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SSB-Filters (PART 4)

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SSB-Filters (PART 4) --------- ELECTRONICS --------------------- SSB-FilterS (PART 4) ELECTRONICS This is the fourth article of a tivity and stability characteristics, with mtmaturization of equipment. This is the fourth article of a series from a training course written but their large size makes them Also to the advantage of the me­ series from a training course written by Collins Radio Company for per­ subject to shock or vibration de· chanical filter is a Q in the order by Collins Radio Company for per- sonnel concerned with single side­ terioration. Also, their cost is of 10,000 which is about 100 times sonnel concerned with single side- band communications. quite high. the Q obtainable with electrical band communications. Both SSB transmitters and SSB Newer crystal filters are being elements. Both SSB transmitters and SSB developed that have extended fre­ receivers require very selective receivers require very selective Although the commercial use of bandpass filters in the region of bandpass filters in the region of quency range and are smaller. These mechanical filters is relatively 100 to 500 kilocycles. 100 to 500 kilocycles. newer crystal filters are more ac­ new, the basic principles on which In receivers, a high order of ad- In receivers, a high order of ad­ ceptable for use in SSB equipment. they are based is well established. jacent channel rejection is required jacent channel rejection is required LC Filters The mechanical filter is a me­ if channels are to be closely spaced if channels are to be closely spaced LC filters have been used at IF chanically resonant device that re­ to conserve spectrum space. In to conserve spectrum space. In frequencies in the region of 20 kilo· ceives electrical energy, converts SSB transmitters, the signal band- SSB transmitters, the signal band­ it into mechanical vibration, then width must be limited sharply in cycles. However, generation of the order to pass the desired sideband width must be limited sharply in SS B signal at this frequency re­ converts the mechanical energy back and reject the other sideband. The order to pass the desired sideband quires an additional mixing stage into the electrical energy at the out­ filter used, therefore, must have and reject the other sideband. The to obtain a transmitting fr equency put. The mechanical filter consists very steep skirt characteristics and filter used, therefore, must have in the high-frequency range. For basically of four elements: a flat bandpass characteristic. very steep skirt characteristics and this reason LC f i 1 t e r s are not 1. An input transducer that con­ These filter requirements are met a flat bandpass characteristic. widely used. verts the electrical input into me­ by LC filters, crystal filters, and These filter requirements are met Mechanical Filters chanical oscillations. mechanical filters. 2. Metal disks that are mechani­ Crystal Filters by LC filters, crystal filters, and The recent advancements in the Until recently crystal filters, mechanical filters. development of the mechanical fil­ cally resonant. used in commercial SSB equipments, Crystal Filters ter have led to its acceptance in 3. Coupling rods that couple the were in the 100-kilocycle range. Until recently crystal filters, SSB equipment. Mechanical filters metal disks. Such filters have excellent selec- used in commercial SSB equipments, have excellent rejection character­ 4. An output transducer that con­ SSB-Filters were in the 100-kilocycle range. istics, are extremely rugged, and verts the mechanical oscillations (PART 4) Such filters have excellent selec· are small enough to be compatible back into electrical oscillations. tivity and stability characteristics, Figure 1 shows the elements of but their large size makes them subject to shock or vibration de- the mechanical filter. Figure 2 ONE SUPPORTING DISK AT EACH [NO....._..,·,,. terioration. Also, their cost is -BIAS WAGNET shows the electrical analogy of the quite high. mechanical filter. In.the electrical Newer crystal filters are being analogy, the series resonant cir­ developed that have extended fre- cuits L1C1 represent the metal quency range and are smaller. These disks, the coupling capacitors cl newer crystal filters are more ac- represent the coupling rods, and ceptable for use in SSB equipment. ' the input and output resistances R LC Filters ' LC filters have been used at IF WAGNETOSTRICTIV[ _/ represent the matching mechanical - {INPUT OR OUTPUT l ORIVI"G ROO frequencies in the region of 20 kilo- loads. cycles. However, generation of the Transducers SSB signal at this frequency re- The transducer, which converts Figure I. Elements of mechanical filter. quires an additional mixing stage electrical energy into mechanica1 to obtain a transmitting frequency in the high-frequency range. For Figure 2. F.leclrical analo� of a mechanical filter. this reason LC filters are not widely used. Mechanical Filters The recent advancements in the development of the mechanical fil- ter have led to its acceptance in SSB