US2243440A - Wave transmission circuits - Google Patents

Description

May 27, 194T.

w. VAN B. ROBERTS WAVE TRANSMISSION CIRCUITS 2 Sheets-Sh et 1 v Original Filed Sept. 50, l936 HIGH P46 PM HIGH l/l/ BAND PASS INVENfOR warm m a. ROBERTS FILTER Ai'ToRNE May 27; 1941 w. VAN B. ROBERTS 2,243,440

WAVE TRANSMISSION CIRCUITS Original Filed se t. 50,, 1936 2 Sheeis-Sheet Huh HII

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INVENTOR nmrm mu meme-22's ATTORNEY STATI. OFFICE WAVE TRAN SMISSION CIRCUITS Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Griginal application September 30, 1936, Serial No. 103,230. Divided and this application September 29, 1937, Serial N 166,248

2 Claims. (Cl. 178-44) My present invention relates to electrical wave Fig. 4 shows a band pass filter employing simitransmission networks, and more particularly to lar reactance networks and a pair of transconhigh frequency networks of the filter type emductances, ploying electronic coupling devices. This ap pli- Fig. 5 shows an element of a section of a band cation is a division of my co-pending application 5 pass filter adapted to have increased uniformity Serial No. 103,230, filed Sept. 30, 1936, now Patent of iterative admittance over a large portion of No. 2,184,400, December 26, 1939. the transmitting band, and a high degree of The main object of this invention is to provide relative attenuation at a pair of frequencies on a wave filter utilizing electronic devices upon either side of the band, and whose transconductances and/or transimped- Fig. 6 shows a receiving system embodying ances the cut-01f frequencies and the attenuation band pass filters constructed in accordance with of the filter depends. the invention.

Such a filter possesses the important advantage Referring now to Fig. 1, there is shown a four over filters of known types that the behavior of terminal network terminated by an impedance is. the filter may be controlled by biasing potentials The shunt elements Z1 and Z2 are pure reactance for the electronic devices. By suitably adjusting networks of any nature. The dotted rectangle Y the biasing potentials of the electronic devices represents transconductance devices carrying out not only can the transmitting range of the filter the following function: A positive potential e1 at be a te ed at Will, but a so t e gain, or, if desired, the left side of Y causes current e1 1112 to fiow into the attenuation of the filter may simultaneously the rectangle from the right without any correand independently e adjusted Since the spending fiowoutward from the left. At the same ation of biasing potentials requires substantially time a positive potential 62,01; th right id of Y no power, the filter characteristics may be auto causes current e2 y21 to flow out of the rectangle matically controlled in accordance, for example, with the strength of the signals traversing the 25 filter. While these filters differ from ordinary or introd cm into a circuit a Volta e rofilters in that there is no reactive coupling bevlces f u g g p portional to current in another circuit, without tween elements, their use is subJect to the same r considerations and will be readily understood by reactwn of any Sort on the first clrcult' In the from the left. In a similar manner rectangle Z represents transimpedance devices; that is, de-

oneiskmed in the art of wave filters. diagram where am is the voltage induced in Z2 The novel features which I believe to be charby unit Current in Z1, and 221 is the Voltage acteristic of my invention are set forth in parduced m Z1 by 111111? current m the arrows Wlth' ticularity in the appended claims; the invention in the rectangle indicate he irection Of the itself, however, as to both its organization and volta e indu ed when the dlrectlons of the curmethod of operation will best be understood by rents are as shown by the solid headed arrows.

reference to the following description taken in The iterative impedance is of the four terminal connection with the drawings in which I have structure is found as in the case of. an ordinary indicated diagrammatically several circuit OI- 3 1' ection; by olving for the input impedganizations Whereby my invention may be Carried ance e1/i1 with is considered as an arbitrary terinto effect. 40

minating impedance, and then equating this input impedance to k. This equation defines lo. Having found It, the current or voltage ratio per transimpedances which are dissimilar section of filter terminated y its iterati.Ve im- Fig. 2 shows a high pass filter section utilizing l5 fi f k 15 P y the 62/61 similar reactance elements-and a pair of screen 22/21 and Subt1tutmg m the expresslon for grid tubes as transconductances either of these ratios the value of already found.

