Radio Broadcast (May 1928-Apr 1929)

158 RADIO BROADCAST JULY, 1928 OUTPUT CHARACTERISTIC BEAT-FREQUENCY OSCILLATOR 20 40 100 200 400 1000 2000 4000 8000 OUTPUT FREQUENCY C.P.S. FIG. 4 This beat-frequency oscillator has a characteristic that makes measurements with it an easy matter — straight from below 40 to above 8000 cycles, and every cycle obtainable from a single dial! denser of 70-mmfd. maximum capacity was used as an external vernier to open up the lower range, resulting in the dotted curve in Fig. 5, The two Figures, 5, in which frequencies are Y lotted logarithmically and 6, in which frequencies are plotted linearly, give a comparison of the types of calibrations that may be expected for various condensers. A National 0.0003-mfd. semicircular plate — straight-line-capacity — condenser gives a frequency calibration that is linear with respect to dial reading, as in the solid curve of Fig. 6. Although the frequency of the oscillator varies inversely as the square root of the capacity, the tuning condenser is only a small portion of the total capacity in the radio-frequency circuit. Its variation swings the frequency over only a small part of the capacity-frequency curve, and to all intents the curve is linear over this small portion. Plotted also in Fig. 6, for comparison when using the linear frequency axis, are curves Nationa 300 A IS.L.C. . WE--—^ * j / y / / / f i Car dwell J9S ■500 MMF E / / / ✓ V I a f 3 t / / / /.--Pilot i ► :l.f. 1 S 1/ J / / / / f 70 M we / / FR / BEAT EQUENCY -FREQUEN calibrate :y oscill DNS VT0R i l 1 40 60 DIAL READING FIG. 5 The calibration curve of the oscillator can be plotted in two ways; one way is with the frequencies plotted logarithmically, or in octaves, as shown here for the 0.0005-mfd. type 192E Cardwell and for the Pilot 0.000070-mfd. straight-line-frequency condenser which was used as an external vernier condenser. The Cardwell condensers were used in the writer's oscillator. The frequency calibrations in all cases were made by comparing the sound output of the beat-frequency oscillator with standard frequencies from a Western Electric 8A oscillator. One pair of phones was connected to the beatfrequency instrument and another pair to the 8A oscillator. For any given setting of the standard, the beat oscillator was brought to approximately the same pitch and then tuned accurately by listening to the beats between the two sounds. As regards the purity of output, the beatfrequency oscillator seemed to be as good as the Western Electric 8A. The most distinct beats were found when the intensities of the two sources were approximately equal. Each time the apparatus is used the variable oscillator is set at the dial setting arbitrarily chosen as zero, and zero beat obtained by varying the trimming condenser, Q. This means that the constancy of the calibration is dependent primarily on the constancy of the tuningcondenser capacity. When a heavy condenser with substantial bearings is used for tuning, the accuracy to which the calibration may be used is limited only by the closeness to which the dial may be set. For the same reason, changes in the various operating voltages have negligible effect on the calibration; and it is not a hardship when a tube, with which the instrument was calibrated, is misplaced. CALIBRATING WITH A PIANO IT IS entirely possible to use a piano as a * source of standard frequency when no other source is available. The range from 30 to 4096 cycles may be covered in this way. Another auxiliary audio-frequency oscillator may then be set to the piano frequencies, and harmonics of it picked out to calibrate the beat-frequency oscillator in the 4000 to 10,000-cycle range. The fundamental frequencies of the notes starting from middle C, are: C 256, D 888, E 320, F 341, G 384, A 487, B 880, and C 5 12. Notes an octave higher are twice the frequency of the corresponding lower note; and likewise, notes an octave low are half the frequency of the higher note. [See Laboratory Sheet No. 52, Radio Broadcast, Dec, 1926, for a chart of piano frequencies. — Editor]. The oscillator is useful for comparison by ear of loud speakers and other electro-acoustical devices. Peaks and dips in the response stand out when the input frequency is varied rapidly over the spectrum, and upper and lower-cutoff frequencies may be approximately determined. A 10,000-ohm potentiometer, such as is used as a volume control, across the output of the oscillator allows any desired magnitude of audiofrequency voltage to be taken out for audiotransformer testing, determination of amplifier characteristics, bridge measurements, and so on. parts employed THE parts used in the writer's oscillator follow. The parts hot mentioned by name are not at all critical and any standard make may be used. The only special apparatus in the entire equipment are the coils which are described in Fig. 1 Ci 2 — Cardwell 500-mmfd. taper-plate condensers type 192E 4 — General Radio ux Sockets G> 3 — 0.00025-mfd. fixed condensers with gridleak mounts C3 4 — 0.002-mfd. Sangamo fixed condensers C4 1 — 0.006-mfd. Sangamo fixed condensers. 8 — General Radio No. 8745 plugs and 874P jacks 2 — lengths bakelite tubing, 55" x 35" Ri 2 — 1 -megohm \ R2 1 — 10-megohm Vmetallized resistors R-; 2 — 0.25-megohm; Ri 1 — Filament rheostat, 10-ohm 10 — Binding Posts 2— Bakelite dials, 4" Seven square feet 1-64" copper sheet Shielding: Sides — 4 pes. 7" x 10" — Turn up \" flange all around Bottom — 1 pc. 10" x 20" — Turn up \" flange all around Top — 1 pc. 9" x 20" Front & Back — 2 pes. 5}" x 20" 1 — Formica panel 6" x 20" Baseboard 9" x 20" x f", wire, screws, etc. The following tubes are required to put the apparatus in operation: 2 201 A type for the oscillators 1 200A type for the detector 1 1 12A type for the amplifier 10,000 8000 .6000 4000 2000 FRE( JUENCY CALIBR/ TIONS / 9 -v 1 J National 300 Mi S.L.C. j MF. <^ * irdivell I9i 500 MMF. / * C 7 E Pilot S.L.F.^ 7OMMK0^> 40 60 DIAL READING 80 100 FIG. 6 The second manner of showing what frequencies the oscillator grinds out — that is, where the frequencies are plotted "straight" and not in octaves — is shown in this curve