US2557122A - Coaxial crystal detector and line - Google Patents

Description

June 19, 1951 J. P. LEIPHART 2,557,,122

COAXIAL CRYSTAL DETECTOR AND LINE TERMINATION DEVICE Filed Sept. 27, 1945 2 Sheets-Sheet 1 Ila-L -INPUT J. PLUMER LEIPHART June 9 J.\P. LEIPHARTI 2,557,122

COAXIAL CRYSTAL DETECTOR AND LINE TERMINATION DEVICE Filed Sept. 27, 1945 2 Sheets-Sheet 2 J. PLUMER LEIPHART @WLW Patented June 19, 1951 UNITED STATES PTENT OFFICE COAXIAL CRYSTAL DETECTOR AND LINE TERMINATION DEVICE amended April 30, 1928; 370 0. G. 757) This invention relates broadly to coaxial lines as used for the transmission of radio-frequency energy and, more particularly, to a coaxial device for simultaneously terminating such a coaxial 2 Since radio-frequency apparatus frequently for its activation requires that the input thereto be of a rectified or direct-current (D. C.) character, it is accordingly another object of this invenline' in a desired impedance and rectifying the 5 tion to provide coaxial terminal rectifying means radio-frequency output thereof. operative to produce, for Whatever utilization In the practice of those electrical arts that purpose and with reasonable accuracy over an contemplate the transmission of radio-frequency unusually wide range of operating frequencies, energy along a coaxial line, there frequently substantially half-Wave rectification of the arises occasion to provide a special termination radio-frequency voltage or current output from for such a coaxial line. This is commonly true, any coaxial line of given cross-sectional dimenfor example, when it is desired to connect extersions and characteristic impedance at any ternal metering or measuring apparatus thereto minal point thereof at which said rectifying for the purpose of measuring the voltage, current, means is utilized. or power output of a radio frequency transmis- An important feature of this invention is that sion system. As is well known, however, unless it provides a compact, coaxially-arranged termithe termination thus effected is such as to prenal connecting and rectifying device by means sent to the coaxial line an impedance substam of which radio-frequency measuring apparatus, tially equal to the characteristic impedance of requiring for its operation half-wave rectified the line, the observed measurements will be dis-- voltages or currents, may be operatively contorted by fi C S a s from pedance mlsnected to any terminal point along a coaxial match, reflection, and standing Waves along the transmission line of given cross-sectional dimencoaxial line. It is known also that when altersions and characteristic impedance; and by natin'g currents are caused to circulate through means of which also any inaccuracies arising a network of co ect lead-S at y igh fre from impedance mismatch, reflection, and standquencies, particularly frequencies in excess of ing Waves, or from lead inductances and stray 100 megacycles per second, the effects of lead iniring capacitances, may be minimized or elimiductances and stray Wiring capao tances ca nated entirely, over an unusually wide range of sume such serious proportions as to necessitate erating frequencies, when measuring radiospecial provision for neutralizing them. For frequency Voltages, currents, power t Said these reasons, therefore, such common methods terminal point of connecting r eq y metering p In accordance with this invention, the device tus to a coaxial transmission line as y means of comprises a parallel network, coaxially mounted Wire leads have been found to introduce serious d arranged adaptably t engage coaxial errors at freque c e pp the llltla-hightransmission line terminal connector, and confr que cy range, unless Step5 are token to sisting of a resistance in parallel with a capacipe sate therefor b the insertion of some tor and with a unidirectional rectifying crystal, justable impedance-matching device, as is known th capacitor and -m1 being connected in in the art. However, the use of such prior imseries, pedanc -m hi devices generally has the In one preferred embodiment of the invention, advantage that their tun tends to Vary the resistance comprises a non-inductive disksiderably Wit e pfi fi frequency; so that shaped resistor having a central hole thereto provide a fixed-tuned coaxial typ telminatthrough, contiguous with which and contiguous ing dev e, p e of Operation CV61 a Wide range also with the outer periphery of one side of the of ultra-hig frequencies, WOuld be of advantage disk are provided concentric annular terminal in the art. means adaptable to engage the inner and outer It is accordingly an object of this invention to conductors of a coaxial receptacle, respectively. provide a simple and compact coaxial terminat- The resistor, capacitor, and crystal each are aring device by means of which any coaxial transranged to be separately replaceable, and to promission line, of given cross-sectional dimensions vide for this contingency a separable coaxial and characteristic impedance and carrying radiomounting is employed in which they are enclosed. frequency energy at frequencies ranging from Said mounting comprises essentially: a coaxial very low to beyond the ultra-high-irequency connector; a coaxial housing so constructed and band, may be terminated in substantially its arranged as to accommodate the aforesaid parcharacteristic impedance at any terminal point allel network and complete the requisite electrialong such line at which the device may be insorted.

