Single sideband modulation
The single-sideband (ESB, now more common than SSB, English single-sideband modulation ) is a spectrum and energy-efficient modulation for voice transmission. It is usually used on analog radio links such as the shortwave range for marine radio , long-range aeronautical radio , in military applications and in amateur radio . SSB was developed by the telecommunications administrations in the 1930s and was initially used for the wired transmission of telephone calls over long distances, and later also for transcontinental radio links. In the area of analog voice radio, as it is still common in amateur radio, amplitude modulation (AM), which occupies two sidebands and thus a larger bandwidth , was almost completely replaced by single sideband modulation in the course of the 1960s.
The form of single-sided modulation in the context of digital signal processing is also known as VSB modulation ( English Vestigial Sideband Modulation ).
properties
Compared to amplitude modulation, SSB does not emit redundant signal components such as a second sideband . Usually, SSB is transmitted with a suppressed carrier , but there are also variants with carriers or carriers with reduced amplitude. When the carrier is suppressed, the entire transmission energy is used for the information content of the signal, which means that greater ranges and better signal-to-noise ratios are achieved for a given transmission power . The interference effects of multipath propagation are also lower with SSB than with amplitude modulation.
Compared to amplitude modulation with a suppressed carrier, SSB has advantages such as halved bandwidth requirements. The main disadvantage of SSB is that analog receivers are more complex in terms of circuit technology and the associated higher costs for SSB-compatible radio devices. A simple AM demodulation with the envelope demodulator is basically not feasible with SSB.
In contrast to the amplitude modulation and the amplitude modulation with suppressed carrier, both of which do not influence the phase position of the carrier frequency, the single sideband modulation has a phase modulation of the carrier as a component. The phase modulation is stronger, the greater the amplitude of the sideband signal to the carrier amplitude. Historically, the SSB belongs to the range of amplitude modulation methods despite modulation of the phase.
Due to the type of modulation, high-frequency baseband signals lead to a large level of modulation in the modulated signal, which is associated with a high crest factor . For this reason, SSB is not particularly suitable for pulse transmissions and as a digital modulation method.
Spectral representation
Designated by the real to be modulated baseband signal as a modulation signal produced during amplitude modulation symmetrically to the carrier frequency ( English Carrier ) two additional frequency ranges. If the modulation frequency fluctuates, for example, in the frequency range between 300 Hz and 4000 Hz, a frequency band with a total width of 8000 Hz is generated and required. The upper occupied frequency range is called USB ( English upper sideband ); the lower frequency range occupied called LSB ( English lower sideband ), both frequency ranges included in an amplitude modulation exactly the same information.
In the case of an SSB with a suppressed carrier, as shown in the figure opposite with a sine wave as the baseband signal that can be changed in frequency, only the spectral component in the upper sideband changes analogously to the baseband signal. The envelope is constant and with a reduced amplitude of the modulation signal, the transmission power therefore also falls in the same ratio.
The transmission power required for this is considerably lower due to the lack of a permanent carrier wave in the middle of the band, it fluctuates between 0% and 12.5% and is therefore only dependent on the modulation signal level that remains at the mixing frequency relative to the middle of the band itself (so-called "differential modulation "). The disadvantage of suppressed carriers is that no information is sent about the spectral location of the carrier frequency required for demodulation, the phase position is not necessary for non-coherent demodulation, and the information whether the broadcast is USB or LSB must be agreed.
In the case of analog broadcasting in amateur radio, when speech is transmitted, the information about the frequency can be determined very easily manually through trial and error, because speech in the event of a deviation of the carrier frequency in the receiver in relation to the carrier frequency used by the transmitter of more than approx. 100 Hz becomes incomprehensible. With Slow Scan Television (SSTV), a regularly transmitted sync pulse, a so-called burst signal , enables the carrier frequency to be adjusted automatically. Music sounds inharmonious even with carrier frequency deviations of just a few Hertz. Therefore, SSB is not used to broadcast high quality music.
generation
An SSB signal can be generated in several ways, the result is identical in all cases:
- The filter method starts with a mixer that suppresses the carrier frequency. As a rule, a Gilbert cell is used for this , which generates an amplitude modulation with a suppressed carrier . Then a narrow-band band- pass filter with a high quality factor , such as a quartz filter , ensures that only one of the two sidebands can be further amplified. Because the frequency of a quartz filter cannot simply be changed in the context of analog circuit technology, this intermediate frequency must be brought to the desired transmission frequency by another mixer.
- The phase method, which corresponds to the IQ method, does not require an expensive filter; instead, two symmetrical mixers are used, the input signals of which are phase-shifted by 90 ° on both the low and high-frequency sides. It is very problematic to generate this value in the entire speech frequency range from 300 Hz to 3500 Hz with the components of analog technology (capacitors and resistors). Any deviation leads to poor suppression of the unwanted sideband. This enables SSB to be generated on the transmission frequency.
