US2426721A

US2426721A - Stroboscopic cathode-ray oscilloscope system for comparing two frequencies

Info

Publication number: US2426721A
Application number: US521568A
Authority: US
Inventors: Robert T Adams
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1944-02-08
Filing date: 1944-02-08
Publication date: 1947-09-02
Anticipated expiration: 1964-09-02

Description

p 1947- R. T. ADAMS STROBOSCOPIC CATHODE RAY OSCILLOSCOPE SYSTEM FOR. COMPARING TWO FREQUENCIES Filed Feb 8, 1944 T? T VAR/ABLELOW F FREQUENCY r= CAL IBRATED l s v OSCILLATOH FIG. 2

I 7 135 U U s s .s 8 u u i s INVENTOR By R. r ADAMS W h 77; m ATTORNEP Patented Sept. 2,1941

' NlT STROBOSEOPEC CATHQDE-RAY OSCELO- SCOPE SYSTEM FOR (DQARING TWO FREQUENCES Robert '1'. Adams, Baltimore, are... assignor to Western Electric Eompany, Incorporated, New York, N. K, a corporation of New York Application February 8, 1944i, Serial No. 521,568

4 Claims. (on. re-e45) In the prior art frequencies have been com pared by causing them to produce stationary,- reentrant patterns known as Lissajous figures.

Where thefrequency ratio is a harmonic ratio which may be represented by small whole numbers, this method is easy to use. However, where the frequencies are nearly equal and in a ratio which must be represented by large whole numbers, the patterns produced often get so compllcated as to become worthless. Nevertheless the simplicity of the direct comparison method is very desirable.

It is the object of this invention to provide a method of and a means for directly comparing Fig. 2 is a series of Lissajous figures illustrating the principles of the invention;

Fig. 3 is a Lissajous-figure which may'result when two frequencies are nearly equal and their ratio is represented by relatively large integers; and,

Fig. 4 shows a simpler pattern derived from the pattern of Fig. 3 in accordance with the teaching of this invention.

In Fig. 1 a standard source of frequency l is applied to one pair of the deflector electrodes t of a cathode ray oscilloscope 3 while an unknown frequency source 2 is applied to the other pair of electrostatic deflector plates of the same oscilloscope. Oscilloscope 3 includes in addition to the electrostatic deflector, plates 6 an electron gun which may comprise an indirectly heated cathode I, an intensity control electrode 8, and one or more other. electrodes represented schematically as 9. While a particular form of oscilloscope tube has been shown schematically in Fig. 1, any of the conventional forms may be employed in the prac-. tice of this invention. For example, instead of the electrostatic deflector plates 6, electromagnectic deflector coils may be used or the beam may be deflected along one axis by electrostatic means and along the other axis at right angles thereto by electromagnetic means.

Connected to the cathode l and intensity control electrode 8 is a variable low frequency cali- 'brated oscillator ill- The voltage output of this oscillator is so adjusted as to periodicall blank or release the beam generated by the electron gun of oscilloscope tube 3.

As is customary in oscilloscope practice, amplifiers may be employed in any of the deflector or control electrode channels. For example, the standard frequency source may be amplified by an amplifier l, the output of which is coupled to the corresponding deflector plates through a coupling condenser and resistor as indicated in Fig. 1. Similar connections through an amplifier 5 are shown for the unknown frequency source 2.

The production of Lissajous figures by the means of a cathode ray oscilloscope is so well known that it need not be described in detail. It need only be remembered that regardless of the two frequencies combined on the deflector plates of the oscilloscope, the pattern will always be reentrant and where the frequencies approach equality the pattern will repeat itself at a rate equal to the difference between the two frequencies. In so far as the effect on the eye is concerned, this pattern becomes entirely unintelligible when the frequency difference is greater than 10 to 15 cycles per second and the frequency ratio must be represented by large numbers. The

Lissajous figures ll represented in Fig. 2 are for I the special eases where the two frequencies are equal but differing in phase as indicated by the -It is a well-known fact that where the frequency difierence is in the order of 2 or 3 cycles per second, or less, and the frequency ratio contains large integers, the discrete Lissajous figures as shown in Fig. 2 appear to successively form'one after another as ifa hoop were being rotated voltages, the standard voltage being used as a reference vector and designated with a reference character S, while the unknown voltage is rep-- resented by the vector 'U. The curved arrow adjacent the unknown voltage vector U represents counterclockwise rotation with respect to the standard frequency reference vector S.

