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AMATEUR RADIO By GARRY CHATT, VK2YBX Single sideband transmission: basic theory & circuits Although it is the most widely used transmission mode on the amateur bands, single sideband (SSB) is perhaps the least understood mode of all. This month's column explains the theory of SSB and shows how it is generated. When a modulating signal is applied to an AM transmitter, four output signals are generated as follows: the original carrier, the original modulating signal, and two sidebands. The two sidebands consist of the sum component, otherwise known as the upper sideband, and the difference component, which is known as the lower side band. For example, let's say that our carrier signal has a frequency fc and that this is modulated by a 2kHz audio signal. The upper sideband will then have a frequency of fc + 2kHz while the lower sideband will be at fc - 2kHz. This is shown graphically in Fig.1. Both the carrier amplitude and frequency are unchanged by the modulation process, while the audio signal is filtered out by the RF output network of the transmitter, leaving the spectral waveforms shown in Fig.1 at the transmitter output. Because all the "intelligence" is contained in the sidebands, the carrier is used only to allow demodulation in the receiver. If the carrier is suppressed at the transmitter, considerable energy can be saved and transmitter efficiency can be vastly improved. The signal can still be 72 SILICON CHIP demodulated at the receiver using a carrier re-insertion technique. Actually, SSB is a derivative of AM modulation. Depending upon which sideband is desired, it can be seen that if the carrier and one sideband is "stripped" from an AM signal (often referred to as a double sideband signal), a single sideband signal remains. It also becomes apparent that this sideband signal, whether upper or lower, occupies far less bandwidth than an AM or "double sideband" signal. In fact, when the correct receiver bandwidth is used to take advantage of an SSB signal, there is an effective improvement of up to 9dB in power over an AM signal having the same peak power. Fig.le shows an SSB signal in which the carrier and lower sideband have been suppressed. (a) le (b) ,___ _.___........___ fe -2kHz le FREQUENCY ___.__ ___ le+2kHz FREQUENCY (e) le+ 2kHz FREQUENCY Balanced modulator Fig.1: the spectral waveforms at the output of an AM transmitter. Fig.l(a) shows the carrier with no modulation while (b) shows the result of single tone sinusoidal modulation. In (c) both the carrier and the lower sideband have been suppressed, leaving only the upper sideband. The most commonly used method of suppressing the carrier signal is to use a balanced modulator. There are several standard types, as shown in Fig. 2. All designs aim to suppress the carrier by 30-60dB, whilst ensuring that the sidebands appear in the output. In all of these designs, there will be no RF output when there is no audio input. When audio is applied, the modulator will become unbalanced (as sum and difference products will be generated), and the sidebands will appear in the output. After nulling out the carrier in the balanced modulator, the DSB FROM CARRIER GENERATOR **GREAT VALUE AND TECHNOLOGY .. BALANCED MODULATOR 4xHPA5082-5826 l: II II II II -. IIII AUDIO INPUT (a) .01i +12vo---------------- CARRIER INPUT vc ~----------"I 3.9k 0.l 3.9k 6 +Vo 511l -Vo ,___-'f MODULATING SIGNAL INPUT 0------<_ _ _ _ _ MC1496, MC1596 2 vs 1k 10k 10k .,. 3 CARRIER NULL (b) +9V ** . 10 PRODUCT OF THE MONTH ... PC FAX SHORT FORM KIT . . . . ONLY $199.00 Featured EA Nov, Dec '89 CONTINUED SAVINGS ON ... 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PRICES SUBJECT TD CHANGE WITHOUT NOTICE. 26 Boron St, Sumner Park, Ken Curry Managing Director · Brisbane, Old 407 4. Ph: (07) 376 2955 WELLINGTON NZ: Ph: (04) 85 87 42 Fax: (04) 82 8850 6.Bk - 8 v o - - -......- - - - - - ~ .,. 0------------------- D.DAUNER 15k ELECTRONIC COMPONENTS 470!! OSCILLATOR INJECTION (1V) 2.2k (c) ~ J 15k . CARRIER / . ON S1 .i .,. DSB OUTPUT (0.2V) Fig.2: three balanced modulator designs. Fig.2(a) shows a ring diode modulator, Fig.2(b) uses the Motorola MC1496 balanced modulatordemodulator IC, and Fig.2(c) is based on varicap diodes D1 & D2. All three designs aim to suppress the carrier by 30-60dB. signal can be fed to a bandpass filter, where one of the sidebands can be filtered out. There are several different types of filters that can be used and depending upon the carrier frequency selected, these can be LG, mechanical, or even made from discrete junkbox crystals. However, in practice this is normally done using a crystal filter which will have sufficiently steep "skirts" to attenuate the unwanted sideband. This is all very well in a transmitter operating on a fixed sideband. However, if the transmitter is designed so that the sideband is selectable, then the design must either use two filters, one for each sideband, or the carrier oscillator must be designed to enable it to be WE STOCK A WIDE RANGE OF ELECTRONIC PARTS for • Development • Repair • Radio Amateur • Industrial Electronic • Analog and Digital WHILE STOCKS LAST Quartz in filter 10.9MHz 6kHz