US2892157A - Modulators - Google Patents

Description

June 23, 1959 K. c. SCHLANSKER ETAL 2,892,157

MODULATORS Filed Oct. 21, 1955 2 Sheets-Sheet 1 OATHODE FOL LOWER WAVE GUIDE INTEGRATING NETWORK sounca OF HIGH I/7 FREQUENCY AMPLIFIER AMPLIF IE R INVENTORS.

ATTORNEY M w w m TRANSDUCER Y TRANSFORMER SOURCE OF HIGH FREQUENCY AMPLIFIER June 2-3, 1959 I c, scHLANSKER ET'AL 2,892,157

I MODULATORS I Filed Oct. 21, 1955 2 sheets-sheet 2 PHASE SPLITTER RISE RATE LIMITER INVENTORS.' KERMIT C. SCHLANSKER MITCHELL M. HANNOOSH ATTORNEY United States Patent Wayne, Ind., assignors to International Telephone and Telegraph Corporatioh Application October 21, 1955, Serial No. 541,995 9 Claims. (Cl. 332-54) This invention relates to modulators and is particularly directed to-means for modulating a high frequency wave with a signal wave by attenuating the high frequency wave.

Microwave energy flowing in a waveguide can be modulated by inserting a vane of lossy material into the waveguide. Attenuation by absorption of the microwave energy is approximately proportional to the depth of penetration of the vane. Attempts have been made to drive the vane mechanically by the signal wave so that the microwave at the output end of the waveguide will have an envelope representative of the signal wave. Unfortunately, such a system requires a mechanically movable vane, with attached parts, which has finite mass and hence has a limited frequency range. Such a system is particularly diflicult .to manage when transients or steep wave fronts appear in the signal circuit. Further, when feedback circuits are coupled for improving the distortion factors, troublesome oscillations are encountered.

The object of this invention .is a high frequency modulator of an absorption type which is substantially free of the disturbing influences of transient signals.

Another object of this invention is a modulator of the absorption type in which feedback circuits are effectively employed for distortion reduction without ringing.

The objects of this invention are attained with a movable attenuating element associated with a source of signal voltage and a source of high frequency carrier waves, withelectro-magnetic means for driving the element. Means are provided for sampling the velocity of said element and a feedback circuit for applying a voltage proportional to the velocity to the input circuit of a signal amplifier. A second amplifier, cascadedwith the first, has the characteristic of attenuating frequencies above the signal frequencies so that steep wave-front signals may not pass to disturb the loop.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent andthe invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a block diagram of the modulator system of this invention;

Fig. 2 is a sectional view of the electro-magnetic elements of the modulator ofthis invention; and

Fig. 3 is a diagram of the essential circuits of Fig. 1.

The particular circuit for the high frequency to be modulated and, shown in the accompanying drawings is a waveguide 1. A vane or card 2 is inserted through and slideable in a slot in the side of the waveguide, the card being of a lossy material which will absorb various amounts of microwave energy moving in the waveguide depending upon the depth of penetration of the card into the waveguide. It will appear that this invention is not necessarily limited to waveguides and to microwave en-, ergy. The energy to be modulated may flow in any type Patented June 23, 195

2v or transmission line with appropriate attenuators in the line which are responsive to movement.

The attenuating paid 2, as best shown in Fig; 3, is reciprocally driven by the armature 3, the power for moving the armature from its mid-position of rest being supplied by the coil 4 which is coupled to the output transformer 17 of a power amplifier for the signal voltage, to be described.

The attenuator card shown has the advantage that the card can be shaped by empirical methods to produce any desired curve of attenuation versus insertion. Once the proper shape of the card is obtained, its characteristics can be repeated in manufacture with a high degree'of accuracy. In one application of this invention it was required that the waveguide power he expressed in decibels and that the decibels vary sinusoidally. The attenuator card was shaped to hold a linear relationship between power, in decibels, and card insertion so that with the application of sinusoidal motion to the card, the desired modulation was attained.

