US2624789A

US2624789A - Peak clipper indicator

Info

Publication number: US2624789A
Application number: US115582A
Authority: US
Inventors: William W H Dean
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1949-09-14
Filing date: 1949-09-14
Publication date: 1953-01-06
Anticipated expiration: 1970-01-06

Description

W. W.-H. DEAN Filed Sept. 14, 1949 T12-- :mmh L PEAK CLIPPER INDICATOR Jan. 6, 1953 Patented Jan. 6,. l1953 UNITED STATES PATENT 'OFFICE PEAK CLlPPER INDICATOR William W. H. ADean, Pointe Claire, Quebec, Canadaassignrto Radio Corporation of America,

a corporation Iof Delaware ApplicationfSeptember 14, 1949Serial No, 115,582

3 Claims.

lThis invention relates to a peak clipper indi-v cator to be used with the type of simple inexpensive peak clipper which uses but two tube structures which may be in a single envelope. `Such a `clipper is simple and of low cost, is highly efficient, linear to the point of Aclippingv and clips cleanly and symmetrically on-both'posi-t-iveand negative peaks.

In order to Yfacilitatel adjustmentof my novel clipper to vthedesired-degree of clipping, I have devised a -simple indicator of lowcurrent drain which-when added -to the clipper load, may still allow joint operation with existing equipment to which it is applied.

My clipper and indicator form -a unitv which may-with advantage replace clippers in existing equipment .using kthe same or -may l be lused -in equipment not-now using a clipper. Then-improvement in communication equal Vtea power increase of ve r ten times is obtainedv with substantially .no 4actual increasey -in power used.

Otheradvantagesto bevattainedby the usel of my invention appear Vfrom a1 detailed: description which follows. Injthe description which follows, reference. will be ymade to vthe accompanying drawing wherein the. single'gure illustrates a peak clipping ampli-ner. arranged4 in accordance with myinvention andthe same..with anim-f proved indicator for indicating thepoint at which clippingtakes place.

`In the drawing, the rectangle HJ designates a prearnpliiierinV which the modulation is amplifled asdesired. Thisampliiier may .be .conventional and includes in its outputa .level control potentiometer l2, the tapon which `isconnected to the control grid 2l of an electron: discharge tube having an anode 23 and a cathode 22. This electron discharge tubemay .be in a` separate envelope .or inthe same envelope with asecond electron discharge ldevice having itsanode 122 connected to a direct current source by a plate load resistor R. TheV cathode return resistor 2li is connected between the :athodesV of Vboth tubes and ground. The control grid 23 ofthe second tube is driven by voltages producedrin the cathode resistor 'iii because such control grid isconnected to the grounded end of resistor .2li-through a resistor 32.

The tube structures, Aas described, constitute a peak limiter of the type disclosed in Crosby Patent 2,276,565, dated March 17, 1942. y In accordance withK onaaspect of myinf/entien,A howeveruthe anodeY of the second tube,` is .coupled byl condenser C and resistors- `3!) and 32 tothe common grounded point and a point between the resistors 30 Yand 32 is connected to the control grid 28 to provide degeneration in the system to improvelinearityvin the amplificationup .to the clipping point. VClipping lintroduces harmonic distortion andthe clipped voltages are fed through a low pass filter LPF1 before further amplification in the modulator and driver MD. The ilteris necessaryto remove higher order harmonics of the speech voltages generated by the fclipper action.

The clipping indicator tube is o1" the type .comprising a double triode in a singleenvelope, although separate tubes may be used here asin the limiter. The control grid GI ofjthernrst section 3B of this tube is connected ytothe movable point on the potentiometer resistor l2. lI'he anode PI is connected by a resistor R2 to the direct current potential source. The cathodes of both tubes 36 and 38 are tied together at .point 4U and connected to groundV by a resistorR3 and potentiometer Rt. A point on the potentiometer R4 is connected by the resistorR to the control grid G2 of tube 36. The anode vP2vof this tube is connected to the direct Acurrent source. An indicator in the form of a neon vtube NT is connected in shunt to the resistorRZ.

