Patent, applied for. Licensed under patents of the A. T. and T. Co.

Patent, applied for. Licensed under patents of the A. T. and T. Co.


"With literally thousands of R-C band switch oscillators in use throughout the industry, we believe it is high time, particularly with the many younger engineers entering industry who may not have had experience with other types, to mention a few favorable words about the

This article is one of the best and fairest summaries that we have seen of the relative performance of beat-frequency oscillators and resistance-capacitance oscillators. It is reprinted, with permission, from Lab Notes, published by Gaw-ler-Knoop Company, Sales Engineers, and was prepared by Clough-Brengle Company, one of the manufacturers whose products are distributed by Gawler-Knoop.


somewhat neglected, yet definitely deserving, beat-frequency audio oscillator.

"Basic advantages of the BFO as compared with the RCO might be summarized as follows:

"1. The BFO output amplitude is constant throughout a frequency range of usually 50 cps. to 10,000 cps. and within 0.5 db to 32 kc. It is not subject to discontinuity of output level resulting from band switching as is necessary with RC types, since no band switching is necessary.

"2. Because of insufficient overlap in band switching, the investigation of important network responses occurring at RCO dial extremes becomes an inconvenient task.

"3. Thermal and warm-up stability of the BFO is far superior to the RCO. At 1,000 cps., a typical drift figure for the BFO from a cold start is in the order of 10 cps. or 1%, but is corrected with simple zero adjustment, so such error is diminished to essentially zero. The RCO has about the same drift with no correction control. At 1,000 cps. the BFO calibration and readability accuracy is in the order of 1% or better, while the RCO is in the order of 2% or better. After a two-hour warm-up, both the BFO and RCO have substantially stabilized. Since BFO frequency drift is a constant value, not a constant percentage, the BFO will perform even better at higher frequencies.

"4. A well-designed BFO will not exhibit frequency change with output attenuator setting changes.

"5. The rated distortion value for the BFO is usually ^ of 1%, while the same figure for the RCO is usually 1%. These figures apply only above 100 cps., however.

"6. The BFO depends upon the stability of Zy rather than R. It consequently exhibits a superior calibration permanence and can take advantage of the further inherent stability resulting from a low L- to C-ratio.

"7. Much has been said of the lock-in tendencies of the BFO at low frequencies (when both high-frequency oscillators are approaching the same frequency). This problem is one of the reasons for BFO's to be more useful above 25 to 50 cps. However, let it be said that a similar problem also exists with the RCO since the tuning capacitor must operate above chassis potential, which causes it to tend to 'lock in' at power line frequency and multiples thereof.

"8. Fairness in making a comparison such as this dictates the comment that the RC circuits are ideal for low-frequency applications, generally below 50 cps., where the design problems of the BFO are extremely severe.

"Beat-frequency audio oscillators are currently manufactured commercially by General Radio Company (who, by the way, hold the basic RC oscillator patent with 34 claims) and Clough-Brengle Co."

Editor's Note: The General Radio Company manufactures two models of the RC-type of oscillator and four of the beat-frequency type. Each is designed for superior performance in its particular field of application.

It is possible, when the application justifies the additional expense, to improve greatly the performance of the RC oscillator. The Type 1301-A Low-Distortion Oscillator, for instance, affords an excellent example of how the properties of the RC-type of circuit can be used to advantage. This oscillator supplies 27 fixed frequencies between 20 and 15,000 cycles for use as test tones in the measurement of harmonic distortion. Here, by careful design of the frequency- and amplitude-controlling circuits and through the use of high-quality components, a total harmonic distortion as low as 0.1% is obtained with a frequency drift of only 0.02% per hour.

The beat-frequency oscillator has an outstanding advantage for audio-frequency testing. The frequency-control dial can be made logarithmic in frequency over a range of three decades, thus permitting the drive to be coupled directly to a recorder for the automatic recording of frequency-dependent phenomena over a range of 20 to 20,000 cycles without range switching. The RC oscillator usually requires three bands to cover this range. This logarithmic, wide-range, single-dial control is a feature of the Type 1304 Beat-Frequency Oscillator, which is widely used for measuring the amplitude-vs.-frequency characteristics of audio-fre-qucncy equipment.

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