BW. $12.50 US Filter capacitor 4µF 3kV. . $15.00 Electromagnetic Airpump for Aquarium . .. . .. . . ... . .. . .. $9.00 Timer Motor 240VAC 6RPH $6.00 Come and see. Showroom: 51 Georges Crescent, Georges Hall, NSW 2198 (Behind Caltex Service Station In Blrdwood Road) Phone 724 6982 TRADING HOURS: Monday to Friday 9.00 a.m. to 4.00 p.m. Saturday from 9.00 a.m. to 12.00 noon. MARCH 1990 73 Fig.4 shows how SSB is generated using the phasing method. Demodulating SSB MIC SPEECH AMPLIFIER LINEAR RF AMPLIFIER FILTER MICO (b) Fig.3: two variations of the filter method of SSB generation. In Fig.3(a), two filters at the output of the balanced modulator are switched to select either the USB or LSB signal while in Fig.3(b) the carrier oscillator is offset by the required amount by selecting one of two crystals. offset by the required amount when the opposite sideband is selected. Fig.3 shows these two selectable sideband schemes. The phasing method Another method of generating a single sideband signal is to use the phasing method. In this scheme, the audio and carrier signals are each fed into 'a 90° phase shift network and applied to balanced modulators. When the outputs of both balanced modulators are combined, one sideband is added or reinforc- ed, while the other sideband is cancelled. For this system to work well, the phase shift and amplitude of both the audio and carrier signals must be very accurate. For this reason, the phasing method became less popular following the introduction of relatively inexpensive crystal filters. The major advantage of the phasing method is that the desired SSB signal can be generated at the operating frequency without using a separate heterodyne oscillator and mixer. The demodulation of an SSB signal requires the reinsertion of a signal at the "carrier" frequency in the receiver. This "carrier" signal is mixed with the incoming sideband signal in a balanced modulator (or demodulator in this case) to provide an audio output signal. In practice, the re-constituted carrier is usually generated by a crystal oscillator. When this is applied to the demodulator (also called a product detector) in conjunction with the incoming sideband signal, demodulation takes place. Fig.5 shows both active and passive product detectors. Active product detectors have the advantage of producing several dB of conversion gain, while passive detectors have the advantage of simplicity and low cost. Typical SSB transceiver A block diagram for a typical HF SSB transceiver is shown in Fig.6. In the transmit mode, signals from the microphone are fed to a balanced modulator where they modulate a carrier signal generated by an offset oscillator to produce a DSB signal (ie, the carrier is suppressed). This DSB signal is then fed to a crystal filter stage with a passband of about 3kHz. Depending on the frequency of the offset oscillator, the filter removes either the upper or lower sideband. The resulting 10.7MHz SSB signal is then amplified and fed to a 2nd mixer stage where it is mixed with a VTO (voltage tuned oscillator) signal. BALANCED MODULATOR SPEECH AMPLIFIER MIC BALANCED MDDULATDII Fig.4: the phasing method of SSB generation. The audio and carrier signals are fed to 90° phase shift networks and the outputs of the balanced modulators combined. This reinforces one sideband and cancels the other. 74 SILICON CHIP 1.Sk RFC 1 + .01 SIG:AL o-ft----'l,-,,.,1,,-11-~ 100k F-o tuuJ!3T .oo,I .,. 10k .o,r (a) BFO INJECTION 01 T2 II SIGNALl',i INPUT JI • Inside a typical SSB transceiver. The large SSB filter can be clearly seen in the centre of the PC board. 2ND MIXER II BFO e (+13dBm) II 1I II II .,. 11 11 II II 11.,. II AUDIO OUTPUT The difference signal produced is then fed to an RF amplifier and applied to the antenna. The receiver is a conventional superheterodyne type with an IF (intermediate frequency) of 10. 7 MHz. As shown in Fig.6, the incoming signal is amplified and fed to an Rx mixer where it is mixed with a local oscillator signal [from the heterodyne mixer) to give a 10.7MHz IF. From there, the signal passes through the noise blanker, SSB amplifier & IF amplifier stages to the SSB detector. HETERODYNE MIXER "f,. . . Tl 0.1-1: (b) Fig.5: an SSB signal is demodulated by mixing it with an injected "carrier" signal in a product detector. Fig.5(a) shows an active product detector while Fig.5(b)is a passive detector. The offset oscillator provides carrier re-insertion at the detector. The USB SW and LSB SW blocks set the frequency of the offset oscillator to give either USB (upper sideband) or LSB (lower sideband) RF AMPLIRER RF AMPLIRER PRE-DRIVER reception as required. Further information on SSB techniques can be obtained from the ARRL Handbook, the Motorola Linear Circuit Databook and the RSGB Handbook. ~ DRIVERS PA LPF METER SSB AMPLIFIER VTO 10.6935MHz CRYSTAL ALTER NOISE BLANKER FREQUENCY COUNTER RX MIXER RX RF AMPLIFIER ATTENUATOR RF IF AMPLIFIER AGC AMPLIFIER SSB DETECTOR RX AUDIO AMPLIFIER DC SUPPLIES AND SWITCH .,. MIC MICROPHONE PREAMPLIRER BALANCED MODULATOR OFFSET OSCILLATOR USB SW LSD SW = + Fig.6: block diagram of a typical HF SSB transceiver. It uses the scheme shown in Fig.3(b) to generate an SSB signal. MARCH 1990 75