In the diagram of the system of Fig. 3, the dipping attenuator card 2 is atached to and driven by a loudspeaker- type transducer 3 and 4. A pick-off coil 5 is wound concentrically with the driving coil 4 of the transducer to produce a feedback voltage proportional to the time rate-of-change of coil flux, and thus proportional to coil velocity. It can be shown mathematically that the feedback voltage V across coil 5 is proportional to XmK w cos wT or K jwX where I X=Vane displacement Xm =Maximum vane displacement K =Coil parameters, including flux density, number of turns, and coil radius T=Time The last expression discloses the fact that the ratio of feedback voltage to motion increases proportionately with frequency, and that the feedback voltage phase leads the phase of motion by For a constant input voltage, motion decreases with frequency and differs in phase from the input voltage by 90. It willbe understood, of course, that the electrical and mechanical gain of the system is complex and is a function of frequency. Normally, this gain, A, is so chosen thatthe approximation AK fw 1 is true near the operating frequency.

Since it is undesirable that the motion differ from the input signal voltage in phase or that its amplitude should change with frequency, a compensating RC network, which differentiates the input signal, is used as the network in the amplifiers of the system.

For example, the signal V is combined with the feedback voltage V in Fig. 1 or 3 to produce the amplifier input voltage V By superposition it can be shown that with respect to the input will be 0 and no change in.

amplitude with frequency will occur. Of course, this isnot true at high or low frequencies where gain A is low in magnitude. It follows that the linearity of mo-- tion and hence linearity of modulation depends on linearity of feedback.

In the expression K J'wX, the feedback voltage V distortion will result if K is not a constant. The most likely cause for variation in K is a change in field strength. In order to prevent coil position from affecting the magnitude of the flux linking the coil, the length of the coil should be greater than that of the magnetic gap in which it moves. In Fig. 2, for example, coils 4 and 5 preferably extend measurable distances beyond the ends of the air gap between the N-S poles.

In Fig. 3 the driving coil 4 is coupled in push-pull to the output of power amplifiers and 11. The grids of 10 and 11 are capacitively coupled to the output cir- (units, respectively, of the amplifiers 12 and 13. Amplifiers 12 and 13 produce push-pull driving voltages for power amplifiers 10 and 11. To obtain the necessary split phase from a single terminal source, the cathodes of amplifiers 12 and 13 are biased by the common cathode resistor 14 so that as the space current of one tube rises, the space current in the other tube falls. In the particular circuit shown, the control grid of the tube 12 is capacitively coupled to the single amplifier 15 while the grid of tube 13 is coupled to the anode of the power amplifiers 10 and 11 by way of coil 16 wound on the core of the push-pull transformer 17. Coil 16 and its feedback coupling to the grid of amplifier 13 constitutes an inside loop for purposes which will presently appear.

The grid of amplifier 15 receives the voltage V which is combined V and the feedback voltage V Since the movable elements of the modulator have finite mass, the attenuator card cannot follow signals with steep wave fronts in the absence of infinite or high power from amplifiers 10 and 11 to drive the moving parts. Where the input signal V may be switched into circuit at any point on the sinusoidal signal wave, source switching transients could result in objectionable long out-ofphase vibration of the attenuating vane. Since in some applications the input signal voltage may be controlled by relays, the input voltage may step suddenly from zero to its maximum value. If the relay closed at 0 or 180 on the signal wave, no transients would result; however, relay closure at 90 or 270 results in a maximum transient. In general, the transient consists of a dip below or above zero in the centerline of the motional sine wave followed by a slow return to a position symmetrical about the zero axis. The duration of the transient seems to be proportional to the magnitude of the velocity feedback.

