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Single side band modulation and demodulation in detail


Posted Date: 25-May-2013  Last Updated:   Category: Electronics Projects    
Author: Member Level: Gold    Points: 60


In this article, I will explain Single side band modulation and demodulation in detail. In this I have also discussed the SSB modulation and demodulation method along with the required circuit diagram and frequency spectrum.



For the electronic transmission of messages is dependent on the message to the baseband and modulation bandwidth must be provided for each transmission channel. The bandwidth of a normal amplitude modulation takes twice the highest frequency to the transmitted information. The information is included in two sidebands equally. By demodulating the upper and lower sideband information alone may be recovered.

The single sideband AM is a method in which only one side band is transmitted. Often, the support is also suppressed. Benefits of SSB are half the bandwidth compared to the normal AM and savings in transmission power. The disadvantage is the higher technical effort in dealing with SSB signals. After modulation and before sending one of the sidebands and the carrier must be filtered. This particularly high selective filters are required. The technically more complex phase method uses two parallel ring modulators. The carrier and the base band in the original phase position, the other of the modulator are supplied to a modulator 90 ° out of phase. After the additive superposition of both AM-modulation products is one of the sidebands counted out. The phase method does not require high slope, high-order filter. Instead, it requires broadband phase shifter to ensure a 90 ° phase rotation to the entire information band. The phase method could not prevail so.

Further problems arise in the demodulation to recover the information. The single sideband modulated signal is normally transmitted with suppressed carrier. An AM signal with reduced carrier is possible, but hardly saves transmission energy and requires filter. For demodulation in the receiver must be produced with a sufficiently large amplitude of the carrier and more particularly a frequency as possible. For signals with reduced carrier that must be filtered with a narrow-band filters and amplifies added before demodulation.

SSB by the filter method


Before applying the filter will take a normal amplitude modulation with the information. If the carriers are suppressed, then provides a diode ring modulator, or four-quadrant multiplier to. In the next step one of the two sidebands is filtered out. The closer together, the two sidebands, the fast rise, the filter needs to be. This property only stage LC filters or quartz filter.
Information with very low frequencies are not suitable for the SSB process. The lowest frequency information determines the spacing of the sidebands. At a frequency of 50 Hz, the distance is just another 100 Hz The filter would have to have an almost rectangular characteristic to achieve the required for voice telephony minimum attenuation of 40 dB.

For the speech band a lower cutoff frequency of 300 Hz is established. The spacing of the sidebands is then 600 Hz filter for a successful use of this absolute distance is not decisive. The higher the carrier frequency, the lower sideband of the relative distance. This requires an even larger filter slope.

Considering the transmission curves monomial LC-ground warp , it is seen that a band-pass of two parallel resonant circuits which are coupled by means of a series resonant circuit, does not reach this slope. This filter chain would be expanded symmetrically with series LC circuits. Filter require complex circuit components with high fidelity and long-term stability.

A cheap filter to use low carrier frequency is not suitable as transmission frequency. In a further modulation stage, the filtered-out sideband AM again, but this time modulated onto a transmission carrier frequency. The carrier can be suppressed. There will again two sidebands, one of which is filtered and sent. The spacing of the side bands to each other is now much larger and simpler bandpass filter range.

If the modulation side band of the first stage is between 10300 Hz and 13500 Hz to a modulated carrier of 500 kHz, so that modulation products are in the cut-off frequencies between the upper and lower side band at 489.7 kHz and 510.3 kHz. The frequency gap is now 20.6 kHz. To filter out one of the sidebands a not so fast rise-lower-order bandpass enough. The following block diagram shows the SSB described modulation in two stages with the corresponding cut-off frequencies.

1

Source of image:

Own created images using CAD software

Demodulating a modulation signal SSB


The demodulation of an SSB signal is in the receiving device after the addition of the original carrier frequency. The subcarrier frequency generated in the receiver should correspond exactly to the carrier frequency used in the transmitter, but the oppressed. This accuracy can only be achieved with quartz oscillators. The phase of the subcarrier is not so important.
In the SSB demodulation of the signal and the subcarrier are added to a linear resistor matrix. The amplitude of the subcarrier should be substantially greater than that of the single-sideband signal. The following figure shows an example of the additive superposition of a 10 kHz subcarrier with an upper single-sideband frequency of 11 kHz side. The timing diagram seems to correspond to a normal AM. The amplitude variations of the envelope to its original frequency of 1 kHz information represents the Fourier analysis indicates that it is not a conventional AM signal. Nevertheless, it can be performed as an AT in the supported envelope detection means and low-pass diode.

2

Source of image:

Own created images using CAD software

Demodulating the amplitude of this auxiliary carrier must be much greater than that of the single-sideband received signal. Too little difference to phase errors become noticeable in a significantly distorted, no longer sinusoidal envelope. The recovered information then comprises a large distortion.

Is the subcarrier frequency slightly above the transmitter carrier frequency, so shifts the recovered information in the direction of lower frequencies. If the subcarrier somewhat too low, the demodulation signal is too high. The amateur radio on shortwave use, among other SSB. At the receiving device, the subcarrier frequency is adjustable. A mismatch makes acoustically as caves or a Mickey Mouse effect.

In another, somewhat more convenient demodulation the received sideband is modulated with the subcarrier modulator at the ring or a four-quadrant multiplier-IC, wherein the carrier is suppressed. The following figure shows the result of the modulation between the 10 kHz subcarrier and the SSB signal with 11 kHz. The timing diagram does not exhibit the typical course of AM with suppressed carrier. The Fourier analysis indicates the two side frequencies, and that is suppressed in the 10 kHz subcarrier. The lower side of the line is the result of difference between subcarrier and SSB. Will provide you with 1 kHz, the original information is supplied, the sum frequency is 21 kHz. Of the modulation signal can be filtered with one simple low pass information.

3

Source of image:

Own created images using CAD software

The relative signal prior to modulation amplitudes are critical. Phase error will not occur in this method. The distortion of the recovered information signal is not larger. The requirement for an accurate subcarrier frequency remains. In the case of the single-sideband demodulation of the phase of the subcarrier is not critical in the two methods.
The demodulation of a suppressed carrier AM phase error responsive to the subcarrier sensitive.

Advantages of the SSB modulation


  • The modulation signal needed only half the bandwidth.

  • The entire transmission line will go to the SSB and not divided into 2 sidebands and the carrier signal.

  • The SSB is insensitive to signal reception loss, because there is no backing, amplitude reductions or cancellations learns through interference of wave propagation.


  • Disadvantages of the SSB modulation


    One of the disadvantages of the SSB modulation include the high demands on the SSB filter to filter out the desired sideband and the most necessary two-time modulation. In the receiver, a quartz-accurate subcarrier frequency must be supplied to the demodulation. Because quartz crystal oscillators are not tunable, the SSB technology has not been enforced in the normal broadcasting.


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