cal connections thereof, and to provide a coaxial receptacle for said coaxial connector such that the outer and inner conductors thereof may be electrically connected across the aforesaid disk 7 resistor; supporting structure for said coaxial mounting; means for making external electrical connections thereto across the said capacitor; and means for attaching said coaxial connector at one end to said coaxial receptacle and at the other end to a coaxial transmission line terminal connector so as to connect the said coaxial line electrically across the aforesaid parallel network.

The foregoing, together with other objects, advantages, and features of the invention, may be understood more fully from the following description of one preferred embodiment thereof when taken in conjunction with the accompanying drawings, wherein a crystal voltmeter arrangement utilizing the invention is illustrated by way of example only, and in which:

Fig. l is a front elevational View of a preferred embodiment of the invention, with rectifying crystal cartridge removed to show the arrangement of certain of the contact fingers;

Fig. 2 is a longitudinal section of the device illustrated in Fig. 1, taken substantially along the line 2-2 of Fig. 1, but with the crystal cartridge shown housed therein in its proper operating position;

Fig. 3 is a front elevational view, on considerably enlarged scale, showing a typical circular dag disk resistor such as employed in the Fig. 1 device; 7 Fig. 4 is a fragmentary sectional View. of the I Fig. 3 resistor, still further enlarged, taken substantially alongthe line 44 of Fig. 3 and showing details of the structure thereof;

Fig. 5 is a schematic diagram of the electrical circuit and associated components of a crystal voltmeter arrangement utilizing the device shown in Figs. 1 and 2; and

Fig. 6 is a side elevational view of the rectifying crystal cartridge.

Referring now to the drawings, there is illustrated therein one practical embodiment of the invention, designed for use in conjunction with a suitably sensitive meter to measure radio-frequency voltages of relatively small magnitude (such as the output voltages from an ultra-highfrequency signal generator) along a standard 50- ohm coaxial transmission line.

As illustrated in Figs. 1 and 2, reference numeral II] generally designates the hollow coaxial housing wherein there is mounted an insertible unidirectional rectifier means: specifically, the stepped cartridge-type crystal II (see Fig. 6), of which one example known to the art is the socalled Western Electric type 1N21 crystal. The housing It is shown as comprising a flat annular metal disk-like front part I2 and a separable coaxial rear part I3; the latter being formed by the hollow metallic cylindrical member I4 and the coaxial connector member I5, coupled together by the metallic coupling ring I6. A flat circular flange I! is provided at one end of the member I4. Disposed between the front disk I2 and the flange H of the separable rear part I8 is an annular wafer-like member I8 of mica or other suitable dielectric material, which together with the disk I2 and the flange I1 forms a capacitor, the purpose of which will become more apparent from the subsequent description of the device. Metal screws I9 secure the members I2, I1 and I8 together, each of these screws being electrically insulated from the front disk I2 by means of an insulating sleeve 20 of vulcanized hard rubber or other suitable insulating material, and being screw-threaded at their inner ends into the flange IT to provide good electrical connection therewith. The two lower screws I9 shown also serve to mount the rear part I3 of the housing Ill securely on (and connect it electrically to) the metal supporting bracket M, which in turn is fixedly mounted on and electrically connected to the metal base 23 by the screws 22.

