Dome Audiophaseshift Network

This circuit arrangement is convenient for obtaining the audio phase shift when it is desired to use a minimum of circuit components and tube elements.

variable and should have a lower value of inductance than that value of inductance which would have the same reactance as resistor R. Inductor L may be considered as being made up of two values of inductance in parallel; (a) a value of inductance which will resonate at the operating frequency with the circuit and tube capacitances, and (b) the value of inductance which is equal in reactance to the resistance R. In a network such as shown in figure 23, equal and opposite 45-degree phase shifts are provided by the RL and RC circuits, thus providing a 90-degree phase difference between the excitation voltages applied to the two balanced modulators.

Audio-Frequency The audio-frequency phase-Phasing shifting networks used in generating a single-sideband signal by the phasing method usually are based on those described by Dome in an article in the December, 1946, Electronics. A relatively simple network for accomplishing the 90-degree phase shift over the range from 160 to 3500 cycles is illustrated in figure 24. The values of resistance and capacitance must be carefully checked to insure minimum deviation from a 90-degree phase shift over the 200 to 3000 cycle range.

Another version of the Dome network is shown in figure 25. This network employs three 12AU7 tubes and provides balanced output for the two balanced modulators. As with the previous network, values of the resistances within the network must be held to very close tolerances. It is necessary to restrict the speech range to 300 to 3000 cycles with this network. Audio frequencies outside this range will not have the necessary phase-shift at the output

12AU7 12AU7

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Degrees Phase Shifter

Figure 25 A VERSION OF THE DOME AUDIO-PHASE-SHIFT NETWORK

Figure 25 A VERSION OF THE DOME AUDIO-PHASE-SHIFT NETWORK

of the network and will show up as spurious emissions on the sideband signal, and also in the region of the rejected sideband. A low-pass 3500 cycle speech filter, such as the Chicago Transformer Co. LPF-2 should be used ahead of this phase-shift network.

A passive audio phase-shift network that employs no tubes is shown in figure 26. This network has the same type of operating restrictions as those described above. Additional information concerning phase-shift networks will be found in Single Sideband Techniques published by the Cowan Publishing Corp., New York, and The Single Sideband Digest published by the American Radio Relay League. A comprehensive sideband review is contained in the December, 1956 issue of Proceedings of the l.R.E-

Comparison of Filter Either the filter or the and Phasing Mel-hods phasing method of of SSB Generation single-sideband gener ation is theoretically capable of a high degree of performance.

In general, it may be said that a high degree of unwanted signal rejection may be attained with less expense and circuit complexity with the filter method. The selective circuits for rejection of unwanted frequencies operate at a relativly low frequency, are designed for this one frequency and have a relatively high order of Q. Carrier rejection of the order of 50 db or so may be obtained with a relatively simple filter and a balanced modulator, and unwanted sideband rejection in the region of 60 db is economically possible.

The phasing method of SSB generation exchanges the problems of high-Q circuits and linear amplification for the problems of accurately controlled phase-shift networks. If the

Ssb Passive Audio Phase Shift
Figure 26 PASSIVE AUDIO-PHASE-SHIFT NETWORK, USEFUL OVER RANGE OF 300 TO 3000 CYCLES.

phasing method is employed on the actual transmitting frequency, change of frequency must be accompanied by a corresponding rebalance of the phasing networks. In addition, it is difficult to obtain a phase balance with ordinary equipment within 2% over a band of audio frequencies. This means that carrier suppression is limited to a maximum of 40 db or so. However, when a relatively simple SSB transmitter is needed for spot frequency operation, a phasing unit will perform in a satisfactory manner.

Where a high degree of performance in the SSB exciter is desired, the filter method and the phasing method may be combined. Through the use of the phasing method in the first balanced modulator those undesired sideband components lying within 1000 cycles of the carrier may be given a much higher degree of rejection than is attainable with the filter method alone, with any reasonable amount of complexity in the sideband filter. Then the sideband filter may be used in its normal way to attain very high attenuation of all undesired sideband components lying perhaps further than 500 cycles away from the carrier, and to restrict the sideband width on the desired side of the carrier to the specified frequency limit.

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