In amateur radio, the practice is to use LSB for emissions below 10 MHz and USB for emissions above 10 MHz. For technical reasons, it had to be done that way when amateurs started using SSB in the 1950's but the practice continues today even though there is no reason to do so other than convention. The reasons for that evolution are beyond the scope of this document. Commercial SSB users, in contrast, have no such convention but they generally use USB.
ISB means Independent sideband. In ISB, the carrier is eliminated. One sideband is modulated with intelligence 'A' and the other sideband is modulated with intelligence 'B'; An example, one side band might carry a tone shifted (AFSK) signal while the second sideband might carry a voice channel.
A3B emission is the mode when two different conversations are transmitted simultaneously with one conversation being modulated on the upper sideband and the other in the lower sideband.
A9B is a SSB frequency-division multiplexed (FDM) emission, another variation of ISB . Each independent side band is used to carry multiplexed emissions. As an example 1 voice circuit plus 2 RTTY circuits can be multiplexed into the upper sideband while the lower sideband could contain 2 voice circuits of 3 KHz each.
This transmission example would only occupy 11 KHz of bandwidth (Upper= 3KHz +1KHz +1KHz + Lower=3 KHz +3 KHz). FDM over SSB can only be accomplished with the use of complex , extremely stable transmitters and receivers.
VSB - In commercial work, a vestigial carrier may be transmitted. Instead of completely canceling the carrier at the balanced modulator, a little of it is allowed to remain in the transmitted signal. At the receiving station the local oscillator is made to lock in, or synchronize on the frequency of the carrier, preventing the distortion that results if the local carrier is a few hertz off frequency. Single sideband with a reduced carrier is called A3A. Asymmetric Sideband Modulation (ASM), was the term used for VSB in the early days.
DSB - Double-sideband suppressed-carrier transmission (DSB-SC) is a transmission mode n which (a) frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier frequency and (b) the carrier level is reduced to the lowest practical level, ideally completely suppressed.
Ray White adds a note about some SSB history. "When SSB was in its infancy, circuit instability plagued both transmitting and receiving equipment. As a workaround until the state of the art improved, SSB was transmitted with a "pilot carrier" plus one sideband. This pilot carrier was not completely suppressed but was reduced to a fairly low db level. It could be heard on a superhet receiver in CW mode and the receiving operator had to zero beat against the reduced carrier to obtain an intelligible signal. Once solid state circuits made their appearance in communications equipment, it no longer became necessary to transmit a pilot carrier".
SUMMARY OF SSB MODES
This table is referenced from Technical Materiel Corp. Field Engineering Bulletin #18. TMC defines carrier levels as follows:
Full carrier: 0 to 6 db
Reduced carrier: -6 to -26 db
Suppressed carrier anything below -26 db
SYMBOL Type of Transmission Supplementary
A3 Telephony Double sideband A3A Telephony Single sideband, reduced carrier A3J Telephony Single sideband, suppressed carrier A3B Telephony Two independent sidebands A4 Facsimile ( with modulation of main carrier either directly of by an FM sub-carrier None A4A Facsimile ( with modulation of main carrier either directly of by an FM sub-carrier Single sideband, reduced carrier A5C Television Vestigial sideband A7A Multichannel, voice frequency telegraphy Single sideband, reduced carrier A9B Cases not covered by the above. ie a combination of telephony and telegraphy Two independent sidebands
Eric Earl remembers the DDR-5 receiver while he served at Coverdale. "I believe that the diversity receiver set we used in Coverdale as part of the North Atlantic HFDF net was the DDR-5K (AN/FRR-74). I remember these receivers being in a separate room. They were a synthesized receiver. One would set the exact frequency with the synthesizer unit then one would use the manual analog tuning control to tune across the set frequency until a phase lock was obtained as indicated by a flickering lamp. Once the phase lock was obtained the operator would turn a knob to the locked position and the receiver master VFO would track the synthesizer and stay dead on. The reason that such accuracy was required at the time is that there could be multiple stations on the HFDF net all transmitting on exactly the same frequency and via ISB one would decode the appropriate tonal pairs. These tonal pairs operated on a very narrow shift. I vividly remember Professor Inch in Gloucester going over these receivers in great detail as the same synthesizer used in the receivers were also used in the transmitters".
Credits and References:
1) TMC DDR-5K photo: http://jptronics.org/radios/TMC/bulletins/RX/tmc.ssb_3028a.pdf
2) Eric Earle KG4OZO Atlanta <eearle(at)adelphia.net>
3) Ray White <r.p.white(at)sympatico.ca>
4) Electronic Communication . Robert Shrader . McGraw Hill Publishing 1959, 1967.
5) DSB Definition: http://en.wikipedia.org/wiki/Double-sideband_suppressed-carrier_transmission
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