The substantially half-wave rectified D. C. output of the device may be removed by means of leads, as shown at 24 and 25, the lead 24 being connected to the front disk I2 by means of a short screw which is sufiiciently short to penetrate only into disk I2. Output lead 25 forms the ground return lead and is connected, in this illustrative example, to the base 23 by means of a screw 22' attached thereto at some convenient point, such as shown in Fig. 2, the electrical circuit being completed to the rear part I3 of the device by way of base 23, bracket 2!, and the two lower screws I9.

thick and presents a capacitance of approximately 100 micromicrofarads. This wafer is provided with suitable marginal perforations and with a central opening, for passage of the screws I9 and insertion of the crystal cartridge II, respectively.

Both the front part I2 and the member I4 of the rear part I3 of the mounting II] are axially perforated and recessed to permit the insertion of crystal cartridge II and provide coaxial cylindrical chambers 26 and 27 espectively, of appropriate size and cross-sectional dimensions.

Annularly arranged metal fingers 28, which protrude outwardly and forwardly from the periphery of the central aperture in the front part I2 of the device, embrace and firmly grip the cylindrical flanged metal cap 29 of the crystal cartridge I I, thereby establishing good electrical conductive contact therewith. Similarly, the internal axially-located and annularly-arranged metal fingers 30 perform a similar function with reference to the cylindrical metal tip 3| of the crystal cartridge I I. Fingers 30 project from and are integral with the metal plug 30, which forms the inner coaxial conductor of member IA. The other end of the plug 30' is of reduced diameter and screw-threaded for attachment to the correspondingly hollowed and threaded central conductor 32 of the coaxial connector I5. When plug 30 is screwed home through the central hole 33 of the disk-shaped, non-inductive resistor 34 (which in this illustration is of the type known to the art as circular dag), a firm contact is established between the central annular metallic terminal 35 of resistor 34 (considerably wards the coaxial connector member I5. The

connector member I5 is provided at one end with a circular flange 36 which is engaged by the coupling nut I6, whereby a firm contact is estab;

lished with the end of the member I4. The member I4 is externally threaded as indicated at 31 to receive said coupling nut i6, whereby the clamping and electrical connection of the circumferential annular metallic terminal 38 of resistor 34 between the flange 36 and the end of the member I 4 is effected when the metallic coupling nut I 6 is screwed down tightly in place.

The input end of the central conductor 32 of coaxial connector I5 is shown in this particular illustration as being of a split-sleeve construction, having spaced fingers 50, while the outer conductor of connector I5 is externally threaded at its input end as indicated at 4 I. This arrangement is designed to accommodate the common type of coaxial transmission line terminal connector (not shown) which includes a central axial metal prong and an external loosely-rotatable threaded sleeve (similar in operation to coupling nut I6) for seating connections firmly. Other types of coaxial line terminal connectors may be accommodated in similar fashion, as by alternatively constructing the coaxial connector member I5 to include such a central axial prong and/or such an external threaded sleeve at its input end as may be necessary to mate the particular terminal connector being used with the coaxial transmission linein fact, in practice a set of various types of interchangeable connectors l5 might profitably be constructed, to cover the various possibilities. Also, while connector member I5 is illustrated in Fig. 2 as being filled with solid polystyrene 45, this is purely for purposes of insulation and support and is not vital to the action of the device. Any suitable dielectric material can be used for the filling 45, or it can be eliminated entirely.

Figs. 3 and 4 illustrate the construction of the circular dag resistor 34, in which Fig. 3 is a front elevational view thereof at approximately 6 times normal size, while Fig. l shows a further enlargedand considerably exaggerated fragmentary cross-section thereof taken along the line 44 of Fig. 3. In this illustrative example, the annular central terminal 35 and circumferential annular terminal 38 each comprise flash coatings of silver, approximately .010 inch thick, impressed on one face of a circular backing disk 42 of Bakelite or other suitable insulating material. The non-inductive intermediate annular resistant section 43 shown in Figs. 3 and 4 comprises a flash coating of carbon, approximately .001 inch thick, impressed on the same face of base 42 throughout the area intervening between annular terminals 35 and 3B, and sufiicient to present an electrical resistance of the order of sixty ohms between said terminals. The resistor 34 also has a central hole 33 of sufiicient diameter to just permit passage of the threaded portion of plu 30, as illustrated in Fig. 2.