The following explanation is offered for these transients. At the instant of switching, the transducer is at rest and is in zero position. Now, when the input signal is switched to a value corresponding to the peak position of the transducer in a length of time so short that it cannot be followed by the transducer, the phase relations of the voltages in the feedback line are meaningless. As soon as the input signal begins to move with a velocity comparable to that which the transducer is able to follow, the transducer commences to move in the correct direction; however, its motion starts at zero position rather than at the peak. Thus, on the first oscillation the transducer swings to one side of zero by an amount equal to the peak-to-peak amplitude, and this constitutes a loss in position equal to the peak amplitude of the signal. The restoring force of the transducer springs (not shown) returns to a position symmetrical about zero. The resistance of velocity feedback to this zero restoration results in a transientlasting several cycles.

According to an important feature of this invention, all signals entering the amplifier at the grid of amplifier 15 have a rise rate which the transducer is able to follow without losing position. The maximum tolerable rise rate depends upon the mass of the moving system and upon the variable peak power for moving that mass.

the rise rate of input signals to have a limitation dictated by these mass and power factors. However, the low distortion and phase-shift requirements preclude any ordinary frequency sensitive elements in the amplifier circuits. According to this invention a pro-amplifier is employed characterized by little phase shift at the operating frequency 'but which limits signal amplitude sharply on higher frequencies.

As shown in Fig. 3, the network 20 in the output circuit of amplifier 21 has decreasing gain with increasing frequency. Amplifier 21 is coupled through frequency insensitive coupling resistors 22 and 23 to the output circuit of amplifier 24. The source of signals and noise voltages or steep-wave front voltages are applied to the grid of amplifier 24. The time constant of network 20 is determined principally by shunt condenser 26 and series resistors 27 and 28, and the constants are so chosen that no limiting occurs in the plate circuit of tube 21 for frequencies within the signal band.

Amplitude limiting may, if desired, be imposed upon the signal to be supplied to the transducer by the voltage clamp including diodes 30 and 31 coupled with reversed polarities between ground and the anode of amplifier 21. If signals of greater amplitude, or higher frequencies at maximum amplitude, enter the rise-rate limiter, as shown, the signals do not pass. The output of the network 20 is applied to the control grid of the cathode follower 40, the cathode of which is tied directly to the cathode of the first amplifier 24 to feedback the limiter output voltage to its input. Since there may be over 60 decibels of feedback in the limiter loop, phase shift and signal distortion are low up to the point at which limiting occurs. Signals at the cathode of tube 40 resulting from a sudden switch closure at the input of tube 24 are sawtooth in shape, the slope of the ramp of the sawtooth being easily adjusted by the network 20 so that the positional phase of the transducer vane is not lost. Since the transducer can follow the initial sawtooth, no motional transient occurs.

When a large amount of feedback is used in the outer loop, it may be necessary to use additional feedback from sensing coil 16 to the input of amplifier 13. The feedback from coil 16 on the core of the tube transformer is conveniently made to the grid of tube 13 of the phase-splitting pair 1213. The input to this inner loop is at the grid of the other amplifier 12. Large capacitors are used in both the inner and outer loops to reduce low-frequency shifts. The transformer feedback makes loop closure simpler in that it lessens high-frequency phase shifts.

' our invention.

What is claimed is:

1. A modulator system comprising a source of highfrequency energy, a movable attenuating element associated with said source for modulating said energy in accordance with the position of the element, electro-mechanical means for driving said element, means for detecting the velocity of said element, a first and second amplifier coupled in cascade between a signal source and said electro-mechanical means, said first amplifier including rise rate-limiting means for limiting the frequency and amplitude of the signal coupled to the second'amplifier, and said second amplifier having an input circuit coupled to the velocity detecting means.

2. A modulator system comprising a high frequency transmission line, a movable attenuating element associated with said line for modulating said high frequency in accordance with the position of said element, electromechanical means for driving said element, amplifier I means coupled between a modulating signal source and In order to minimize peak power requirements of the amplifier coupled to driving winding 4, it is desirable for said electro-mechanical means for driving the same, rise rate-limiting means coupled to said amplifier means; said rise rate-limiting means including a network for limiting the frequency and amplitude response thereof, and a feedback loop in said rise rate-limiting means for minimizing distortion of the signal which is within the aforesaid frequency and amplitude response.