The resulting image'on the oscilloscope screen will appear similar to that shown in Fig, 3 whenever the frequency ratio .contains large numbers the eye.

and the frequency difference exceeds about 10 cycles per second. In the previous simplified description of the production of these Lissajous figures, it must be remembered that figures ll shown in Fig. 2 are the figures that would be produced should the phase angle remainv fixed. at theflangles indicated. The actual path of the beam is a little more complicated by reason of the fact that the phase angle is uniformly shifting. If the frequencies arelarge enough, the pattern may become reentrant at a, rate more rapid than the persistence of vision and consequently the reentrant Lissajous figure'appears stationary to When the two frequencies are nearly equal, the pattern becomes reentrant once for each complete cycle of frequency difference between the standard and unknown frequency sources. Consequently, if the trace canbe blanked out once for each cycle of frequency difference,

and, for the major portion of' that cycle, substantially only one epoch of the complicated Lissajous figure l3 -of Fig. 3 willappear on the. screen-as shown at M- in Fig. 4. Furthermore,

if this blanking frequency differs slightly from the difference frequency between, the standard and unknown sources, this figure I4 of Fig. '4

will not remain stationary but will'seem to go of standard frequency to one of the beam defleeting means, means connecting the source of unknown frequency to the other beam ,d efl ecting means, means connecting the calibrated variable frequency source to the beam intensity control electrode, and a bias source for said beam in-v Y tensity control electrode, said source being adthrough the various phases indicated at l I in Fig.

2. actly equal to the difference frequency, this stroboscopic image [4 shown in Fig. 4 will be stationary.

The action just described is preferably produced by the apparatus ofthis invention by em-- ployin a variable low frequency calibrated oscillator ID as shown in Fig. 1 which produces a peaked output as indicated at Ill. The circuits When the blanking frequency is made ex-' are preferably so arranged that the beam is blanked out except during the peaked periods of 1 the peaked voltages Ill from the variable low frequency calibrated oscillator l0. By reason of the stroboscopic action produced by this low fre' quency oscillator, it is evident that the frequency thereof will be equal to the difference frequency between thestandard source I and the unknown frequency source 2 when the stroboscopic figure is made stationary; Consequently the unknown and standard frequency sources may be applied to the beam deflector electrodes of the oscilloscope 3 and the variable low. frequency calibrated oscillator adjusted until the stroboscopic image appears stationary'on the screen as shown at 84 in Fig. 4. The frequency as then indicated by the calibrated low frequency oscillator will be equal justed to normally suppress the beam until the positive peak voltage from said variable frequency source exceeds a predetermined limit whereupon said beam may be interrupted at a frequency.

equal to the difference between the standard and unknown frequencies.

3. An apparatus for ,comparing the" frequency of a source of unknownfrequency with a source of standard frequency comprising in combination a cathode ray oscilloscopeincluding horizontal and vertical beam deflectin means and a-beam intensity contro1 electrode, acalibrated variable frequency source of the type 1.-which produces peaked'voltage pulses of short'duration'relative v t0 the period of the. cycle, means connecting said source of standard frequency to one of the beam deflecting means, means connecting the source of j unknown frequency to the other beam deflecting-3 means and means connectin the calibratedvariable frequency source to the beam intensity control electrode, whereby said beam is caused to form a Lissajous figure only during the short duration of said peak voltage from the variable frequency source.

4. An apparatus for comparin t frequency of a Source of unknown frequency with a source of standard frequency, comprising in combination a cathode ray oscilloscope including hori- Zontal and-vertical beam deflecting means and a beam intensity contro1 electrode,'a calibrated variable frequency source of the type whichproduces peakedvoltage pulses of short duration relative to the period of the cycle, means connecting said source of standard frequency to one of the beam deflecting means; meansconnecting the source of unknown frequency to the other to the diiference frequency between the standard and the unknown sources.

What is claimed is:

1. The method of comparing an unknown frequency with a standard frequency comprising causing said frequencies to cooperate to produce a Lissajous figure, stroboscopically interrupting the production of said figure by 'employinga variable frequency of known magnitudaand,

adjusting the magnitude of said variable frequency until said interrupted figure appears sta- I tionary, whereupon the magnitude of said variable frequency.is a measure of the difference between the standard and unknown frequencies. 2. An apparatus for comparing the frequency of a source of unknown frequency with'a source f standard frequency comprising in combination a cathode ray oscilloscope including horizontal and vertical beam deflecting means and a beam intensity, control electrode, a calibrated variable frequency source, means connecting said sou'rce beam deflecting means, means" connecting the calibrated variable frequency' 'source to the beam intensity control electrode and a bias source for said beam intensity control electrode, saidsource being adjusted to normally suppress the beam until the positive peak voltage from said variable frequency source exceeds a predetermined limit, whereby said beam may be caused to produce the Lissajous figure onl during said peakvoltage periods of short duration.

ROBERT T. ADz s.

REFERENCES CITED The following references are of record nfile of this patent:

. UNITED STATES- PATENT Von Duhn May 27, 1941