equipment. Mechanical filters have excellent rejection character- istics, are extremely rugged, and are small enough to be compatible ONE SUPPORTING DISK AT EACH END COUPLING RODS BIAS MAGNET TRANSDUCER COIL ELECTRICAL SIGNAL (INPUT OR OUTPUT ) UNIVERSITY 0 F CALl FORN lA 30 MAGNETOSTRICTIVE BuShips Journal DRIVING ROD ELECTRICAL SIGNAL (INPUT OR OUTPUT) Figure 1. Elements of mechanical filter. Figure 2. Electrical analogy of a mechanical filter. with miniaturization of equipment. Also to the advantage of the me- chanical filter is a Q in the order of 10,000 which is about 100 times the Q obtainable with electrical elements. Although the commercial use of mechanical filters is relatively new, the basic principles on which they are based is well established. The mechanical filter is a me- chanically resonant device that re- ceives electrical energy, converts it into mechanical vibration, then converts the mechanical energy back into the electrical energy at the out- put. The mechanical filter consists basically of four elements: 1. An input transducer that con- verts the electrical input into me- chanical oscillations. 2. Metal disks that are mechani- cally resonant. 3. Coupling rods that couple the metal disks. 4. An output transducer that con- verts the mechanical oscillations back into electrical oscillations. Figure 1 shows the elements of the mechanical filter. Figure 2 shows the electrical analogy of the mechanical filter. In.the electrical analogy, the series resonant cir- cuits L,C, represent the metal disks, the coupling capacitors C, represent the coupling rods, and the input and output resistances R represent the matching mechanical loads. Transducers The transducer, which converts electrical energy into mechanical 30 BuShips Journal ELECTRONICS -------- energy and vice versa, may be ical filters will without doubt bring impedances are largely resistive either a magnetostrictive device or an even faster rate of cutoff. and range between 1,000 ohms and an electrostrictive device. Coupling 50,000 ohms. The magnetostrictive transducer In the equivalent circuit, the The insertion loss is measured is based on the principle that cer­ coupling capacitors cl represent with both the source and load im­ ELECTRONICS tain materials elongate or shorten the rods that couple the disks. By energy and vice versa, may be pedance matched to the input and either a magnetostrictive device or when in the presence of a magnetic varying the mechanical coupling output impedance of the filter. The an electrostrictive device. field. Therefore, if an electrical between the disks, that is, by mak­ value of insertion loss ranges be· The magnetostrictive transducer signal is sent through a coil, which ing the coupling rods larger or tween 2 decibels and 16 decibels, is based on the principle that cer- contains the magnetostrictive ma­ smaller, the bandwidth of the filter depending on the type of transducer. tain materials elongate or shorten terial as the core, the electrical is varied. The transmission loss is an in­ when in the presence of a magnetic oscillation will be converted into Because the bandwidth varies dication of the filter loss with field. Therefore, if an electrical mechanical oscillation. The me­ approximately as the total area of source and load impedances mis­ signal is sent through a coil, which chanical oscillation can then be the coupling wires, the bandwidth matched. The transmission loss is contains the magnetostrictive ma- terial as the core, the electrical used to drive the mechanical ele­ can be increased by using either of importance when using a mechan­ oscillation will be converted into ments of the filter. larger or more coupling rods. Me­ ical filter in pentode IF amplifiers mechanical oscillation. The me- The electrostrictive transducer chanical filters with bandwidths where both source and load imped· chanical oscillation can then be is based on the principle that cer­ as narrow as 0.5 kilocycle and as ance are much greater than the fil­ used to drive the mechanical ele- tain materials, such as piezo­ wide as 35 kilocycles are practical ter impedances. ments of the filter. electric crystals, will compress in the 100- to 500-kilocycle range. The transfer impedance is useful The electrostrictive transducer when subjected to an electric Passband is based on the principle that cer- to determine the overall gain of a tain materials, such as piezo- current. Although an ideal filter would pentode amplifier stage that makes electric crystals, will compress In practice, the magnetostrictive have a flat "nose" or passband, use of a mechanical filter. The when subjected to an electric transducer is more commonly used. practical limitations prevent the transfer impedance of the filter mul­ current. The transducer not only converts ideal from being attained. The tiplied by the transconductance of In practice, the magnetostrictive electrical energy into mechanical term "ripple amplitude" or "peak­ the pentode gives the gain of the transducer is more commonly used. energy an d vice versa; it also pro­ to-valley ratio" is used to specify amplifier stage.
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