Fig 3 Shows a band pass filt r Section utilizing The carrying out of the calculation above outimilar reaetance tw rk and pair of screen lined is simple but tedious, and it will be sufiicient grid tubes so connected as to perform the funchere to note that as a result there is secured the tions of transimpedances, I following:

In the drawings: Fig. 1 shows schematically a filter section utilizing reactance elements, transconductances and Now since both Z1 and Z2 are pure reactances, the first term in the denominator of Equation 2 is pure imaginary. On the other hand, all the quantities under the radical sign of Equation2 which is the current, or voltage, ratio per section for the range of frequencies specified by requiring the quantity under the radical of Equation 2 to be positive.

In the case of ordinary filters composed of inductances and capacities only, the theoretical filtering action that occurs per section of a filter having an infinite number of sections may be fairly clearly approximated in a filter having only a few sections provided the latter is terminated by a resistance which is preferably taken equal to the value of the iterative impedance of the last filter section evaluated at the mid-frequency of the band in the case of a band filter, or at a very low frequency in the case of a low pass filter, or at a very high frequency in the case of a high pass filter. It will be understood that the same considerations apply in the case of the present invention, and that preferably when the iterative impedance of the filter section (given by Equation 1) is changed the terminating resistance will be correspondingly changed.

To provide a true filter action the ratio given by Equation 3 should be independent of frequencies over a range of frequencies. If 1 12 and 1/21 and em and 221 are each independent of frequency, the ratio is also independent if yizzzi yzizm, or if y12=y21=0 or if z12=z21=0. The first of these conditions leads to structures having two transmission bands, or, as a special case, a structure amplifying all frequencies uniformly. The second and third conditions, however, lead to structures much simpler, and of more practical importance, some of which will be described more in detail.

Fig. 2 shows a high-pass structure where z1z=z21=0 and Z1=Z2=1/y. In this case the current ratio per section is simply:

'yiz y PW/21 ti /121121 which is constant for all frequencies from infin ity (where 1:0) down to the frequency at which lyl l/ 21122121 and throughout the range of frequencies so defined has an absolute value adjusted to a point of maximum reversed plate 7 current so that the internal plate resistance is.

extremely high, while the transconductance between control grid and plate current is negative.

Of course, in any of the filters of the invention any suitable negative transconductance device may be used. In fact, an ordinary tube may be used if means are provided for exciting its grid with voltage proportional to e2 of Fig. l, but opposite in phase from e2. However, as the particular devices that may be used in the electronic portions of the filter are not part of the present invention, they will be indicated merely symbolically by a circle including a control element and a current electrode. Whatever kind of device is used, means for supplying operating and control voltages, and for blocking off undesired direct current potentials where necessary, will be understood to be supplied in any of the well known ways.

Since the cut-off frequency of the high pass filter of Fig. 2 is determined by the value of while the gain per section is this filter may be controlled both as to gain and cut-off by suitable variation of 11 12 and 11 21. If capacitive admittances are substituted for the inductive admittances y of Fig. 2, the filter becomes a low pass filter, and may advantageously be used in the audio frequency system of a radio receiver where the audio frequency gain and cut-off frequency, or both, are desired to be controlled automatically.

For example, in receiving weak signals accompanied by considerable noise it is desirable to lower the cut-off frequency of the audio system. This may be done automatically by controlling either 11 12 or 1 21, or both, by a voltage derived from rectification of the incoming carrier. If both 3112 and 11121 are varied by similar factors, the gain will remain unaltered. However, if a control voltage developed from rectification of the audio output of the detector is applied to superpose an opposite variation of 3112 and 1/21, the band width will not be affected by this superposed variation but the gain will be altered in accordance with the average strength of audio signals. This control may be used either for compressing or expanding the range of audio intensities.