The equivalent circuit for the preferred model herein discussed is shown schematically in Fig. 5, with connections to the coaxial transmission line terminal T and output meter M indicated by dotted lines, and. operates as follows to provide a substantially half-wave rectified D. C. output across terminals 26 and 22'. Radio-frequency impulses from coaxial transmission line T impressed at the input end of connector I5, as at points 40 and 4 I, effectively see the parallel network comprisingthe non-inductive circular dag resistor 34 across the series-parallel combination of unidirectional rectifying crystal II in series with radio-frequency by-pass capacitor I 8 and meter M in parallel. For an impressed impulse of one polarity (which for purposes of discussion we shall call positive) the D. C. resistance RX ofiered by crystal II is quite low (of the order of 200 to 300 ohms in a practical case of the Type 1N21 crystal cited), whereas for impressed impulses of the opposite (or negative) polarity its D. C. resistance Rx is very high (probably of the order of several thousand ohms) Thus the magnitudes of the currents and resultant voltage drops across the various elements of the parallel network will differ widely, according as they arise from impulses of positive or negative polarity impressed at input points 40 and. 4|. They also will vary with frequency, but to a much less extent. In this illustration, the positive direction of current fiow through the crystal I I is assumed to be from tip 3| to cap 29, as is commonly the case in practice.

It may help to clarify the discussion if we assign definite values, of an order of magnitude we might expect to meet in a practical application, to certain of the variables involved. Accordingly, let us assume, in addition to the already-mentioned 50-ohm characteristic impedance of the coaxial transmission line, that the actual frequency 7 is 1000 megacycles per second, that the resistances offered by crystal II at that frequency are Rx=250 ohms in the positive direction and RX=2500 ohms in the negative direction, that the magnitudes of resistance 34 and capacitance I8 are R=62.5 ohms and C=l00 micromicrofarads, respectively, and that the resistance Rm of meter M, looking in from terminals 26 and 22', is ohms. Then the magnitude of the reactance [X01 offered by capacitor I3 at frequency f will be, employing the usual formula, quite small:

, far as any negative radio-frequency impulses that might slip through crystal 1 I are concerned, the magnitude of the combined impedance of meter M and capacitor I 8 working out in the example cited to be approximately 1.07 oh ms at frequency 5'. Furthermore, since the magnitude R of resistance 3 is only 62.5 ohms, whereas the combined impedance of the other branches of the parallel network will be approximately 2500 ohms in the negative direction, it is clear that more than 97% of all negative radio-frequency impulses impressed at input points 1 3 and ll in this illustrative example will be by-passed through resistor 34. Also, the combined impedance of the parallel network in the negative direction will have a magnitude of approximately 61 ohms.

On the other hand, positive impulses impressed at the input points ii) and 48 will effectively see the 50-ohm characteristic impedance of the coaxial line, since the magnitude of the combined impedance [Z] of the parallel network will then be approximately:

Thus, the impedance of the coaxial line will be perfectly matched for all positive impulses, and nearly so for all negative impulses, thereby virtually eliminating the undesirable effects of impedance mismatch, reflection, or standing waves that so frequently arise when terminal connections are made to a coaxial transmission line.