3. A modulator system comprising a high frequency transmission line, a movable attenuating element associated with said line for modulating said high frequency in accordance with the position of said element, electromechanical means for driving said element, amplifier means coupled between a modulating signal source and said electro-mechanical means for driving the same, rise rate-limiting means coupled to said amplifier means; said rise rate-limiting means including an amplifier having an output circuit, a condenser in shunt with the output circuit, a resistor in series with said output circuit, and a voltage clamp in shunt with said output circuit whereby the signals appearing in said output circuit will be limited in frequency and amplitude.

4. A modulator system comprising a high frequency transmission line, a movable attenuating element associated with said line for modulating said high frequency in accordance with the position of said element, electromechanical means for driving said element, amplifier means coupled between a modulating signal source and said electro-mechanical means for driving the same, rise rate-limiting means coupled to said amplifier means; said rise rate-limiting means including an amplifier having an output circuit, a condenser in shunt with the output circuit, a resistor in series with said output circuit, a voltage clamp in shunt with said output circuit whereby the signals appearing in said output circuit will be limited in frequency and amplitude, said amplifier also having an input circuit, and a feedback loop coupled between said output and input circuits whereby distortion in the signals appearing in said output circuit will possess minimum distortion.

5. A modulator system comprising a high frequency transmission line, a movable attenuating element associated with said line for modulating said high frequency in accordance with the position of said element, electro-mechanical means for driving said element, amplifier means coupled between a modulating signal source and said electro-mechanical means for driving the same, rise ratelimiting means coupled to said amplifier means; said rise rate-limiting means including an amplifier having an output circuit, an integrating network in said output circuit, a voltage clamp in shunt with said integrating network whereby the signals appearing in said output circuit will be limited in frequency and amplitude, said amplifier also having an input circuit, and a feedback loop coupled between said output and input circuits whereby distortion in the signals appearing in said output circuit will possess minimum distortion.

6. The system of claim 5 and including a cathode follower coupled to said output circuit and a resistor common to both said cathode follower and said amplifier, said resistor constituting a part of said feedback loop.

7. A modulator system comprising a high frequency transmission line, a movable attenuating element associated with said line for modulating said high frequency in accordance with the position of said element, electro mechanical means for driving said element, amplifier means coupled between a modulating signal source and said electro-mechanical means for driving the same, rise rate-limiting means coupled to said amplifier means; said rise rate-limiting means including an amplifier having an output circuit, a second amplifier coupled to said first amplifier, a frequency and amplitude-limiting circuit coupled to said second amplifier, a cathode follower coupled to this last-mentioned circuit and having a cathode resistor, said resistor being coupled to said first amplifier for providing a feedback loop thereto.

8. The system of claim 7 wherein said frequency and amplitude-limiting circuit comprises an integrating network which is shunted by a voltage clamp whereby the signals which are supplied to the cathode follower are limited in frequency and amplitude.

9. The system of claim 7 wherein said frequency and amplitude-limiting circuit comprises an integrating network which is shunted by a unidirectionally conducting device whereby the signals which are supplied to the cathode follower are limited in frequency and amplitude.

References Cited in the file of this patent UNITED STATES PATENTS 2,078,302 Wolfe Apr. 27, 1937 2,266,168 Crabtree Dec. 16, 1941 2,414,546 Nagel Jan. 21, 1947 2,481,533 Pratt Sept. 13, 1949 2,505,557 Lyman Apr. 25, 1950 2,574,253 Duffy Nov. 6, 1951 2,697,168 Spaulding Dec. 14, 1954 2,750,565 Mercer et a1 June 12, 1956 2,777,993 Braden Jan. 15, 1957

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