As an illustration of one of the many k types of special equipment built by the General Radio Company, we are showing a photograph of a new self-developing string oscillograph.
This oscillograph automatically develops its own photographic records and, accordingly, is of unusual value in commercial research or in adjustment of control circuits wrhere the results of any change must be seen at once.
The sensitized paper is fed into a constantly revolving cylinder, carried past the shutter, and then on through
A three-element camera oscillograph with the self-developing feature both developing anil fixing solutions, so that the record is available within a few seconds of the time of exposure. A wide range of operating speed is available and satisfactory oscillograms can be made with paper speeds up to about 15 inches per second.
The action is controlled by a two-position lever which feeds the paper in the first position, and cuts and stops the paper in the second. A three-string harp is standard, and the camera, driving motor, light source, timing units, and controls are all mounted on a portable table.
We have available at the present time two extra oscillographs of this type, designed for operation from 110 volts, d-c. They are priced at $3000.00 each, subject to prior sale.
A FREQUENCY DEVIATION INDICATOR FOR TRANSMITTERS
General Order No. 116 of the Federal Radio Commission, which requires radio broadcasting stations to hold their transmitter frequencies to within ±50 cycles per second of the assigned channels, places on the station frequency monitoring equipment more rigid requirements than have heretofore been necessary.
A highly stable piezo-electric oscillator for use as a monitoring standard of frequency was described by James K. Clapp in the last issue of the Experi-rn enter.
While the frequency standard is the most important element of the monitoring system, General Order No. 116 requires, by implication, that an accurate means be available for comparing the frequency of the transmitter with that of the monitoring standard. Under the old 500-cycle tolerance, a zero audible beat or any audible beat note below 500 cycles was sufficient, while under the new order, the beat-frequency indicator should be accurate to within a few cycles per second.
The design of a frequency meter to operate from zero to 50 or 100 cycles per second is extremely difficult since it involves the measurement of both audio and sub-audio frequencies, and, if it actually operates down to zero, it must cover an infinite frequency range. Even if it operates only between one and 50 cycles per second, the frequency
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Figure 1. The frequency monitor consisting of a Type 575-D Piezo-Electric Oscillator and a Type 581-A Frequency Deviation Meter
Figure 1. The frequency monitor consisting of a Type 575-D Piezo-Electric Oscillator and a Type 581-A Frequency Deviation Meter range covered lias a ratio of 50 to 1. In general, an accurate frequency meter can cover only a narrow range of frequency ; and, as the range becomes smaller, the accuracy increases accordingly.
Since the problem of measuring the deviation of a radio transmitter from a known standard is concerned with the actual deviation in cycles rather than the percentage deviation, it is immaterial what actual value of beat frequency corresponds to zero deviation as long as the variations above and below this value can be measured. A practical answer, then, is to move the normal operating point up in the audiofrequency spectrum until 50-cyele deviations on either side correspond to smaller percentage changes in tbe audio frequency. This can be realized by using as a monitoring standard a crystal whose frequency differs from the assigned broadcast channel by, say, 1000 cycles per second. The frequency meter can then be designed to read 50
cycles per second above and below 1000. The 1000-cycle difference need not appear on the frequency indicator which can be arranged to read zero at 1000 cycles per second.
General Radio Type 581-A Frequency Deviation Meter is specifically designed to meet these requirements. It consists of a 1000-cycle frequency meter, preceded by a detector and a two-stage audio amplifier.
Voltages derived from the unmodulated master oscillator of the transmitter and from the monitoring standard are impressed on the detector and the resulting audio-frequency beat is amplified and applied to the frequency meter. The frequency indicator is a large pointer-type meter reading zero at 1000 cycles per second and indicating deviations of 100 cycles above and below this value. The scale is sufficiently open to indicate changes of one cycle per second.
The frequency meter itself is a tuned circuit arrangement, as are nearly all
such instruments which cover narrow frequency ranges.
Several new features are involved which permit high accuracy to be achieved at low cost, a factor which is important if the meter is to he commercially acceptable. Another advantage lies in the fact that it indicates continuously the direction, as well as the magnitude of the frequency deviation. A glance at the meter tells the operator what adjustments he must make to bring the station to the proper frequency.
Type 581-A Frequency Deviation Meter is intended for use with the Type 575-D Piezo-Electric Oscillator. An assembly of the two instruments is shown in Figure 1.
A functional block diagram of the frequency meter assembly is shown at the right of Figure 2. A schematic diagram of the frequency meter itself is given in Figure 3. It consists of two tuned circuits, and two rectifiers connected in opposing directions. The difference of the currents from these rectifiers is indicated by a meter which is calibrated directly in cycles per second.
Figure 2 is a block diagram of the entire monitoring system. In this diagram, f0 is the assigned channel frequency and / is the deviation of the
Figure 3. Schematic diagram for the frequency indicating element of the deviation meter
Figure 3. Schematic diagram for the frequency indicating element of the deviation meter transmitter from that frequency. The transmitter frequency is accordingly fo =t /. The crystal oscillator operates at a frequency 1000 cycles per second above the assigned channel and its frequency may be expressed as fQ -+-1000.
When the crystal oscillator and transmitter voltages are impressed on the detector, the resulting audiofrequency beat tone is 1000 cycles =Ff.*
If / is zero, that is, if the transmitter is on frequency, the beat is 1000 cycles per second and the indicator is at zero. An increase in transmitter frequency produces a deflection to the right; a decrease one to the left. The scale is 200 cycles wide, that is, deviations of 100 cycles either side of zero can be read on the meter.
*The crystal frequency can, of course, be either above or below the channel frequency. The sign of the deviation indication can be reversed by reversing the leads to the frequency indicator.
THE "CLASS B" MODULATOR FOR AMATEUR PHONE TRANSMITTERS
By means of "the so-called Class B" type of amplifier, the power output from an amplifier using standard types of tubes may be greatly increased over that possible in the conventional amplifier system.
The circuit arrangement is that known as a push-pull amplifier. The
Schematic diagram for a low-power amateur phone transmitter utilizing General Radio
Type 292 Transformers in a "Class B" Modulator
Schematic diagram for a low-power amateur phone transmitter utilizing General Radio
Type 292 Transformers in a "Class B" Modulator gain in power is obtained by a shift in the operating point of the tube on its characteristic so that grid current is taken. The plate current cuts off entirely in each tube during one-half of the cycle. The current in the B lead is therefore not constant, as is the case with the standard push-pull amplifier, but varies cyclically. This difference is of importance in considering power-supply design.
One of the most interesting applications of the "Class B" amplifier is in radio-phone transmitters of moderate power. By means of this circuit, tubes of the 210-type can be made to pro duce sufficient power for 100% modulation of 50-watt tubes.
The circuit is shown above. Two new General Radio transformers are announced for use in this circuit. One is used as an input push-pull transformer between two 245 and 210 stages and the other is used as the output transformer, coupling the "Class B" modulating amplifier to the radio-frequency amplifier.
The new transformers are: Type 292-A Input Transformer, price $7.00. Type 292-B Output Transformer, price $10.00.
rriHE GENERAL RADIO COMPANY mails the Experimenter, without charge, each month to engineers? scientists, and others interested in commun-ication-frequency measurement and control problems. Please send requests for subscriptions and address-change notices to the
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