Fig. 3,shows a low impedance band pass filter where Z1=Z2=Z, y1z=y21=0, and 212 and 221 are independent of frequency. Both the tubes I and 2 are ordinary screen grid tubes. The voltage on the grid of tube I is directly proportional to input current and inversely proportional to frequency, while the induced voltage in the output circuit is directly proportional to plate current of tube I and to frequency. Likewise, the voltage on the grid of tube 2 is proportional to output current and inversely to frequency while the voltage induced in the input circuit by the plate current of tube 2 is proportional to its current and to frequency. Hence, the induced voltage in either circuit is proportional to current in the other circuit, and independent of frequency. As a practical matter, to avoid increasing reactive coupling through unavoidable tube capacity, it is advisable to connect the grid of the tube 2 acting as 221 across only a fraction of the total capacity reactance of the output branch as shown in Fig. 3. In this. case, from Equations 1, 2 and and the current ratio per section is over the band of frequencies where and the current, or voltage ratio per section is over the band of frequencies wherein Fig. shows a particular structure that may be substituted for the reactance networks of Fig. 4 to provide a more uniform value of iterative impedance over the transmitted band together with high relative attenuation of frequencies just outside of the band. The high attenuation is obtained by arranging the structure to be series resonant at frequencies just outside the band on either side thereof. The uniform value of 1/10 is obtained by making the admittance of the structure low relative to over most of the band, the approach of resonance at the outlying frequencies, however, insuring that the admittance rises rapidly near cut-ofi to the value vymyzi which it must have for cut-off. In each of Figs. 2, 3 and 4 there is shown the terminating resistance K across the output terminals of the filter; the manner of choosing the proper termination has been discussed previously.

In Fig. 6 there is shown a superheterodyne type of radio receiver which employs an I. F. amplifier consisting of a two section band pass filter; the filter sections being essentially those of Fig. 4. The amplifier .tube II! has its input electrodes coupled across the resonant input circuit I I, the latter being tuned to the operating I. F. which may be chosen from a range of 75 to 450 k. c. The I. F. energy source may be any desired type of converter, or first detector, network; those skilled in the art are fully aware of the superheterodyne type of construction wherein the I. F. amplifier is preceded by one, or more, stages of tunable radio frequency amplification and a first detector. The resonant circuit I2, tuned to the I. F. is connected between the output electrodes of the amplifier Ill. The negative tramsconductance element of the first filter section comprises the .tube I3, whose anode is connected to the high alternating potential side of input circuit II. The anode of tube I3 is maintained at the desired positive voltage by connecting the low alternating side of input circuit II to the source of positive voltage B (not shown). A direct current blocking condenser I4 is connected between the grid of tube I0 and the anode connection to 7 circuit II. The cathodes of tubes Ill and I3 are grounded, and initial negative grid biasing sources I5 and I6 furnish the grid biases for tubes I0 and I3 respectively. I

The input electrodes of tube I3 are coupled to the circuit I 2 by means of the reversing transformer II; the secondary winding II of the latter bein connected in series, between the grid of tube I3 and the negative terminal of bias source I6, with a pulsating current filter resistor I8. The section comprising circuits II, I2 and tubes I0, I3 provides a high impedance band pass filter. The grid of tube I3 is excited with a voltage degrees out of phase with the output voltage developed across circuit I2 by virtue of the reversing transformer I1. Hence, there is simulated the eifect of negative transconductance between the input and output circuits I I, I2.

The following band pass filter section includes amplifier I9 whose input electrodes are connected across the circuit I2. The negative transconductance element for this second section is provided by tube 20 having its grid connected to the secondary winding 22' of reversing transformer 22. The filter resistor I8 is connected in series between the winding 22 and the negative terminal of bias source I6. The anode of tube 20 is connected to the positiv voltage source B through the coil of circuit I2. The output circuit 2| is tuned to the operating I. F., and it is connected between the output electrodes of amplifier I9. The sources I5 and I6 provide the normal negative biases for the grids of tubes I9 and 20 respectively; the cathode of the latter tubes being grounded. The reactances of circuit I2 are one half as large as the reactances of the end circuits II and 2I of the filter.