Furthermore, it is evident that the current flowing through crystal H in the positive direction will be substantial, being about one-fifth of the total such current furnished by the coaxial line. Hence, in spite of the shunting efiect of capacitor i8, adequate half-wave rectified D. 0. current will reach meter M to actuate a sensitive movement. For example, if the magnitude of the radio frequency voltage across input points 49 and 4! in the foregoing illustration is assumed to be two volts, a current of the order of 40 milliamperes will fiowacross the 50-ohm parallel network in the positive, direction, of which approximately one-fifth, or 8 milliamperes, will pass through crystal 2 I. Approximately 100 of the 8 milliamperes, or 80 microamperes of half-wave rectified D. C. current, will then flow through meter M, which is more than required by, say, a 50-microampere movement. Also, since the current in leads 24 and 25 will be essentially D. C. in character, the efiects of lead inductance and stray capacitance introduced by such leads will be negligible.

The device as illustrated and discussed herein is suitable for connection to any terminal T along any50-ohm coaxial transmission line of given cross-sectional dimensions. It is to be noted, however, that the invention and the principles involved are not limited in their practical application to 50-ohm coaxial lines of given dimensions, nor to voltage measurements. For example, by properly choosing the depth of the flash carbon coating 43, resistances 34 can be constructed of sufficient range of magnitude R. to make it possible to present, in combination with the other elements of the parallel'network, a combined input impedance of any magnitude likely to be desired in practice. Coaxial transmission lines of different cross-sectional dimensions would simply require that correspondingly different dimensions be adopted in the construction of the coaxial elements of the device. It is also evident that meter M can be calibrated in terms of the known impedance of the parallel network to read voltages, currents, or power. Furthermore, terminals 25 and 22 can be left open if for any reason it is desired merely to terminate the coaxial line (either in approximately its own characteristic impedance or in a different impedance) without using the rectified output; or any other device requiring for its operation substantially half-wave rectified D. C. currents or voltages may be connected to said terminals.

The eifective frequency range of the device is determined chiefly by the reactance lXc! of capacitor it, since its magnitude varies markedly with frequency, and to a lesser extent by probable coincident variations in the magnitude of the resistance Rx of crystal H in the positive direction. Considerably larger magnitudes C of by-pass capacitance it will be required in order properly to shunt meter M at low radio frequencies; while marked variations in the magnitude of Ex will require ofisetting changes in the magnitude R of resistance 34. By so choosing the values as to strike an average for a particular band of frequencies, measurements ,of considerable accuracy :can be obtained over that band, the accuracy improving as the Width of the operating band of frequencies is cut down. A rough working model, constructed approximately as described herein, was found to give voltage readings accurate to within 10% over the wide frequency range of 10 kilocycles to 1000 megacycles per second.

The efiective voltage or current range of the device depends at the low end largely on the sensitivity of the meter M used therewith, and at the high end on the burn-out point of the crystal H. The above-mentioned working model was found to read down to .01 volt and up to at least 2 volts.

It remains merely to point out that the device is not limited to the use of type 1N21 crystalsany type of unidirectional rectiiying crystal ll having a suitably high back-to-iront resistance ratio and mounted in a metal-ended cartridge may be employed, provided that the coaxial crystal housing l0, including fingers 23 and 30, is

constructed to fit it.

It will be obvious to those skilled in the art that further changes or modifications may be made in the preferred embodiment of the invention herein described without departing from the spirit thereof, and it is to be distinctly understood, thereiore, that no limitations are intended other than are imposed by the scope of the appended claim as limited by the prior art.

The invention described herein may be manu iactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

An output termination for a coaxial line of'the internal conductor and sheath type comprising a coaxial coupler for receiving the end of the line and having internal and external conductors terminating in the same plane, a flat annular resistor abutting in said'plane against the conductors to terminate the coaxial line and coupler with the desired impedance, a central cavity member in electric contact with the outer portion of the resistor and opposite the external coupler conductor, a crystal rectifierunit in the cavity, wholly outside the coaxial coupler, and spaced from the resistor, a conductor member connecting the crystal unit to the center of the resistor and the internal coupler conductor, and a conductive cap member contacting the crystal unit and capacitatively coupled to the cavity member, the cap and cavity members constituting output electrodes for the line.

J. PLUMIER LEIPHART.

REFERENCES CITED 7 The following references are of record in the file of this patent:

UNITED STATES PATENTS 2,498,335 Hunt Feb. 21, 1950

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