yA bypass capacitonlil, having a capacitance value of at least 0.25 microfarad, is connected between common cathode point 49 and ground, or-in other words across the common indicator cathode resistors R3 and Re. This bypassing of the indicator cathode resistors eliminates a feedback circuit which would be formed by the common impedance of such resistors (when unie-ypassed) between the input grid 2| and the output plate 2s of theclipper tube;A thus; oscillation-in the clipper tube is prevented. A

In operation, the peak clipper -isvinphysical make-up-similarfinp-art at least, to thelimiting ampliiier described by Crosbyin-hisabove-identied patent. In the description thatvfollowsfit willbeY assumed that thel readerisfamilar With the operation oi Crosbys limiter. My4 clipper is to be used on speech signals and theoperation obviously must belinear upto thezclippingpoint. To obtain such operation, a .degenerative feedback circuit for theA last tubefstage includ-ing resistors 30 and 32 is provided. .Theresistorsl 32 are unbypassed so that signal voltagesr are -fed to Ygrid 28. This reducesv the distortion-which would take place in the clipper without-.such feedback. The degree of clippngisadjustedtby movement 0f the tap point; 0n. potentiometer. i2. as clippnestartswhenthe plateciirrentp the tubes becomes zero. The use of feedback extends the linear portion of the tube characteristic as zero plate current is approached. In operation, then, both grids are at ground potential in the absence of signals, since both tubes then are conducting, as governed by the bias placed on the grids by cathode resistor 24. The current in the tubes is then insufcient to bias them to cut oi. When signal is applied, the positive cycles on grid 2l .cause the cathode current to increase and the increase in potential drop in resistor 24 makes the common cathodes more positive relative to ground, which has the same effect as making the grid 28 more negative and grid limiting in the positive halves of the modulation cycles takes place in this tube structure. The negative potential of the modulation cycles drives grid 2| to cut oil, so that full wave limiting takes place in the tube. The amplification, however, is linear, for reasons pointed out above, up to the point at which clipping takes place. As grid 2B approaches the negative potential at which cutoi would take place because of the potential drop in resistor 24, the plate current variation is almost linear with respect to the grid voltage variation because of the feedback from anode 29 to grid 28. The rst tube is connected as a cathde follower and hence 100 per cent feedback lexists at grid 2l; therefore, 1inearity on negative half cycles is good.

The extent of clipping is indicated as follows: If no signal is applied the adjustment at potentiometer R4 is such that grid Gl of tube 33 is biased to cut-ofi and tube 35 has a positive potential on its grid G2 and draws a small amount of current through cathode resistors R3 and Ril. This is because the entire potential drop in these resistors is effective in fixing the bias on GI whose direct current circuit includes part of potentiometer I2 also. Only part of this potential drop is effective on the grid G2, so that G2 has a smaller negative bias than Gl.

Signal is fed from the preamplifier output, as adjusted in level at the potentiometer i2, to the grid Gl. Clipped signal is fed from the anode 29 of the clipper stage to the grid G2 through condenser C and an uniiunibered condenser. The preampliied signal on the grid Gl is ineiective to overcome the cut-oit bias, until it reaches a predetermined peak value, because the amplied clipped signal on the grid G2 keeps tube 36 conductive and the amplied current is rectiiied in the plate circuit of tube 35 to produce a potential drop in R3 and R4 which holds the grid Gi past ,cut-off with negative bias which is even higher than that when no signal is applied. The amplied clipped signal on the grid G2 is of fixed magnitude or level, so that as the signal fed from the preamplifier output at l2 to the grid Gl increases a point will be reached at Which the negative bias on the grid Gl is overcome and current will flow to plate Pl, through the high resistor R2. The potential drop in resistor R2 is sufficient to ash the neon bulb NT. The point at which the signal from the preamplifier l0 overcomes the bias on Gl is determined by the diierential between the no-signal biases on grids Gl and G2, which is controlled by adjustment of the potentiometer R4, and on the amplitude of the clipped wave applied to G2. The latter depends on the gain characteristic of the clipper stage. The potentiometer R4 may then be calibrated to show the differential between the biases on grids GI and G2 and with this set as desired, the level controi potentiometer l2 is adjusted so that the signal magnitude is such that peaks thereof drive tube 38 and the neon tube NT flashes on the signal peaks which exceed the desired level.