- In today's radio equipment, the SSB is mostly implemented using digital signal processing and software-defined radio (SDR). For this purpose, the real-valued modulation signal is transformed into a complex baseband signal, a so-called analytical signal , by means of the Hilbert transformation. The actual Hilbert transformation can be realized in specially designed FIR filters . The complex modulation signal formed in this way is then mixed with two, phase-shifted carriers and then these two components are added, as shown in the adjacent figure. Due to the phase shift between the two carrier waves, the carrier signal is canceled out during the addition. The final addition also suppresses one of the two sidebands and the result is a single sideband signal with a suppressed carrier. After a digital-to-analog conversion, this sum signal can be output as an analog, now purely real signal and, if necessary, shifted to a higher output frequency by repeated analog mixing. First work on this process goes back to Donald K. Weaver in the 1950s.
Demodulation
Although the amplitude of the SSB signal has some similarity to the modulating low frequency, SSB cannot be demodulated with a simple envelope demodulator, as is possible with the non-coherent demodulation of AM.
The SSB signal is received in an analog SSB receiver, as is common in amateur radio, by first mixing the received and preselected signal in a mixer to a fixed intermediate frequency (IF) and using a steep-edged filter such as a quartz filter , as is the case with other receivers is freed of interfering signals on neighboring radio frequencies - this is the so-called superheterodyne principle in reception technology. Another mixer stage, known for historical reasons as a product detector , takes over the demodulation by adding a carrier generated locally in the receiver. The oscillator for local carrier generation is as English Beat Frequency Oscillator , BFO short, or as English carrier insertion oscillator , short CIO, called
After filtering, the mixed product is the modulation signal in the baseband position; in amateur radio it is usually speech in the frequency range from 300 Hz to 4 kHz. Frequency deviations of the BFO lead to a frequency offset and can then be fine-tuned manually by the user of the radio receiver using the speech intelligibility ( English pitch ). A frequency offset of typically ± 20 Hz is considered tolerable. Some amateur radio receivers have an LSB / USB switch that sets the frequency of the BFO to the lower or the upper end of the transmission curve of the intermediate frequency filter.
A disadvantage of carrierless SSB is the difficult implementation of automatic gain control (AGC) and automatic frequency control (AFC) in the receiver. One way out is SSB with lowered carrier and its regeneration in the receiver. Further procedures are the transmission of an agreed pilot tone from which the carrier and the control variables for the AGC and AFC can be derived. In the absence of such information, z. B. the spectrum of the SSB detector with that of an IF envelope detector (diode) can be made to coincide by using speaker-typical frequency peaks that occur regardless of the fundamental frequency of the voice.
In receiver technology, too, analog signal processing, which requires numerous electronic components, some of which have been adjusted by hand, has been gradually replaced by cheaper digital signal processing since the mid-1990s . In the shortwave frequency range, fully digital receiver concepts are common, in which, apart from a preselection in front of the AD converter, no analog components are used for signal processing.
Miscellaneous, use, special forms
In the broadcasting sector, the introduction of SSB as a replacement for AM was discussed for decades, but ended after the development of digital transmission methods ( DRM and DAB ). In principle, the transmission of stereophony signals in the context of AM stereo with single sideband modulation on a radio frequency is possible, but is not used. The main reason is that the achievable music quality is unsatisfactory because of the narrow bandwidth of the AM channels of only 9 kHz and the receiver must be tuned to a few Hertz.
Programs that are broadcast in single sideband modulation cannot be reproduced in radio receivers with normal AM demodulation. But there is also a special form of single sideband modulation that can also be reproduced by normal radio sets with AM demodulators (AM-compatible single sideband modulation; the carrier is also transmitted here). This type of modulation was used between 1953 and 1962 on a long-wave transmitter operated by Deutschlandfunk .
A widespread special form of single sideband modulation is what is known as residual sideband modulation , used in the analog transmission of television signals , which also achieves a considerable increase in energy and spectrum efficiency compared to double sideband AM. In contrast to pure single sideband modulation, however, a reduced carrier and part of the second sideband is transmitted here. As a result, the demodulation of the signal on the receiving end (for example a TV set) is considerably simplified and made cheaper.
Web links
- History of single sideband modulation SSB: "The history of single sideband modulation (SSB)" (German)
- History of single sideband: "When Did Single Side Band Appear in radio?" (English)
- Single sideband and residual sideband modulation ( PDF ; 583 kB)
Individual evidence
- ↑ Dietmar Rudolph: single sideband & residual sideband modulation. TFH Berlin - Telekom TT - IBH, accessed on April 30, 2020 .
- ↑ Donald K. Weaver, Jr .: A Third Method of Generation and Detection of Single-Sideband Signals. In: Proceedings of the IRE. Vol. 44, No. 12, 1956, ISSN 0096-8390 , pp. 1703-1705, doi : 10.1109 / JRPROC.1956.275061 .
- ↑ https://patents.google.com/patent/US4596046 Julian H. Richardson, Bruce C. Eastmond: Split loop AFC system for a SSB receiver , Patent US4596046A, October 1984
- ↑ https://www.electronics-notes.com/articles/radio/modulation/single-sideband-ssb-demodulation-reception.php Ian Poole: Electronics Notes on SSB Demodulation, accessed on Aug. 15, 2018
- ↑ https://ieeexplore.ieee.org/document/1090695/ O. Villard: Sideband-Operated Automatic Frequency Control for Reception of Suppressed-Carrier SSB Voice Signals in IEEE Transactions on Communication Technology, Vol. 19, Issue 5, October 1971