Архитектура -> Simple coaxial reflectometer
DOUBLE-BALANCED -fjJ MODULATOR
TWO TYPES OF DIODE BALANCED MODULATOR
Such balanced modulaior circuits are com-manly used in carrier telephone work and in single-sideband systems where the carrier frequency and modulating frequency are relatively close together. Vacuum diodes, copper-oxide rectifiers, or crystal diodes may be used in the circuits.
17-3 Carrier Elimination Circuits
Various circuits may be employed to eliminate the carrier to provide a double sideband signal. A selective filter may follow the carrier elimination circuit to produce a single sideband signal.
Two modulated amplifiers may be connected with the carrier inputs 180° out of phase, and with the carrier outputs in parallel. The car-
rier will be balanced out of the output circuit, leaving only the two sidebands. Such a circuit is called a balanced modulator.
Any non-linear element will produce modulation. That is, if two signals are put in, sum and difference frequencies as well as the original frequencies appear in the output. This phenomenon is objectionable in amplifiers and desirable in modulators or mixers.
In addition to the sum and difference frequencies, other outputs (such as twice one frequency plus the other) may appear. All combinations of all harmonics of each input frequency may appear, but in general these are of decreasing amplitude with increasing order of harmonic. These outputs are usually rejected by selective circuits following the modulator. All modulators are not alike in the magnitude of these higher order outputs. Balanced diode rings operating in the square law region are fairly good and pentagrid converters much poorer. Excessive carrier level in tube mixers will increase the relative magnitude of the higher order outputs. Two types of triode balanced modulators are shown in figure
8, and two types of diode modulators in figure
9. Balanced modulators employing vacuum tubes may be made to work very easily to a point. Circuits may be devised wherein both input signals may be applied to a high impedance grid, simplifying isolation and loading problems. The most important difficulties with these vacuum tube modulator circuits are: (1) Balance is not independent of signal level. (2) Balance drifts with time and environment. (3) The carrier level for low high-order output is critical, and (4) Such circuits have limited dynamic range.
A number of typical circuits are shown in figure 10. Of the group the most satisfactory performance is to be had from plate modulated triodes.
PLATE MODULATED BALANCED TRIODE MODULATOR
BALANCED TRIODE MODULATOR WITH SINGLE ENDED INPUT CIRCUITS
Figure TO BALANCED MODULATORS
BALANCED PENTAGRID CONVERTER MODULATOR
HIGH Z MODULATING VOLTAGE
DOUBLE-BALANCED RING MODULATOR SHUNT-QUAD MODULATOR
Figure 11 DIODE RING MODULATORS
LOW Z MODULATING VOLTAGE
Diode Ring Modulation in telephone car-Modulators rier equipment has been very successfully accomplished with copper-oxide double balanced ring modulators. More recently, germanium diodes have been applied to similar circuits. The basic diode ring circuits are shown in figure 11. The most widely applied is the double balanced ring (A). Both carrier and input are balanced with respect to the output, which is advantageous when the output frequency is not sufficiently different from the inputs to allow ready separation by filters- It should be noted that the carrier must pass through the balanced input and output transformers. Care must be taken in adapting this circuit to minimize the carrier power that will be lost in these elements. The shunt and series quad circuits are usable when the output frequencies are entirely different (i.e.: audio and r.f.). The shunt quad (B) is used with high source and load impedances and the series quad (C) with low source and load impedances. These two circuits may be adapted to use only two diodes, substituting a balanced transformer for one side of the bridge, as shown in figure 12. It should be noted that these circuits present a half-wave load to the carrier source. In applying any of these circuits, r-f chokes and capacitors must be employed to control the path of signal and carrier currents. In the shunt pair, for example, a blocking capacitor is used to prevent the r-f load from shorting the audio input.
To a first approximation, the source and load impedances should be an arithmetical mean of the forward and back resistances of the diodes employed. A workable rule of thumb is that the source and load impedances be ten to twenty times the forward resistance for semi-conductor rings. The high frequency limit of operation in the case of junction and copper-oxide diodes may be appreciably extended by the use of very low source and load impedances.
Copper-oxide diodes suitable for carrier
work are normally manufactured to order. They offer no particular advantage to the amateur, though their excellent long-term stability is important in commercial applications. Rectifier types intended to be used as meter rectifiers are not likely to have the balance or high frequency response desirable in amateur SSB transmitters.
Vacuum diodes such as the 6AL5 may be used as modulators. Balancing the heater-cathode capacity is a major difficulty except when the 6AL5 is used at low source and load impedance levels- In addition, contact potentials of the order of a few tenths of a volt may also disturb low level applications (figure 13).
The double diode circuits appear attractive, but in general it is more difficult to balance a transformer at carrier frequency than an additional pair of diodes. Balancing potentiometers may be employed, but the actual cause of the unbalance is far more subtile, and cannot be adequately corrected with a single adjustment.
A signal produced by any of the above circuits may be classified as a double sideband, suppressed-carrier signal.
6 0 CARRIER VOLTACE
DOUBLE-DIODE PAIRED MODULATORS
(a) series-balanced diode modulator using 6al5 tube
(B) ring-diode modulator using 6AL5 tube
VACUUM DIODE MODULATOR CIRCUITS
17-4 Generation of
In general, there are two commonly used methods by which a single-sideband signal may be generated. These systems are: (1) The Filter Method, and (2) The Phasing Method. The systems may be used singly or in combination, and either method, in theory, may be used at the operating frequency of the transmitter or at some other frequency with the signal at the operating frequency being obtained through the use of frequency changers (mixers).
The Filter The filter method for obtaining Method a SSB signal is the classic meth-
od which has been in use by the telephone companies for many years both for
-e -s -4 -3 -г -1 о +1 kilocycles deviation
Figure 15 BANDPASS CHARACTERISTIC OF BURNELL S-15000 SINGLE SIDEBAND FILTER
land-line and radio communications. The mode of operation of the filter method is diagrammed in figure 14, in terms of components and filters which normally would be available to the amateur or experimenter. The output of the speech amplifier passes through a conventional speech filter to limit the frequency range of the speech to about 200 to 3000 cycles. This signal then is fed to a balanced modulator along with a 50,000-cycle first carrier from a self-excited oscillator. A low-frequency balanced modulator of this type most conveniently may be made up of four diodes of the vacuum or crystal type cross connected in a balanced bridge or ring modulator circuit. Such a modulator passes only the sideband components resulting from the sum and difference between the two signals being fed to the balanced modulator. The audio signal and the 50-kc. carrier signal from the oscillator both cancel out in the balanced modulator so that a band of frequencies between 47 and 50 kc. and another band of frequencies between 50 and 53 kc. appear in the output.
The signals from the first balanced modulator are then fed through the most critical
so RC. OSCILLATOR
irSO-1950 KC. OSCILLATOR
FOR OUTPUT IN l 00-aOOOKC.BAND
BLOCK DIAGRAM OF FILTER EXCITER EMPLOYING A 50-K.C.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 [ 108 ] 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266