The terminatin resistance of the filter comprises the diode rectifiers 24 and 25, the load resistors 24' and 25' thereof, and the audio utilization network. The diode 24 has its anode connected to the high alternating potential side of circuit 2I through the condenser 26. The diode cathode is at ground potential. The load resistor 24 is connected between the electrodes of diode 24, and the audio frequency component of rectified I. F. current is impressed upon the grid of audio tube 311 through condenser 3I connected to the anode side of resistor 24'. The gain of each of amplifiers I0 and I9 is varied automatically by connecting the grids of the amplifiers, through proper pulsating current filter resistors 32, to the AVG connection 23 leading to bias source I5. The positive terminal of source I5 is connected to resistor 24' by an adjustable tap element; 49. In this way the magnitude of the AVG bias can be selected; the source I5 providing the maximum amplification bias for each amplifier III and I9.

The :diode 25 is connected in reverse manner across circuit 2|. Condenser 4I connects the diode cathode to the terminal 42 of circuit 2|, Whereas the diode anode is grounded. The load resistor 25 is connected between the electrodes of diode 25 and the audio voltage component of rectified current, flowing through resistor 25', is impressed upon the grid of audio tube 59 by condenser 5|; the latter connecting the grid of tube 50 to the positive side of resistor 25. The audio amplifier tubes 30 and 59 are connected in pushpull relation; the manner of connecting the diodes 24 and 25 to circuit 2I making it possible to operate the audio amplifier grids in push-pull from the diode load resistors.

The gain of each of tubes I3 and 20 is automatically varied by connecting the ABC lead 23 (these letters designating the automatic band width control circuit) to a desired point on load resistor 25. The grids of tubes I3 and 29 are connected, through bias source I6, to the adjustable tap element 60, and the latter can be varied to adjust the magnitude of the band width control bias to be impressed on tubes I3 and 20. The source It provides cut-off bias for tubes I3 and 20 in the absence of received signal energy. When signal energy is rectified by diode 25, a positive direct current voltage is developed for overcoming the initial cut-ofi bias due to source I6. The positive bias applied to tubes I3 and 20 increases with signal carrier amplitude increase, and, hence, the transconductance of each tube I3 and 20 increases.

The audio output of the push-pull stage 3B-5ll may be utilized in any desired manner. For example, one, or more, audio amplifiers may follow, and a final reproducer will terminate the receiver system. The anode side of resistor 24' is preferably connected to control the gain of one, or more, of the signal transmission tubes preceding I. F. amplifier I0. Since the AVG connection to the pre-I. F. stages will furnish effective gain control, the tap 40 can be set at an intermediate point on resistor 24'.

The taps 40 and 60 are related in adjustment in a predetermined manner. In general, the most effective band width control is secured by adjusting tap 60 towards the positive end of resistor 25, and setting tap 40 adjacent the cathode terminal of resistor 24'. This is because of the fact that the band width. in the filter depends on the square root of the product of the positive and negative transconductances of the two tubes in each section; whereas the amplification of each section depends on the square root of the ratio of said transconductances. From this it follows that moving tap 60 to the upper end of resistor 25, with similar moving of tap 40 to the lower end of resistor 24, results in an increase in the band widening factor and a decrease in amplification as the signal amplitude increases. The receiver may be operated without any ABC by moving slider 60 to the grounded end of resistor 25'.

However, if freedom of adjustment is desired to secure other relations between band width and amplification, the taps may be permitted to remain independently adjustable. The diodes 24 and 25 may be a tube of the 61-16 type if desired, since the latter comprises a common tube casing housing the electrodes of two diodes. Further, the condensers 26 and II may be adjusted in magnitude to maintain a proper impedance match between the filter and its terminating resistance, as the product of the transconductances is varied.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A wave filter composed of sections each of which consists of a pair of pure reactance networks having positive one-way transconductance therebetween in one direction, and a negative one-way transconductance therebetween in the opposite direction, said filter being terminated by substantially its iterative impedance.

2. A filter as defined in claim 1, wherein each reactance network is arranged to provide rapid cut-off and high attenuations of frequencies close to but outside the transmitted band, and also a substantially constant iterative impedance over the greater portion of the'transmitted band.

WALTER VAN B. ROBERTS.

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