Using the constants listed below the clipper stage has a voltage gain (below clipping) of 5 and the indicator ashes at approximately 20 db of clipping. By varying the tap point of G2 on the cathode resistor R4 dierent clipping levels and different clipper stage gains may be handled.

Resistor 12 megohm-- 1 Resistor R ohms-- 47,000 Resistor 30 do 270,000 Resistor 32 do 2,700 Resistor 24 do 2,700 Resistor R2 do 270,000 Resistor R3 do 100,000 Resistor R4 do 2,000

Clipper tube type 6SL7GT. Indicator tube type 6SN7GT. NT is a 1A; watt neon tube.

The primary advantage claimed for the indicator of this invention is that 'it takes its reierence from the signal itself and thus directly compares tlie amplitudes o1' the clipped and unclipped waves. No elaborate voltage stabilization is required.

'Wnat I claim to be my invention is as follows:

l. In apparatus to indicate the amount of clipping which takes piace in a clipper amplifier having an input on which voltages representing speech are impressed and an output irom which clipped speech voltages are derived, two tube structures each having electrodes including an anode, a cathode and a control grid, bias circuits for tile grids including a cathode return resistor a portion ol' which is common to both grid circuits, tlie arrangement being such that one tube structure is conductive and the other nonconductive, connections for applying unclipped signal voltages to the grid of the nonconuuctive tube structure and signal voltages from the output of the clipper amplifier to the grid of the conductive tube structure, whereby the negative bias on the grid of the nonconductive tube structure increases due to the potential drop in the common portion oi said resistor until clipping starts in said clipper amplifier and the voltage on the grid of the conductive tube structure becomes xed, so that conduction starts in the nonconductive tube structure when the signal voltage reaches a critical amplitude, a neon tube connected to the anode 0i' said last tube structure, said neon tube flashing when conduction starts in such tube structure, and means for adjusting the resistance value of the resistor common to the grid circuits to thereby adjust the point at which the cut-off potential on the grid of the nonconductive tube structure is overcome by the unclipped signal voltages.

2. In apparatus to indicate the amount of clipping which takes place in a clipper amplier having an input on which voltages to be clipped are impressed and having also an output from which clipped voltages are derived, two electron control devices each having an anode, a cathode and a control electrode, means for supplying unidirectional potential to the anode-cathode circuits of the two devices, a common impedance in the anode-cathode circuits of the two devices through which current flows when said devices conduct, means for applying different portions of the potential developed across said impedance as biases to the control electrodes of said two devices, thereby to bias one device to conduction and the other to non-conduction, an impedance and a gas tube in parallel in the anode-cathode circuit of one device, connections for applying unclpped signal voltages to the control electrode of the non-conductive device and clipped signal voltages to the control electrode of the conductive device, and means for adjusting the bias on at least one of said devices to change the diierence between the biases on the two devices to thereby adjust the point at which the cutoff potential on the control electrode of the non-conductive device is overcome by the unclipped signal voltages.

3. In yapparatus to indi-cate the amount of clipping which takes place in a clipper amplifier having an input on which voltages to be clipped are impressed and having also an output from which clipped voltages are derived, two electron control devices each having an anode, a cathode and a control electrode, an indicator connected to the anode-cathode circuit of one device and actuated by current flow in such device, connections for applying unclipped voltages obtained from the amplier input to the control electrode of said one device and clipped voltages obtained from the amplifier output to the control electrode of the WILLIAM vv. H. DEAN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,143,094 Swift J-an. 10, 1939 2,213,099 Adorjan Aug. 27, 1940 2,403,988 Likel July 16, 1946